From 36c788701f98c60fbea417198dbbb3407fa599bc Mon Sep 17 00:00:00 2001
From: Vladimir Sitnikov <sitnikov@netcracker.com>
Date: Thu, 6 Jun 2024 20:26:14 +0300
Subject: [PATCH 1/5] WIP: fixup nullability annotations and generic variance

---
 .../main/java/net/jqwik/api/Arbitraries.java  | 78 +++++++++----------
 .../main/java/net/jqwik/api/Arbitrary.java    |  4 +-
 api/src/main/java/net/jqwik/api/Builders.java |  8 +-
 .../main/java/net/jqwik/api/Combinators.java  | 78 +++++++++----------
 .../java/net/jqwik/api/RandomGenerator.java   |  3 +-
 .../main/java/net/jqwik/api/Shrinkable.java   |  2 +-
 api/src/main/java/net/jqwik/api/Tuple.java    | 34 ++++----
 .../jqwik/api/arbitraries/ArrayArbitrary.java |  6 +-
 .../api/arbitraries/FunctionArbitrary.java    |  4 +-
 .../api/arbitraries/IteratorArbitrary.java    |  6 +-
 .../jqwik/api/arbitraries/ListArbitrary.java  |  6 +-
 .../jqwik/api/arbitraries/MapArbitrary.java   |  4 +-
 .../jqwik/api/arbitraries/SetArbitrary.java   |  6 +-
 .../api/arbitraries/SizableArbitrary.java     |  4 +-
 .../api/arbitraries/StreamArbitrary.java      |  6 +-
 .../api/arbitraries/StreamableArbitrary.java  |  4 +-
 .../api/arbitraries/TraverseArbitrary.java    |  4 +-
 .../jqwik/api/arbitraries/TypeArbitrary.java  |  4 +-
 .../api/statistics/StatisticsCoverage.java    |  2 +-
 .../customized-parameter-generation.md        | 10 +--
 .../StatisticsCoverageExamples.java           |  5 +-
 .../engine/facades/ArbitrariesFacadeImpl.java | 28 +++----
 .../engine/facades/CombinatorsFacadeImpl.java |  2 +-
 .../statistics/StatisticsCollectorImpl.java   |  6 +-
 .../arbitraries/DefaultArrayArbitrary.java    |  2 +-
 .../arbitraries/DefaultFunctionArbitrary.java | 14 ++--
 .../arbitraries/DefaultIteratorArbitrary.java |  2 +-
 .../arbitraries/DefaultListArbitrary.java     |  8 +-
 .../arbitraries/DefaultSetArbitrary.java      |  2 +-
 .../arbitraries/DefaultStreamArbitrary.java   |  2 +-
 .../arbitraries/EdgeCasesSupport.java         |  2 +-
 .../arbitraries/FrequencyOfArbitrary.java     |  4 +-
 .../FromGeneratorWithSizeArbitrary.java       |  4 +-
 .../properties/arbitraries/LazyArbitrary.java |  4 +-
 .../arbitraries/LazyOfArbitrary.java          |  6 +-
 .../arbitraries/OneOfArbitrary.java           |  5 +-
 .../arbitraries/RecursiveArbitrary.java       |  6 +-
 .../combinations/DefaultListCombinator.java   |  2 +-
 .../exhaustive/ExhaustiveGenerators.java      |  2 +-
 .../FlatMappedExhaustiveGenerator.java        |  6 +-
 .../randomized/AbstractFunctionGenerator.java |  6 +-
 .../randomized/ConstantFunctionGenerator.java |  2 +-
 .../randomized/FrequencyGenerator.java        |  2 +-
 .../randomized/FunctionGenerator.java         |  2 +-
 .../randomized/RandomGenerators.java          |  4 +-
 .../shrinking/ChooseValueShrinkable.java      |  4 +-
 .../support/ChooseRandomlyByFrequency.java    |  6 +-
 .../statistics/StatisticsCoverageTests.java   |  8 +-
 kotlin/build.gradle                           |  1 +
 .../jqwik/kotlin/api/BuildersExtensions.kt    |  6 +-
 .../net/jqwik/kotlin/api/SequenceArbitrary.kt |  4 +-
 .../kotlin/api/SizableArbitraryExtensions.kt  |  2 +-
 .../jqwik/kotlin/CollectionExtensionsTests.kt |  4 +-
 .../net/jqwik/kotlin/InternalModifierTests.kt |  6 +-
 54 files changed, 231 insertions(+), 211 deletions(-)

diff --git a/api/src/main/java/net/jqwik/api/Arbitraries.java b/api/src/main/java/net/jqwik/api/Arbitraries.java
index b46c0943c..cefb5d81a 100644
--- a/api/src/main/java/net/jqwik/api/Arbitraries.java
+++ b/api/src/main/java/net/jqwik/api/Arbitraries.java
@@ -25,13 +25,13 @@ public static abstract class ArbitrariesFacade {
 			implementation = FacadeLoader.load(ArbitrariesFacade.class);
 		}
 
-		public abstract <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> all);
+		public abstract <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> all);
 
 		public abstract <M> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> actionArbitrary);
 
-		public abstract <T> Arbitrary<T> frequencyOf(List<Tuple2<Integer, Arbitrary<T>>> frequencies);
+		public abstract <T extends @Nullable Object> Arbitrary<T> frequencyOf(List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies);
 
-		public abstract <@Nullable T> Arbitrary<T> just(@Nullable T value);
+		public abstract <T extends @Nullable Object> Arbitrary<T> just(T value);
 
 		public abstract IntegerArbitrary integers();
 
@@ -57,36 +57,36 @@ public static abstract class ArbitrariesFacade {
 
 		public abstract <T> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<TypeUsage, Arbitrary<Object>> noDefaultResolver);
 
-		public abstract <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier);
+		public abstract <T extends @Nullable Object> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier);
 
 		public abstract <T> TypeArbitrary<T> forType(Class<T> targetType);
 
-		public abstract <K, V> MapArbitrary<K, V> maps(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
+		public abstract <K extends @Nullable Object, V extends @Nullable Object> MapArbitrary<K, V> maps(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
 
-		public abstract <K, V> Arbitrary<Map.Entry<K, V>> entries(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
+		public abstract <K extends @Nullable Object, V extends @Nullable Object> Arbitrary<Map.Entry<K, V>> entries(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary);
 
-		public abstract <T> Arbitrary<T> recursive(
-			Supplier<Arbitrary<T>> base,
-			Function<Arbitrary<T>, Arbitrary<T>> recur,
+		public abstract <T extends @Nullable Object> Arbitrary<T> recursive(
+			Supplier<? extends Arbitrary<T>> base,
+			Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 			int minDepth,
 			int maxDepth
 		);
 
-		public abstract <T> Arbitrary<T> lazyOf(List<Supplier<Arbitrary<T>>> suppliers);
+		public abstract <T extends @Nullable Object> Arbitrary<T> lazyOf(List<? extends Supplier<? extends Arbitrary<T>>> suppliers);
 
 		public abstract <T> TraverseArbitrary<T> traverse(Class<T> targetType, Traverser traverser);
 
 		public abstract Arbitrary<Character> of(char[] chars);
 
-		public abstract <T> Arbitrary<T> of(Collection<T> values);
+		public abstract <T extends @Nullable Object> Arbitrary<T> of(Collection<T> values);
 
-		public abstract <T> Arbitrary<T> create(Supplier<T> supplier);
+		public abstract <T extends @Nullable Object> Arbitrary<T> create(Supplier<T> supplier);
 
-		public abstract <T> Arbitrary<List<T>> shuffle(List<T> values);
+		public abstract <T extends @Nullable Object> Arbitrary<List<T>> shuffle(List<T> values);
 
-		public abstract <T> Arbitrary<T> fromGenerator(IntFunction<RandomGenerator<T>> generatorSupplier);
+		public abstract <T extends @Nullable Object> Arbitrary<T> fromGenerator(IntFunction<? extends RandomGenerator<T>> generatorSupplier);
 
-		public abstract <T> Arbitrary<T> frequency(List<Tuple2<Integer, T>> frequencies);
+		public abstract <T extends @Nullable Object> Arbitrary<T> frequency(List<? extends Tuple2<Integer, T>> frequencies);
 	}
 
 	private Arbitraries() {
@@ -99,7 +99,7 @@ private Arbitraries() {
 	 * @param <T>       The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <T> Arbitrary<T> fromGenerator(RandomGenerator<T> generator) {
+	public static <T extends @Nullable Object> Arbitrary<T> fromGenerator(RandomGenerator<T> generator) {
 		return fromGeneratorWithSize(ignore -> generator);
 	}
 
@@ -111,7 +111,7 @@ public static <T> Arbitrary<T> fromGenerator(RandomGenerator<T> generator) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = EXPERIMENTAL, since = "1.8.0")
-	public static <T> Arbitrary<T> fromGeneratorWithSize(IntFunction<RandomGenerator<T>> generatorSupplier) {
+	public static <T extends @Nullable Object> Arbitrary<T> fromGeneratorWithSize(IntFunction<RandomGenerator<T>> generatorSupplier) {
 		return ArbitrariesFacade.implementation.fromGenerator(generatorSupplier);
 	}
 
@@ -123,7 +123,7 @@ public static <T> Arbitrary<T> fromGeneratorWithSize(IntFunction<RandomGenerator
 	 * @param <T>       The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <T> Arbitrary<T> randomValue(Function<Random, T> generator) {
+	public static <T extends @Nullable Object> Arbitrary<T> randomValue(Function<Random, T> generator) {
 		IntFunction<RandomGenerator<T>> generatorSupplier = ignore -> random -> Shrinkable.unshrinkable(generator.apply(random));
 		return fromGeneratorWithSize(generatorSupplier);
 	}
@@ -153,7 +153,7 @@ public static Arbitrary<Random> randoms() {
 	 * @return a new arbitrary instance
 	 */
 	@SafeVarargs
-	public static <T> Arbitrary<T> of(T... values) {
+	public static <T extends @Nullable Object> Arbitrary<T> of(T... values) {
 		return of(Arrays.asList(values));
 	}
 
@@ -172,7 +172,7 @@ public static <T> Arbitrary<T> of(T... values) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.1")
-	public static <@Nullable T> Arbitrary<T> of(Collection<T> values) {
+	public static <T extends @Nullable Object> Arbitrary<T> of(Collection<T> values) {
 		return ArbitrariesFacade.implementation.of(values);
 	}
 
@@ -192,7 +192,7 @@ public static <T> Arbitrary<T> of(T... values) {
 	 */
 	@API(status = MAINTAINED, since = "1.3.0")
 	@SafeVarargs
-	public static <T> Arbitrary<T> ofSuppliers(Supplier<T>... valueSuppliers) {
+	public static <T extends @Nullable Object> Arbitrary<T> ofSuppliers(Supplier<T>... valueSuppliers) {
 		return of(valueSuppliers).map(Supplier::get);
 	}
 
@@ -211,7 +211,7 @@ public static <T> Arbitrary<T> ofSuppliers(Supplier<T>... valueSuppliers) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.1")
-	public static <T> Arbitrary<T> ofSuppliers(Collection<Supplier<T>> valueSuppliers) {
+	public static <T extends @Nullable Object> Arbitrary<T> ofSuppliers(Collection<Supplier<T>> valueSuppliers) {
 		return of(valueSuppliers).map(Supplier::get);
 	}
 
@@ -247,7 +247,7 @@ public static <T extends Enum<T>> Arbitrary<T> of(Class<T> enumClass) {
 	 */
 	@SuppressWarnings("unchecked")
 	@SafeVarargs
-	public static <T> Arbitrary<T> oneOf(Arbitrary<? extends T> first, Arbitrary<? extends T>... rest) {
+	public static <T extends @Nullable Object> Arbitrary<T> oneOf(Arbitrary<? extends T> first, Arbitrary<? extends T>... rest) {
 		List<Arbitrary<? extends T>> all = new ArrayList<>();
 		all.add(first);
 		for (Arbitrary<?> arbitrary : rest) {
@@ -264,7 +264,7 @@ public static <T> Arbitrary<T> oneOf(Arbitrary<? extends T> first, Arbitrary<? e
 	 * @return a new arbitrary instance
 	 */
 	@SuppressWarnings("unchecked")
-	public static <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> choices) {
+	public static <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> choices) {
 		if (choices.isEmpty()) {
 			String message = "oneOf() must not be called with no choices";
 			throw new JqwikException(message);
@@ -285,7 +285,7 @@ public static <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> choices)
 	 * @return a new arbitrary instance
 	 */
 	@SafeVarargs
-	public static <T> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
 		return frequency(Arrays.asList(frequencies));
 	}
 
@@ -297,7 +297,7 @@ public static <T> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
 	 * @param <T>         The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <@Nullable T> Arbitrary<T> frequency(List<Tuple2<Integer, T>> frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequency(List<? extends Tuple2<Integer, T>> frequencies) {
 		return ArbitrariesFacade.implementation.frequency(frequencies);
 	}
 
@@ -311,7 +311,7 @@ public static <T> Arbitrary<T> frequency(Tuple2<Integer, T>... frequencies) {
 	 */
 	@SuppressWarnings("unchecked")
 	@SafeVarargs
-	public static <T> Arbitrary<T> frequencyOf(Tuple2<Integer, Arbitrary<? extends T>>... frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequencyOf(Tuple2<Integer, Arbitrary<? extends T>>... frequencies) {
 		List<Tuple2<Integer, Arbitrary<T>>> all = new ArrayList<>();
 		for (Tuple2<Integer, Arbitrary<? extends T>> frequency : frequencies) {
 			all.add(Tuple.of(frequency.get1(), (Arbitrary<T>) frequency.get2()));
@@ -327,7 +327,7 @@ public static <T> Arbitrary<T> frequencyOf(Tuple2<Integer, Arbitrary<? extends T
 	 * @param <T>         The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <@Nullable T> Arbitrary<T> frequencyOf(List<Tuple2<Integer, Arbitrary<T>>> frequencies) {
+	public static <T extends @Nullable Object> Arbitrary<T> frequencyOf(List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
 		// Simple flatMapping is not enough because of configurations
 		return ArbitrariesFacade.implementation.frequencyOf(frequencies);
 	}
@@ -454,7 +454,7 @@ public static CharacterArbitrary chars() {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.1.1")
-	public static <T> Arbitrary<T> create(Supplier<T> supplier) {
+	public static <T extends @Nullable Object> Arbitrary<T> create(Supplier<T> supplier) {
 		return ArbitrariesFacade.implementation.create(supplier);
 	}
 
@@ -468,7 +468,7 @@ public static <T> Arbitrary<T> create(Supplier<T> supplier) {
 	 * @return a new arbitrary instance
 	 */
 	@SafeVarargs
-	public static <T> Arbitrary<List<T>> shuffle(T... values) {
+	public static <T extends @Nullable Object> Arbitrary<List<T>> shuffle(T... values) {
 		return shuffle(Arrays.asList(values));
 	}
 
@@ -481,7 +481,7 @@ public static <T> Arbitrary<List<T>> shuffle(T... values) {
 	 * @param <T>    The type of values to generate
 	 * @return a new arbitrary instance
 	 */
-	public static <T> Arbitrary<List<T>> shuffle(List<T> values) {
+	public static <T extends @Nullable Object> Arbitrary<List<T>> shuffle(List<T> values) {
 		return ArbitrariesFacade.implementation.shuffle(values);
 	}
 
@@ -591,7 +591,7 @@ public static <T> TraverseArbitrary<T> traverse(Class<T> targetType, Traverser t
 	 * @see #recursive(Supplier, Function, int)
 	 * @see #lazyOf(Supplier, Supplier[])
 	 */
-	public static <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier) {
+	public static <T extends @Nullable Object> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		return ArbitrariesFacade.implementation.lazy(arbitrarySupplier);
 	}
 
@@ -609,9 +609,9 @@ public static <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier) {
 	 * @return a new arbitrary instance
 	 * @see #lazy(Supplier)
 	 */
-	public static <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+	public static <T extends @Nullable Object> Arbitrary<T> recursive(
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int depth
 	) {
 		return ArbitrariesFacade.implementation.recursive(base, recur, depth, depth);
@@ -633,9 +633,9 @@ public static <T> Arbitrary<T> recursive(
 	 * @see #lazy(Supplier)
 	 */
 	@API(status = MAINTAINED, since = "1.6.4")
-	public static <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+	public static <T extends @Nullable Object> Arbitrary<T> recursive(
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int minDepth,
 		int maxDepth
 	) {
@@ -671,7 +671,7 @@ public static <T> Arbitrary<T> recursive(
 	@SuppressWarnings("unchecked")
 	@SafeVarargs
 	@API(status = MAINTAINED, since = "1.3.4")
-	public static <T> Arbitrary<T> lazyOf(Supplier<Arbitrary<? extends T>> first, Supplier<Arbitrary<? extends T>>... rest) {
+	public static <T extends @Nullable Object> Arbitrary<T> lazyOf(Supplier<Arbitrary<? extends T>> first, Supplier<Arbitrary<? extends T>>... rest) {
 		List<Supplier<Arbitrary<T>>> all = new ArrayList<>();
 		all.add(() -> (Arbitrary<T>) first.get());
 		for (Supplier<Arbitrary<? extends T>> arbitrarySupplier : rest) {
diff --git a/api/src/main/java/net/jqwik/api/Arbitrary.java b/api/src/main/java/net/jqwik/api/Arbitrary.java
index f76f8c7b4..9f0c5ed08 100644
--- a/api/src/main/java/net/jqwik/api/Arbitrary.java
+++ b/api/src/main/java/net/jqwik/api/Arbitrary.java
@@ -6,7 +6,7 @@
 
 import com.google.errorprone.annotations.*;
 import org.apiguardian.api.*;
-import org.jspecify.annotations.Nullable;
+import org.jspecify.annotations.*;
 
 import net.jqwik.api.arbitraries.*;
 
@@ -20,7 +20,7 @@
  */
 @API(status = STABLE, since = "1.0")
 @CheckReturnValue
-public interface Arbitrary<@Nullable T> {
+public interface Arbitrary<T extends @Nullable Object> {
 
 	@API(status = INTERNAL)
 	abstract class ArbitraryFacade {
diff --git a/api/src/main/java/net/jqwik/api/Builders.java b/api/src/main/java/net/jqwik/api/Builders.java
index 3a7e317d4..b96949449 100644
--- a/api/src/main/java/net/jqwik/api/Builders.java
+++ b/api/src/main/java/net/jqwik/api/Builders.java
@@ -53,7 +53,7 @@ private BuilderCombinator(Supplier<B> starter, List<Tuple3<Double, Arbitrary<Obj
 		 * @param <T>
 		 * @return new {@linkplain CombinableBuilder} instance
 		 */
-		public <@Nullable T> CombinableBuilder<B, T> use(Arbitrary<T> arbitrary) {
+		public <T extends @Nullable Object> CombinableBuilder<B, T> use(Arbitrary<T> arbitrary) {
 			return new CombinableBuilder<>(this, 1.0, arbitrary);
 		}
 
@@ -167,7 +167,7 @@ BuilderCombinator<B> withMutator(double probabilityOfUse, Arbitrary<Object> arbi
 	 *
 	 * @param <B> The builder's type
 	 */
-	public static class CombinableBuilder<B, T> {
+	public static class CombinableBuilder<B, T extends @Nullable Object> {
 		private final BuilderCombinator<B> combinator;
 		private final double probabilityOfUse;
 		private final Arbitrary<T> arbitrary;
@@ -201,7 +201,7 @@ public CombinableBuilder<B, T> withProbability(double probabilityOfUse) {
 		 * @return new {@linkplain BuilderCombinator} instance
 		 */
 		@SuppressWarnings("unchecked")
-		public BuilderCombinator<B> in(BiFunction<B, @Nullable T, B> toFunction) {
+		public BuilderCombinator<B> in(BiFunction<? super B, ? super T, ? extends B> toFunction) {
 			if (probabilityOfUse == 0.0) {
 				return combinator;
 			}
@@ -217,7 +217,7 @@ public BuilderCombinator<B> in(BiFunction<B, @Nullable T, B> toFunction) {
 		 * @param setter Use value provided by arbitrary to change a builder's property.
 		 * @return new {@linkplain BuilderCombinator} instance with same embedded builder
 		 */
-		public BuilderCombinator<B> inSetter(BiConsumer<B, T> setter) {
+		public BuilderCombinator<B> inSetter(BiConsumer<? super B, ? super T> setter) {
 			BiFunction<B, T, B> toFunction = (b, t) -> {
 				setter.accept(b, t);
 				return b;
diff --git a/api/src/main/java/net/jqwik/api/Combinators.java b/api/src/main/java/net/jqwik/api/Combinators.java
index da7011726..a8a21dfdc 100644
--- a/api/src/main/java/net/jqwik/api/Combinators.java
+++ b/api/src/main/java/net/jqwik/api/Combinators.java
@@ -68,7 +68,7 @@ public abstract <T1, T2, T3, T4, T5, T6, T7, T8> Combinator8<T1, T2, T3, T4, T5,
 			Arbitrary<T8> a8
 		);
 
-		public abstract <T> ListCombinator<T> combineList(List<Arbitrary<T>> listOfArbitraries);
+		public abstract <T> ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries);
 	}
 
 	private Combinators() {
@@ -79,7 +79,7 @@ private Combinators() {
 	 *
 	 * @return Combinator2 instance which can be evaluated using {@linkplain Combinator2#as}
 	 */
-	public static <@Nullable T1, @Nullable T2> Combinator2<T1, T2> combine(Arbitrary<T1> a1, Arbitrary<T2> a2) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object> Combinator2<T1, T2> combine(Arbitrary<T1> a1, Arbitrary<T2> a2) {
 		return CombinatorsFacade.implementation.combine2(a1, a2);
 	}
 
@@ -88,7 +88,7 @@ private Combinators() {
 	 *
 	 * @return Combinator3 instance which can be evaluated using {@linkplain Combinator3#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3> Combinator3<T1, T2, T3> combine(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> Combinator3<T1, T2, T3> combine(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3) {
 		return CombinatorsFacade.implementation.combine3(a1, a2, a3);
 	}
 
@@ -97,7 +97,7 @@ private Combinators() {
 	 *
 	 * @return Combinator4 instance which can be evaluated using {@linkplain Combinator4#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> Combinator4<T1, T2, T3, T4> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> Combinator4<T1, T2, T3, T4> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 		Arbitrary<T4> a4
 	) {
@@ -109,7 +109,7 @@ private Combinators() {
 	 *
 	 * @return Combinator5 instance which can be evaluated using {@linkplain Combinator5#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> Combinator5<T1, T2, T3, T4, T5> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> Combinator5<T1, T2, T3, T4, T5> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 		Arbitrary<T4> a4, Arbitrary<T5> a5
 	) {
@@ -121,7 +121,7 @@ private Combinators() {
 	 *
 	 * @return Combinator6 instance which can be evaluated using {@linkplain Combinator6#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> Combinator6<T1, T2, T3, T4, T5, T6> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> Combinator6<T1, T2, T3, T4, T5, T6> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 		Arbitrary<T4> a4, Arbitrary<T5> a5, Arbitrary<T6> a6
 	) {
@@ -133,7 +133,7 @@ private Combinators() {
 	 *
 	 * @return Combinator7 instance which can be evaluated using {@linkplain Combinator7#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> Combinator7<T1, T2, T3, T4, T5, T6, T7> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> Combinator7<T1, T2, T3, T4, T5, T6, T7> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2,
 		Arbitrary<T3> a3, Arbitrary<T4> a4, Arbitrary<T5> a5, Arbitrary<T6> a6, Arbitrary<T7> a7
 	) {
@@ -145,7 +145,7 @@ private Combinators() {
 	 *
 	 * @return Combinator8 instance which can be evaluated using {@linkplain Combinator8#as}
 	 */
-	public static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> combine(
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> combine(
 		Arbitrary<T1> a1, Arbitrary<T2> a2,
 		Arbitrary<T3> a3, Arbitrary<T4> a4, Arbitrary<T5> a5, Arbitrary<T6> a6, Arbitrary<T7> a7, Arbitrary<T8> a8
 	) {
@@ -157,14 +157,14 @@ private Combinators() {
 	 *
 	 * @return ListCombinator instance which can be evaluated using {@linkplain ListCombinator#as}
 	 */
-	public static <@Nullable T> ListCombinator<T> combine(List<Arbitrary<T>> listOfArbitraries) {
+	public static <T extends @Nullable Object> ListCombinator<T> combine(List<? extends Arbitrary<T>> listOfArbitraries) {
 		return CombinatorsFacade.implementation.combineList(listOfArbitraries);
 	}
 
 	/**
 	 * Combinator for two values.
 	 */
-	public interface Combinator2<@Nullable T1, @Nullable T2> {
+	public interface Combinator2<T1 extends @Nullable Object, T2 extends @Nullable Object> {
 
 		/**
 		 * Combine two values.
@@ -173,7 +173,7 @@ public interface Combinator2<@Nullable T1, @Nullable T2> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F2<T1, T2, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F2<T1, T2, R> combinator);
 
 		/**
 		 * Filter two values to only let them pass if the predicate is true.
@@ -191,7 +191,7 @@ public interface Combinator2<@Nullable T1, @Nullable T2> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F2<T1, T2, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F2<T1, T2, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -199,7 +199,7 @@ public interface Combinator2<@Nullable T1, @Nullable T2> {
 	/**
 	 * Combinator for three values.
 	 */
-	public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
+	public interface Combinator3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> {
 
 		/**
 		 * Combine three values.
@@ -208,7 +208,7 @@ public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F3<T1, T2, T3, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F3<T1, T2, T3, R> combinator);
 
 		/**
 		 * Filter three values to only let them pass if the predicate is true.
@@ -226,7 +226,7 @@ public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F3<T1, T2, T3, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F3<T1, T2, T3, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -235,7 +235,7 @@ public interface Combinator3<@Nullable T1, @Nullable T2, @Nullable T3> {
 	/**
 	 * Combinator for four values.
 	 */
-	public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> {
+	public interface Combinator4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> {
 
 		/**
 		 * Combine four values.
@@ -244,7 +244,7 @@ public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F4<T1, T2, T3, T4, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F4<T1, T2, T3, T4, R> combinator);
 
 		/**
 		 * Filter four values to only let them pass if the predicate is true.
@@ -262,7 +262,7 @@ public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F4<T1, T2, T3, T4, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F4<T1, T2, T3, T4, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -271,7 +271,7 @@ public interface Combinator4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for five values.
 	 */
-	public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> {
+	public interface Combinator5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> {
 
 		/**
 		 * Combine five values.
@@ -280,7 +280,7 @@ public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F5<T1, T2, T3, T4, T5, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F5<T1, T2, T3, T4, T5, R> combinator);
 
 		/**
 		 * Filter five values to only let them pass if the predicate is true.
@@ -298,7 +298,7 @@ public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F5<T1, T2, T3, T4, T5, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F5<T1, T2, T3, T4, T5, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -306,7 +306,7 @@ public interface Combinator5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for six values.
 	 */
-	public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> {
+	public interface Combinator6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> {
 
 		/**
 		 * Combine six values.
@@ -315,7 +315,7 @@ public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F6<T1, T2, T3, T4, T5, T6, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F6<T1, T2, T3, T4, T5, T6, R> combinator);
 
 		/**
 		 * Filter six values to only let them pass if the predicate is true.
@@ -333,7 +333,7 @@ public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F6<T1, T2, T3, T4, T5, T6, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F6<T1, T2, T3, T4, T5, T6, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -342,7 +342,7 @@ public interface Combinator6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for seven values.
 	 */
-	public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> {
+	public interface Combinator7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> {
 
 		/**
 		 * Combine seven values.
@@ -351,7 +351,7 @@ public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F7<T1, T2, T3, T4, T5, T6, T7, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F7<T1, T2, T3, T4, T5, T6, T7, R> combinator);
 
 		/**
 		 * Filter seven values to only let them pass if the predicate is true.
@@ -369,7 +369,7 @@ public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F7<T1, T2, T3, T4, T5, T6, T7, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F7<T1, T2, T3, T4, T5, T6, T7, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -378,7 +378,7 @@ public interface Combinator7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 	/**
 	 * Combinator for eight values.
 	 */
-	public interface Combinator8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> {
+	public interface Combinator8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> {
 
 		/**
 		 * Combine eight values.
@@ -387,7 +387,7 @@ public interface Combinator8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator);
 
 		/**
 		 * Filter eight values to only let them pass if the predicate is true.
@@ -405,7 +405,7 @@ public interface Combinator8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(F8<T1, T2, T3, T4, T5, T6, T7, T8, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F8<T1, T2, T3, T4, T5, T6, T7, T8, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -422,7 +422,7 @@ public interface ListCombinator<@Nullable T> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<@Nullable R> Arbitrary<R> as(Function<List<T>, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(Function<List<T>, R> combinator);
 
 		/**
 		 * Filter list of values to only let them pass if the predicate is true.
@@ -440,50 +440,50 @@ public interface ListCombinator<@Nullable T> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <@Nullable R> Arbitrary<R> flatAs(Function<List<T>, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(Function<List<T>, Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F2<@Nullable T1, @Nullable T2, @Nullable R> {
+	public interface F2<T1 extends @Nullable Object, T2 extends @Nullable Object, @Nullable R> {
 		R apply(T1 t1, T2 t2);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F3<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable R> {
+	public interface F3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, @Nullable R> {
 		R apply(T1 t1, T2 t2, T3 t3);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable R> {
+	public interface F4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, @Nullable R> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable R> {
+	public interface F5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable R> {
+	public interface F6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5, T6 t6);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable R> {
+	public interface F7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5, T6 t6, T7 t7);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8, @Nullable R> {
+	public interface F8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4, T5 t5, T6 t6, T7 t7, T8 t8);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/RandomGenerator.java b/api/src/main/java/net/jqwik/api/RandomGenerator.java
index 694d66742..b47f3871e 100644
--- a/api/src/main/java/net/jqwik/api/RandomGenerator.java
+++ b/api/src/main/java/net/jqwik/api/RandomGenerator.java
@@ -5,11 +5,12 @@
 import java.util.stream.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = STABLE, since = "1.0")
-public interface RandomGenerator<T> {
+public interface RandomGenerator<T extends @Nullable Object> {
 
 	@API(status = INTERNAL)
 	abstract class RandomGeneratorFacade {
diff --git a/api/src/main/java/net/jqwik/api/Shrinkable.java b/api/src/main/java/net/jqwik/api/Shrinkable.java
index c99d9bd3c..c7a4c87db 100644
--- a/api/src/main/java/net/jqwik/api/Shrinkable.java
+++ b/api/src/main/java/net/jqwik/api/Shrinkable.java
@@ -10,7 +10,7 @@
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = STABLE, since = "1.0")
-public interface Shrinkable<T> extends Comparable<Shrinkable<T>> {
+public interface Shrinkable<T extends @Nullable Object> extends Comparable<Shrinkable<T>> {
 
 	@API(status = INTERNAL)
 	abstract class ShrinkableFacade {
diff --git a/api/src/main/java/net/jqwik/api/Tuple.java b/api/src/main/java/net/jqwik/api/Tuple.java
index 438d36e99..6b97d3cc1 100644
--- a/api/src/main/java/net/jqwik/api/Tuple.java
+++ b/api/src/main/java/net/jqwik/api/Tuple.java
@@ -37,23 +37,23 @@ static Tuple0 empty() {
 		return Tuple.of();
 	}
 
-	static <@Nullable T1> Tuple1<T1> of(T1 v1) {
+	static <T1 extends @Nullable Object> Tuple1<T1> of(T1 v1) {
 		return new Tuple1<>(v1);
 	}
 
-	static <@Nullable T1, @Nullable T2> Tuple2<T1, T2> of(T1 v1, T2 v2) {
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object> Tuple2<T1, T2> of(T1 v1, T2 v2) {
 		return new Tuple2<>(v1, v2);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3> Tuple3<T1, T2, T3> of(T1 v1, T2 v2, T3 v3) {
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> Tuple3<T1, T2, T3> of(T1 v1, T2 v2, T3 v3) {
 		return new Tuple3<>(v1, v2, v3);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> Tuple4<T1, T2, T3, T4> of(T1 v1, T2 v2, T3 v3, T4 v4) {
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> Tuple4<T1, T2, T3, T4> of(T1 v1, T2 v2, T3 v3, T4 v4) {
 		return new Tuple4<>(v1, v2, v3, v4);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> Tuple5<T1, T2, T3, T4, T5> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> Tuple5<T1, T2, T3, T4, T5> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -63,7 +63,7 @@ static Tuple0 empty() {
 		return new Tuple5<>(v1, v2, v3, v4, v5);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> Tuple6<T1, T2, T3, T4, T5, T6> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> Tuple6<T1, T2, T3, T4, T5, T6> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -74,7 +74,7 @@ static Tuple0 empty() {
 		return new Tuple6<>(v1, v2, v3, v4, v5, v6);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> Tuple7<T1, T2, T3, T4, T5, T6, T7> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> Tuple7<T1, T2, T3, T4, T5, T6, T7> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -86,7 +86,7 @@ static Tuple0 empty() {
 		return new Tuple7<>(v1, v2, v3, v4, v5, v6, v7);
 	}
 
-	static <@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> Tuple8<T1, T2, T3, T4, T5, T6, T7, T8> of(
+	static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> Tuple8<T1, T2, T3, T4, T5, T6, T7, T8> of(
 		T1 v1,
 		T2 v2,
 		T3 v3,
@@ -122,10 +122,10 @@ public String toString() {
 		}
 	}
 
-	class Tuple1<@Nullable T1> extends Tuple0 {
+	class Tuple1<T1 extends @Nullable Object> extends Tuple0 {
 		final T1 v1;
 
-		private Tuple1(@Nullable T1 v1) {
+		private Tuple1(T1 v1) {
 			super();
 			this.v1 = v1;
 		}
@@ -163,7 +163,7 @@ public String toString() {
 		}
 	}
 
-	class Tuple2<@Nullable T1, @Nullable T2> extends Tuple1<T1> {
+	class Tuple2<T1 extends @Nullable Object, T2 extends @Nullable Object> extends Tuple1<T1> {
 		final T2 v2;
 
 		private Tuple2(T1 v1, T2 v2) {
@@ -200,7 +200,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple3<@Nullable T1, @Nullable T2, @Nullable T3> extends Tuple2<T1, T2> {
+	class Tuple3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> extends Tuple2<T1, T2> {
 		final T3 v3;
 
 		private Tuple3(T1 v1, T2 v2, T3 v3) {
@@ -238,7 +238,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple4<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4> extends Tuple3<T1, T2, T3> {
+	class Tuple4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> extends Tuple3<T1, T2, T3> {
 		final T4 v4;
 
 		private Tuple4(T1 v1, T2 v2, T3 v3, T4 v4) {
@@ -277,7 +277,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple5<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5> extends Tuple4<T1, T2, T3, T4> {
+	class Tuple5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> extends Tuple4<T1, T2, T3, T4> {
 		final T5 v5;
 
 		private Tuple5(T1 v1, T2 v2, T3 v3, T4 v4, T5 v5) {
@@ -317,7 +317,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple6<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6> extends Tuple5<T1, T2, T3, T4, T5> {
+	class Tuple6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> extends Tuple5<T1, T2, T3, T4, T5> {
 		final T6 v6;
 
 		private Tuple6(T1 v1, T2 v2, T3 v3, T4 v4, T5 v5, T6 v6) {
@@ -358,7 +358,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple7<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7> extends Tuple6<T1, T2, T3, T4, T5, T6> {
+	class Tuple7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> extends Tuple6<T1, T2, T3, T4, T5, T6> {
 		final T7 v7;
 
 		private Tuple7(
@@ -408,7 +408,7 @@ public int hashCode() {
 		}
 	}
 
-	class Tuple8<@Nullable T1, @Nullable T2, @Nullable T3, @Nullable T4, @Nullable T5, @Nullable T6, @Nullable T7, @Nullable T8> extends Tuple7<T1, T2, T3, T4, T5, T6, T7> {
+	class Tuple8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> extends Tuple7<T1, T2, T3, T4, T5, T6, T7> {
 		final T8 v8;
 
 		private Tuple8(
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java
index 7a93878bf..cee278c1b 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/ArrayArbitrary.java
@@ -12,9 +12,11 @@
 /**
  * Fluent interface to add functionality to arbitraries that generate instances
  * of Arrays
+ *
+ * @param <A> type of the resulting array class
  */
 @API(status = MAINTAINED, since = "1.4.0")
-public interface ArrayArbitrary<@Nullable T, A> extends StreamableArbitrary<T, A> {
+public interface ArrayArbitrary<T extends @Nullable Object, A> extends StreamableArbitrary<T, A> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -76,6 +78,6 @@ default ArrayArbitrary<T, A> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	ArrayArbitrary<T, A> uniqueElements(Function<T, Object> by);
+	ArrayArbitrary<T, A> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
index 85bec6760..808963692 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
@@ -17,7 +17,7 @@
  * @param <R> The return type of the functional interface
  */
 @API(status = MAINTAINED, since = "1.3.0")
-public interface FunctionArbitrary<F, @Nullable R> extends Arbitrary<F> {
+public interface FunctionArbitrary<F extends @Nullable Object, R extends @Nullable Object> extends Arbitrary<F> {
 
 	/**
 	 *
@@ -28,6 +28,6 @@ public interface FunctionArbitrary<F, @Nullable R> extends Arbitrary<F> {
 	 * @param answer A function that produces a concrete answer or throws an exception
 	 * @return A new instance of function arbitrary
 	 */
-	<F_ extends F> FunctionArbitrary<F_, R> when(Predicate<List<Object>> parameterCondition, Function<List<Object>, R> answer);
+	<F_ extends F> FunctionArbitrary<F_, R> when(Predicate<? super List<?>> parameterCondition, Function<? super List<?>, ? extends R> answer);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java
index d854a02b4..0d6d8e16c 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/IteratorArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Iterator}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface IteratorArbitrary<T> extends Arbitrary<Iterator<T>>, StreamableArbitrary<T, Iterator<T>> {
+public interface IteratorArbitrary<T extends @Nullable Object> extends Arbitrary<Iterator<T>>, StreamableArbitrary<T, Iterator<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -76,6 +78,6 @@ default IteratorArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	IteratorArbitrary<T> uniqueElements(Function<T, Object> by);
+	IteratorArbitrary<T> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
index ed7ae6a8e..4c2e69655 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
@@ -16,7 +16,7 @@
  * of type {@linkplain List}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface ListArbitrary<T> extends Arbitrary<List<T>>, StreamableArbitrary<T, List<T>> {
+public interface ListArbitrary<T extends @Nullable Object> extends Arbitrary<List<T>>, StreamableArbitrary<T, List<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -72,7 +72,7 @@ default ListArbitrary<T> ofSize(int size) {
 	 * @return arbitrary of a list of Us
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	<U> Arbitrary<List<U>> flatMapEach(BiFunction<List<T>, T, Arbitrary<U>> flatMapper);
+	<U> Arbitrary<List<U>> flatMapEach(BiFunction<? super List<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper);
 
 	/**
 	 * Add the constraint that elements of the generated list must be unique,
@@ -100,5 +100,5 @@ default ListArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	ListArbitrary<@Nullable T> uniqueElements(Function<@Nullable T, Object> by);
+	ListArbitrary<T> uniqueElements(Function<? super T, ?> by);
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java
index fb0ec32c4..1f9ef22dc 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/MapArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Map}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface MapArbitrary<K, V> extends Arbitrary<Map<K, V>>, SizableArbitrary<Map<K, V>> {
+public interface MapArbitrary<K extends @Nullable Object, V extends @Nullable Object> extends Arbitrary<Map<K, V>>, SizableArbitrary<Map<K, V>> {
 
 	/**
 	 * Fix the size to {@code size}.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
index d8a66e983..f9a9fe1b5 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Set}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface SetArbitrary<T> extends Arbitrary<Set<T>>, StreamableArbitrary<T, Set<T>> {
+public interface SetArbitrary<T extends @Nullable Object> extends Arbitrary<Set<T>>, StreamableArbitrary<T, Set<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -94,6 +96,6 @@ default SetArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	SetArbitrary<T> uniqueElements(Function<T, Object> by);
+	SetArbitrary<T> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java
index 0035cab95..8f270a035 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/SizableArbitrary.java
@@ -4,13 +4,15 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
  * Fluent interface to configure arbitraries that have size constraints for generated values, e.g. collections and arrays.
  */
 @API(status = MAINTAINED, since = "1.0")
-public interface SizableArbitrary<U> extends Arbitrary<U> {
+public interface SizableArbitrary<U extends @Nullable Object> extends Arbitrary<U> {
 
 	/**
 	 * Fix the size to {@code size}.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java
index 1c5d93d3c..c200f6b37 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/StreamArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -14,7 +16,7 @@
  * of type {@linkplain Stream}
  */
 @API(status = MAINTAINED, since = "1.3.2")
-public interface StreamArbitrary<T> extends Arbitrary<Stream<T>>, StreamableArbitrary<T, Stream<T>> {
+public interface StreamArbitrary<T extends @Nullable Object> extends Arbitrary<Stream<T>>, StreamableArbitrary<T, Stream<T>> {
 
 	/**
 	 * Fix the size to {@code size}.
@@ -76,6 +78,6 @@ default StreamArbitrary<T> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	StreamArbitrary<T> uniqueElements(Function<T, Object> by);
+	StreamArbitrary<T> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
index 4f8c70f52..d45d63565 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
@@ -17,7 +17,7 @@
  * can be streamed, e.g. {@link List}, {@link Set}, {@link Stream} and Arrays
  */
 @API(status = MAINTAINED, since = "1.2.1")
-public interface StreamableArbitrary<T, U> extends SizableArbitrary<U> {
+public interface StreamableArbitrary<T extends @Nullable Object, U> extends SizableArbitrary<U> {
 
 	/**
 	 * Given an {@code initial} argument use {@code accumulator} to produce
@@ -79,6 +79,6 @@ default StreamableArbitrary<T, U> ofSize(int size) {
 	 * @return new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.7.3")
-	StreamableArbitrary<@Nullable T, U> uniqueElements(Function<@Nullable T, Object> by);
+	StreamableArbitrary<T, U> uniqueElements(Function<? super T, ?> by);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java
index adc5cfe64..5a7ec3c20 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/TraverseArbitrary.java
@@ -8,6 +8,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.providers.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -15,7 +17,7 @@
  * of a given type {@code T} from the type's available constructors and factory methods.
  */
 @API(status = MAINTAINED, since = "1.8.0")
-public interface TraverseArbitrary<T> extends Arbitrary<T> {
+public interface TraverseArbitrary<T extends @Nullable Object> extends Arbitrary<T> {
 
 	/**
 	 * A traverser describes how to travers a given type by providing a hook to
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
index d5101fc32..0ea63796a 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -24,7 +26,7 @@
  * @see net.jqwik.api.domains.DomainContext
  */
 @API(status = MAINTAINED, since = "1.2")
-public interface TypeArbitrary<T> extends Arbitrary<T> {
+public interface TypeArbitrary<T extends @Nullable Object> extends Arbitrary<T> {
 
 	/**
 	 * Add public constructors of class {@code T} to be used
diff --git a/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java b/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
index e841d5978..03856d196 100644
--- a/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
+++ b/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
@@ -80,7 +80,7 @@ interface CoverageChecker {
 	 * @param query A {@link Predicate} that takes the collected values as parameter
 	 *              and returns true if the specific values shall be counted.
 	 */
-	CoverageChecker checkQuery(Predicate<? extends List<?>> query);
+	CoverageChecker checkQuery(Predicate<? super List<?>> query);
 
 	/**
 	 * Match collected values against a regular expression.
diff --git a/documentation/src/docs/include/customized-parameter-generation.md b/documentation/src/docs/include/customized-parameter-generation.md
index 4970f3270..264bf2a8c 100644
--- a/documentation/src/docs/include/customized-parameter-generation.md
+++ b/documentation/src/docs/include/customized-parameter-generation.md
@@ -1051,14 +1051,14 @@ either [annotating parameters with `@UniqueElements`](#unique-elements)
 or by using one of the many `uniqueness(..)` configuration methods for 
 collections and collection-like types:
 
-- `ListArbitrary<T>.uniqueElements(Function<T, Object>)`
+- `ListArbitrary<T>.uniqueElements(Function<? super T, ?>)`
 - `ListArbitrary<T>.uniqueElements()`
-- `SetArbitrary<T>.uniqueElements(Function<T, Object>)`
-- `StreamArbitrary<T>.uniqueElements(Function<T, Object>)`
+- `SetArbitrary<T>.uniqueElements(Function<? super T, ?>)`
+- `StreamArbitrary<T>.uniqueElements(Function<? super T, ?>)`
 - `StreamArbitrary<T>.uniqueElements()`
-- `IteratorArbitrary<T>.uniqueElements(Function<T, Object>)`
+- `IteratorArbitrary<T>.uniqueElements(Function<? super T, ?>)`
 - `IteratorArbitrary<T>.uniqueElements()`
-- `ArrayArbitrary<T, A>.uniqueElements(Function<T, Object>)`
+- `ArrayArbitrary<T, A>.uniqueElements(Function<? super T, ?>)`
 - `ArrayArbitrary<T, A>.uniqueElements()`
 - `MapArbitrary<K, V>.uniqueKeys(Function<K, Object>)`
 - `MapArbitrary<K, V>.uniqueValues(Function<V, Object>)`
diff --git a/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java b/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java
index 36762d99d..298371e65 100644
--- a/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java
+++ b/documentation/src/test/java/net/jqwik/docs/statistics/StatisticsCoverageExamples.java
@@ -57,7 +57,10 @@ void queryStatistics(@ForAll int anInt) {
 		Statistics.collect(anInt);
 
 		Statistics.coverage(coverage -> {
-			Predicate<List<Integer>> isZero = params -> params.get(0) == 0;
+			// Note: coverage.checkQuery is untuped as it requires Predicate that must support **any** object
+			// So we can't have Predicate<List<Integer>> here as checkQuery would not be able to call our
+			// predicate with Object values.
+			Predicate<List<?>> isZero = params -> (Integer) params.get(0) == 0;
 			coverage.checkQuery(isZero).percentage(p -> p > 5.0);
 		});
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
index 00eab6625..f4a318285 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
@@ -16,6 +16,8 @@
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 import net.jqwik.engine.properties.stateful.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.properties.arbitraries.ArbitrariesSupport.*;
 
 /**
@@ -24,12 +26,12 @@
 public class ArbitrariesFacadeImpl extends Arbitraries.ArbitrariesFacade {
 
 	@Override
-	public <T> Arbitrary<T> just(T value) {
+	public <T extends @Nullable Object> Arbitrary<T> just(T value) {
 		return new JustArbitrary<>(value);
 	}
 
 	@Override
-	public <T> Arbitrary<T> oneOf(Collection<Arbitrary<? extends T>> choices) {
+	public <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> choices) {
 		return new OneOfArbitrary<>(choices);
 	}
 
@@ -39,8 +41,8 @@ public <M> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> a
 	}
 
 	@Override
-	public <T> Arbitrary<T> frequencyOf(List<Tuple.Tuple2<Integer, Arbitrary<T>>> frequencies) {
-		List<Tuple.Tuple2<Integer, Arbitrary<T>>> aboveZeroFrequencies =
+	public <T> Arbitrary<T> frequencyOf(List<? extends Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
+		List<Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> aboveZeroFrequencies =
 			frequencies.stream().filter(f -> f.get1() > 0).collect(Collectors.toList());
 
 		if (aboveZeroFrequencies.size() == 1) {
@@ -105,12 +107,12 @@ public CharacterArbitrary chars() {
 	}
 
 	@Override
-	public <T> Arbitrary<T> lazy(Supplier<Arbitrary<T>> arbitrarySupplier) {
+	public <T> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		return new LazyArbitrary<>(arbitrarySupplier);
 	}
 
 	@Override
-	public <T> Arbitrary<T> lazyOf(List<Supplier<Arbitrary<T>>> suppliers) {
+	public <T> Arbitrary<T> lazyOf(List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		int hashIdentifier = calculateIdentifier(suppliers.size());
 		return LazyOfArbitrary.of(hashIdentifier, suppliers);
 	}
@@ -142,12 +144,12 @@ public <T> Arbitrary<List<T>> shuffle(List<T> values) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> fromGenerator(IntFunction<RandomGenerator<T>> generatorSupplier) {
+	public <T> Arbitrary<T> fromGenerator(IntFunction<? extends RandomGenerator<T>> generatorSupplier) {
 		return new FromGeneratorWithSizeArbitrary<>(generatorSupplier);
 	}
 
 	@Override
-	public <T> Arbitrary<T> frequency(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	public <T> Arbitrary<T> frequency(List<? extends Tuple.Tuple2<Integer, T>> frequencies) {
 		List<Tuple.Tuple2<Integer, T>> frequenciesAbove0 = frequencies.stream()
 																	  .filter(f -> f.get1() > 0)
 																	  .collect(Collectors.toList());
@@ -259,9 +261,9 @@ private static Set<Arbitrary<?>> createDefaultArbitraries(
 	}
 
 	@Override
-	public <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+	public <T extends @Nullable Object> Arbitrary<T> recursive(
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int minDepth,
 		int maxDepth
 	) {
@@ -285,8 +287,8 @@ public <T> Arbitrary<T> recursive(
 	}
 
 	private <T> Arbitrary<T> recursive(
-		Supplier<Arbitrary<T>> base,
-		Function<Arbitrary<T>, Arbitrary<T>> recur,
+		Supplier<? extends Arbitrary<T>> base,
+		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int depth
 	) {
 		return new RecursiveArbitrary<>(base, recur, depth);
diff --git a/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
index d11d16de1..27b8dfaa0 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
@@ -81,7 +81,7 @@ public <T1, T2, T3, T4, T5, T6, T7, T8> Combinators.Combinator8<T1, T2, T3, T4,
 	}
 
 	@Override
-	public <T> Combinators.ListCombinator<T> combineList(List<Arbitrary<T>> listOfArbitraries) {
+	public <T> Combinators.ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries) {
 		return new DefaultListCombinator<>(listOfArbitraries);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java b/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
index 95b5725b6..cee4275a6 100644
--- a/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
@@ -80,7 +80,7 @@ private StatisticsEntry statisticsEntry(Object[] values) {
 				   .orElse(StatisticsEntryImpl.nullFor(key));
 	}
 
-	private StatisticsEntry query(Predicate<List<Object>> query, int countAll) {
+	private StatisticsEntry query(Predicate<? super List<?>> query, int countAll) {
 		return statisticsEntries()
 				   .stream()
 				   .filter(entry -> {
@@ -198,9 +198,9 @@ public CoverageChecker check(Object... values) {
 
 		@SuppressWarnings("unchecked")
 		@Override
-		public CoverageChecker checkQuery(Predicate<? extends List<?>> query) {
+		public CoverageChecker checkQuery(Predicate<? super List<?>> query) {
 			int countAll = countAllCollects();
-			StatisticsEntry entry = query((Predicate<List<Object>>) query, countAll);
+			StatisticsEntry entry = query(query, countAll);
 			return new CoverageCheckerImpl(label, entry, countAll);
 		}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
index 0f6e55912..4b7fadece 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
@@ -95,7 +95,7 @@ public ArrayArbitrary<T, A> uniqueElements() {
 	}
 
 	@Override
-	public ArrayArbitrary<T, A> uniqueElements(Function<T, Object> by) {
+	public ArrayArbitrary<T, A> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (ArrayArbitrary<T, A>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java
index a275971e0..e21459f08 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultFunctionArbitrary.java
@@ -13,7 +13,7 @@ public class DefaultFunctionArbitrary<F, R> extends TypedCloneable implements Fu
 
 	private final Class<F> functionalType;
 	private final Arbitrary<R> resultArbitrary;
-	private final List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions = new ArrayList<>();
+	private final List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions = new ArrayList<>();
 
 	public DefaultFunctionArbitrary(Class<F> functionalType, Arbitrary<R> resultArbitrary) {
 		this.functionalType = functionalType;
@@ -52,15 +52,15 @@ public boolean equals(Object o) {
 	}
 
 	private boolean conditionsAreEqual(
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> left,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> right
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> left,
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> right
 	) {
 		if (left.size() != right.size()) {
 			return false;
 		}
 		for (int i = 0; i < left.size(); i++) {
-			Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> leftCondition = left.get(i);
-			Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> rightCondition = right.get(i);
+			Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> leftCondition = left.get(i);
+			Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> rightCondition = right.get(i);
 			if (!LambdaSupport.areEqual(leftCondition.get1(), rightCondition.get1())) {
 				return false;
 			}
@@ -94,14 +94,14 @@ private ConstantFunctionGenerator<F, R> createConstantFunctionGenerator(final in
 	}
 
 	@Override
-	public <F_ extends F> FunctionArbitrary<F_, R> when(Predicate<List<Object>> parameterCondition, Function<List<Object>, R> answer) {
+	public <F_ extends F> FunctionArbitrary<F_, R> when(Predicate<? super List<?>> parameterCondition, Function<? super List<?>, ? extends R> answer) {
 		DefaultFunctionArbitrary<F_, R> clone = typedClone();
 		clone.conditions.addAll(this.conditions);
 		clone.addCondition(Tuple.of(parameterCondition, answer));
 		return clone;
 	}
 
-	private void addCondition(Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> condition) {
+	private void addCondition(Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> condition) {
 		conditions.add(condition);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
index ffef597f4..cd14bc789 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
@@ -60,7 +60,7 @@ public IteratorArbitrary<T> withSizeDistribution(RandomDistribution distribution
 	}
 
 	@Override
-	public IteratorArbitrary<T> uniqueElements(Function<T, Object> by) {
+	public IteratorArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (IteratorArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
index 0691b3587..389031013 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
@@ -12,7 +12,7 @@
 
 import org.jspecify.annotations.*;
 
-public class DefaultListArbitrary<T> extends MultivalueArbitraryBase<T, List<T>> implements ListArbitrary<T> {
+public class DefaultListArbitrary<T extends @Nullable Object> extends MultivalueArbitraryBase<T, List<T>> implements ListArbitrary<T> {
 
 	public DefaultListArbitrary(Arbitrary<T> elementArbitrary) {
 		super(elementArbitrary);
@@ -68,9 +68,9 @@ public <U> Arbitrary<List<U>> mapEach(BiFunction<List<T>, T, U> mapper) {
 
 	// TODO: Remove duplication with DefaultSetArbitrary.flatMapEach()
 	@Override
-	public <U> Arbitrary<List<U>> flatMapEach(BiFunction<List<T>, T, Arbitrary<U>> flatMapper) {
+	public <U> Arbitrary<List<U>> flatMapEach(BiFunction<? super List<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper) {
 		return this.flatMap(elements -> {
-			List<Arbitrary<U>> arbitraries =
+			List<? extends Arbitrary<U>> arbitraries =
 				elements.stream()
 						.map(e -> flatMapper.apply(elements, e))
 						.collect(Collectors.toList());
@@ -79,7 +79,7 @@ public <U> Arbitrary<List<U>> flatMapEach(BiFunction<List<T>, T, Arbitrary<U>> f
 	}
 
 	@Override
-	public ListArbitrary<@Nullable T> uniqueElements(Function<@Nullable T, Object> by) {
+	public ListArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (ListArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
index ad48dd029..5ede6e77f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
@@ -93,7 +93,7 @@ public SetArbitrary<T> uniqueElements() {
 	}
 
 	@Override
-	public SetArbitrary<T> uniqueElements(Function<T, Object> by) {
+	public SetArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (SetArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
index fa289d647..c706cdb5a 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
@@ -62,7 +62,7 @@ public StreamArbitrary<T> withSizeDistribution(RandomDistribution distribution)
 	}
 
 	@Override
-	public StreamArbitrary<T> uniqueElements(Function<T, Object> by) {
+	public StreamArbitrary<T> uniqueElements(Function<? super T, ?> by) {
 		FeatureExtractor<T> featureExtractor = by::apply;
 		return (StreamArbitrary<T>) super.uniqueElements(featureExtractor);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
index 2bb22e671..1f0ad50c9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
@@ -52,7 +52,7 @@ public static <T> EdgeCases<T> choose(final List<T> values, int maxEdgeCases) {
 		return EdgeCasesSupport.fromShrinkables(shrinkables);
 	}
 
-	public static <T> EdgeCases<T> concatFrom(final List<Arbitrary<T>> arbitraries, int maxEdgeCases) {
+	public static <T> EdgeCases<T> concatFrom(final List<? extends Arbitrary<T>> arbitraries, int maxEdgeCases) {
 		List<Shrinkable<Arbitrary<T>>> shrinkables = new ArrayList<>();
 		for (Arbitrary<T> arbitrary : arbitraries) {
 			shrinkables.add(new ChooseValueShrinkable<>(arbitrary, arbitraries));
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
index 4c884eca7..2fb69bcc5 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
@@ -13,10 +13,10 @@
 
 public class FrequencyOfArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
 
-	private final List<Tuple2<Integer, Arbitrary<T>>> frequencies;
+	private final List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies;
 	private final boolean isGeneratorMemoizable;
 
-	public FrequencyOfArbitrary(List<Tuple2<Integer, Arbitrary<T>>> frequencies) {
+	public FrequencyOfArbitrary(List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
 		this.frequencies = frequencies;
 		this.isGeneratorMemoizable = frequencies.stream().allMatch(t -> t.get2().isGeneratorMemoizable());
 		if (this.frequencies.isEmpty()) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
index 9497e236d..38971e03b 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
@@ -7,9 +7,9 @@
 
 public class FromGeneratorWithSizeArbitrary<T> implements Arbitrary<T> {
 
-	private final IntFunction<RandomGenerator<T>> supplier;
+	private final IntFunction<? extends RandomGenerator<T>> supplier;
 
-	public FromGeneratorWithSizeArbitrary(IntFunction<RandomGenerator<T>> generatorSupplier) {
+	public FromGeneratorWithSizeArbitrary(IntFunction<? extends RandomGenerator<T>> generatorSupplier) {
 		this.supplier = generatorSupplier;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
index 49dd53c5a..9da910f76 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
@@ -8,11 +8,11 @@
 import net.jqwik.api.providers.*;
 
 public class LazyArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
-	private final Supplier<Arbitrary<T>> arbitrarySupplier;
+	private final Supplier<? extends Arbitrary<T>> arbitrarySupplier;
 	private final List<Tuple.Tuple2<ArbitraryConfigurator, TypeUsage>> configurations = new ArrayList<>();
 	private Arbitrary<T> arbitrary;
 
-	public LazyArbitrary(Supplier<Arbitrary<T>> arbitrarySupplier) {
+	public LazyArbitrary(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		this.arbitrarySupplier = arbitrarySupplier;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
index 4cd3798f1..9afd129f8 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
@@ -24,13 +24,13 @@ private static Store<Map<Integer, LazyOfArbitrary<?>>> arbitrariesStore() {
 	}
 
 	@SuppressWarnings("unchecked")
-	public static <T> Arbitrary<T> of(int hashIdentifier, List<Supplier<Arbitrary<T>>> suppliers) {
+	public static <T> Arbitrary<T> of(int hashIdentifier, List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		// It's important for good shrinking to work that the same arbitrary usage is handled by the same arbitrary instance
 		LazyOfArbitrary<?> arbitrary = arbitrariesStore().get().computeIfAbsent(hashIdentifier, ignore -> new LazyOfArbitrary<>(suppliers));
 		return (Arbitrary<T>) arbitrary;
 	}
 
-	private final List<Supplier<Arbitrary<T>>> suppliers;
+	private final List<? extends Supplier<? extends Arbitrary<T>>> suppliers;
 
 	private final Deque<Set<LazyOfShrinkable<T>>> generatedParts = new ArrayDeque<>();
 
@@ -45,7 +45,7 @@ private Store<Map<Integer, RandomGenerator<T>>> createGeneratorsStore() {
 		}
 	}
 
-	public LazyOfArbitrary(List<Supplier<Arbitrary<T>>> suppliers) {
+	public LazyOfArbitrary(List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		this.suppliers = suppliers;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
index 8c3eacb7b..a928246f6 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
@@ -16,8 +16,9 @@ public class OneOfArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary
 	private final boolean isGeneratorMemoizable;
 
 	@SuppressWarnings("unchecked")
-	public OneOfArbitrary(Collection<Arbitrary<? extends T>> choices) {
+	public OneOfArbitrary(Collection<? extends Arbitrary<? extends T>> choices) {
 		for (Arbitrary<? extends T> choice : choices) {
+			// TODO: double-check cast
 			all.add((Arbitrary<T>) choice);
 		}
 		isGeneratorMemoizable = all.stream().allMatch(Arbitrary::isGeneratorMemoizable);
@@ -39,7 +40,7 @@ public boolean isGeneratorMemoizable() {
 	}
 
 	private RandomGenerator<T> rawGeneration(int genSize, boolean withEmbeddedEdgeCases) {
-		List<Tuple2<Integer, Arbitrary<T>>> frequencies =
+		List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies =
 			all.stream()
 			   .map(a -> Tuple.of(1, a))
 			   .collect(Collectors.toList());
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
index c713a968b..1494c3b00 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
@@ -7,8 +7,8 @@
 import net.jqwik.api.support.*;
 
 public class RecursiveArbitrary<T> implements Arbitrary<T> {
-	private final Supplier<Arbitrary<T>> base;
-	private final Function<Arbitrary<T>, Arbitrary<T>> recur;
+	private final Supplier<? extends Arbitrary<T>> base;
+	private final Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur;
 	private final int depth;
 
 	// Not used for exhaustive generation
@@ -16,7 +16,7 @@ public class RecursiveArbitrary<T> implements Arbitrary<T> {
 
 	private boolean isGeneratorMemoizable = true;
 
-	public RecursiveArbitrary(Supplier<Arbitrary<T>> base, Function<Arbitrary<T>, Arbitrary<T>> recur, int depth) {
+	public RecursiveArbitrary(Supplier<? extends Arbitrary<T>> base, Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur, int depth) {
 		this.base = base;
 		this.recur = recur;
 		this.depth = depth;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
index 8d4c84f5d..5c661c4f7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
@@ -15,7 +15,7 @@ public class DefaultListCombinator<T> implements Combinators.ListCombinator<T> {
 	protected final Arbitrary<T>[] arbitraries;
 
 	@SuppressWarnings("unchecked")
-	public DefaultListCombinator(List<Arbitrary<T>> listOfArbitraries) {
+	public DefaultListCombinator(List<? extends Arbitrary<T>> listOfArbitraries) {
 		this(listOfArbitraries.toArray(new Arbitrary[0]));
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
index e5281d736..2c8b50ca7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
@@ -97,7 +97,7 @@ public static <T> Optional<ExhaustiveGenerator<List<T>>> shuffle(List<T> values,
 
 	public static <U, T> Optional<ExhaustiveGenerator<U>> flatMap(
 		ExhaustiveGenerator<T> base,
-		Function<T, Arbitrary<U>> mapper,
+		Function<? super T, ? extends Arbitrary<U>> mapper,
 		long maxNumberOfSamples
 	) {
 		Optional<Long> optionalMaxCount = FlatMappedExhaustiveGenerator.calculateMaxCounts(base, mapper, maxNumberOfSamples);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
index 8241a4ada..3db50eaa9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
@@ -10,11 +10,11 @@
 public class FlatMappedExhaustiveGenerator<U, T> implements ExhaustiveGenerator<U> {
 	private final ExhaustiveGenerator<T> baseGenerator;
 	private final long maxCount;
-	private final Function<T, Arbitrary<U>> mapper;
+	private final Function<? super T, ? extends Arbitrary<U>> mapper;
 
 	public static <T, U> Optional<Long> calculateMaxCounts(
 		ExhaustiveGenerator<T> baseGenerator,
-		Function<T, Arbitrary<U>> mapper,
+		Function<? super T, ? extends Arbitrary<U>> mapper,
 		long maxNumberOfSamples
 	) {
 		long choices = 0;
@@ -31,7 +31,7 @@ public static <T, U> Optional<Long> calculateMaxCounts(
 		return Optional.of(choices);
 	}
 
-	public FlatMappedExhaustiveGenerator(ExhaustiveGenerator<T> baseGenerator, long maxCount, Function<T, Arbitrary<U>> mapper) {
+	public FlatMappedExhaustiveGenerator(ExhaustiveGenerator<T> baseGenerator, long maxCount, Function<? super T, ? extends Arbitrary<U>> mapper) {
 		this.baseGenerator = baseGenerator;
 		this.maxCount = maxCount;
 		this.mapper = mapper;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java
index 67fcb862d..9edab461f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/AbstractFunctionGenerator.java
@@ -13,12 +13,12 @@
 abstract class AbstractFunctionGenerator<F, R> implements RandomGenerator<F> {
 	final Class<F> functionalType;
 	final RandomGenerator<R> resultGenerator;
-	final List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions;
+	final List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions;
 
 	AbstractFunctionGenerator(
 		Class<F> functionalType,
 		RandomGenerator<R> resultGenerator,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions
 	) {
 		this.functionalType = functionalType;
 		this.resultGenerator = resultGenerator;
@@ -70,7 +70,7 @@ private Object handleToStringOfConstantMethod(final R constant) {
 	// Returns result wrapped in array to allow null as result
 	protected Optional<Object[]> conditionalResult(Object[] args) {
 		Optional<Object[]> conditionalResult = Optional.empty();
-		for (Tuple2<Predicate<List<Object>>, Function<List<Object>, R>> condition : conditions) {
+		for (Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>> condition : conditions) {
 			List<Object> params = Arrays.asList(args);
 			if (condition.get1().test(params)) {
 				Object[] result = new Object[]{condition.get2().apply(params)};
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java
index 31743d2b8..4343f8304 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ConstantFunctionGenerator.java
@@ -11,7 +11,7 @@ public class ConstantFunctionGenerator<F, R> extends AbstractFunctionGenerator<F
 	public ConstantFunctionGenerator(
 		Class<F> functionalType,
 		RandomGenerator<R> resultGenerator,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions
 	) {
 		super(functionalType, resultGenerator, conditions);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
index 6f0ccd185..cedb08da0 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
@@ -8,7 +8,7 @@
 
 class FrequencyGenerator<T> extends ChooseRandomlyByFrequency<T> implements RandomGenerator<T> {
 
-	FrequencyGenerator(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	FrequencyGenerator(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		super(frequencies);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java
index f1750e2f5..12022209d 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FunctionGenerator.java
@@ -18,7 +18,7 @@ public class FunctionGenerator<F, R> extends AbstractFunctionGenerator<F, R> {
 	public FunctionGenerator(
 		Class<F> functionalType,
 		RandomGenerator<R> resultGenerator,
-		List<Tuple2<Predicate<List<Object>>, Function<List<Object>, R>>> conditions
+		List<Tuple2<Predicate<? super List<?>>, Function<? super List<?>, ? extends R>>> conditions
 	) {
 		super(functionalType, resultGenerator, conditions);
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
index cf1a30fd2..5e6fb3a1e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
@@ -174,12 +174,12 @@ public static <T> RandomGenerator<T> samples(T[] samples) {
 		return samplesFromShrinkables(shrinkables);
 	}
 
-	public static <T> RandomGenerator<T> frequency(List<Tuple2<Integer, T>> frequencies) {
+	public static <T> RandomGenerator<T> frequency(List<? extends Tuple2<Integer, ? extends T>> frequencies) {
 		return new FrequencyGenerator<>(frequencies);
 	}
 
 	public static <T> RandomGenerator<T> frequencyOf(
-			List<Tuple2<Integer, Arbitrary<T>>> frequencies,
+			List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies,
 			int genSize,
 			boolean withEmbeddedEdgeCases
 	) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
index 96ee7c9ca..c3bf061a7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
@@ -7,9 +7,9 @@
 
 public class ChooseValueShrinkable<T> extends AbstractValueShrinkable<T> {
 
-	private final List<T> values;
+	private final List<? extends T> values;
 
-	public ChooseValueShrinkable(T value, List<T> values) {
+	public ChooseValueShrinkable(T value, List<? extends T> values) {
 		super(value);
 		this.values = values;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java b/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
index 891abaf78..1b1faf86e 100644
--- a/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
+++ b/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
@@ -11,7 +11,7 @@ public class ChooseRandomlyByFrequency<T> implements Function<Random, T> {
 	private int size = 0;
 	private List<T> valuesToChooseFrom;
 
-	public ChooseRandomlyByFrequency(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	public ChooseRandomlyByFrequency(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		calculateUpperBorders(frequencies);
 		if (size <= 0) {
 			throw new JqwikException(String.format(
@@ -25,11 +25,11 @@ protected List<T> possibleValues() {
 		return valuesToChooseFrom;
 	}
 
-	private void calculateUpperBorders(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	private void calculateUpperBorders(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		List<T> values = new ArrayList<>(frequencies.size());
 		// Zero-frequency elements are possible, so we start with a list, and transform to array later
 		List<Integer> upperBounds = new ArrayList<>(frequencies.size());
-		for (Tuple.Tuple2<Integer, T> tuple : frequencies) {
+		for (Tuple.Tuple2<Integer, ? extends T> tuple : frequencies) {
 			int frequency = tuple.get1();
 			if (frequency <= 0)
 				continue;
diff --git a/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java b/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java
index fede04bb1..1e5cf24f7 100644
--- a/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java
+++ b/engine/src/test/java/net/jqwik/api/statistics/StatisticsCoverageTests.java
@@ -240,7 +240,7 @@ void checkCountForQuery(@ForAll @IntRange(min = 1, max = 100) int anInt) {
 			Statistics.collect(anInt);
 
 			Statistics.coverage(coverage -> {
-				Predicate<List<Integer>> query = params -> params.get(0) <= 10;
+				Predicate<List<?>> query = params -> (Integer) params.get(0) <= 10;
 				coverage.checkQuery(query).count(c -> c == 10);
 			});
 		}
@@ -250,7 +250,7 @@ void checkPercentageForQuery(@ForAll @IntRange(min = 1, max = 100) int anInt) {
 			Statistics.collect(anInt);
 
 			Statistics.coverage(coverage -> {
-				Predicate<List<Integer>> query = params -> params.get(0) <= 50;
+				Predicate<List<?>> query = params -> (Integer) params.get(0) <= 50;
 				coverage.checkQuery(query).percentage(p -> {
 					assertThat(p).isCloseTo(50.0, Offset.offset(0.1));
 				});
@@ -263,7 +263,7 @@ void checkCountsAreAddedUp(@ForAll @IntRange(min = 1, max = 100) int anInt) {
 			Statistics.label("twice").collect(anInt);
 
 			Statistics.label("twice").coverage(coverage -> {
-				Predicate<List<Integer>> query = params -> params.get(0) <= 10;
+				Predicate<List<?>> query = params -> (Integer) params.get(0) <= 10;
 				coverage.checkQuery(query).count(c -> c == 20);
 			});
 		}
@@ -274,7 +274,7 @@ void queryWithWrongTypeFails(@ForAll int anInt) {
 			Statistics.collect(anInt);
 
 			Statistics.coverage(coverage -> {
-				Predicate<List<String>> query = params -> !params.get(0).isEmpty();
+				Predicate<List<?>> query = params -> !((String)params.get(0)).isEmpty();
 				coverage.checkQuery(query).count(c -> c == 10);
 			});
 		}
diff --git a/kotlin/build.gradle b/kotlin/build.gradle
index 311c9d46a..1b849e9f8 100644
--- a/kotlin/build.gradle
+++ b/kotlin/build.gradle
@@ -78,6 +78,7 @@ signing {
 tasks.withType(KotlinCompile).configureEach {
 	compilerOptions {
 		freeCompilerArgs = [
+			"-Xnullability-annotations=@org.jspecify.annotations:strict",
 			"-Xemit-jvm-type-annotations" // Required for annotations on type variables
 		]
 		apiVersion = KotlinVersion.KOTLIN_1_9
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
index a25baa473..b8a2771a8 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
@@ -9,10 +9,8 @@ import java.util.function.BiFunction
  * Convenience function for Kotlin to not use backticked `in` function.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <B, T> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
+fun <B, T: Any?> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
     where B : Any {
-    // This explicit conversion form combinator to toFunction is necessary
-    // since the implicit conversion (just handing in combinator) creates nullability warning
-    val toFunction: BiFunction<B, T, B> = BiFunction<B, T, B> { b, t -> combinator(b, t) }
+    val toFunction = BiFunction<B, T, B> { b, t -> combinator(b, t) }
     return this.use(arbitrary).`in`(toFunction)
 }
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
index 9a0b023e5..1b75d2170 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
@@ -15,7 +15,7 @@ import java.util.function.Function
  * of type [Sequence]
  */
 @API(status = EXPERIMENTAL, since = "1.6.0")
-class SequenceArbitrary<T>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<Sequence<T>>(),
+class SequenceArbitrary<T: Any?>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<Sequence<T>>(),
     SizableArbitrary<Sequence<T>> {
 
     private var listArbitrary: ListArbitrary<T>
@@ -80,7 +80,7 @@ class SequenceArbitrary<T>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<
      *
      * @return new arbitrary instance
      */
-    fun uniqueElements(by: Function<T, Any>): SequenceArbitrary<T> {
+    fun uniqueElements(by: Function<in T, *>): SequenceArbitrary<T> {
         val clone = typedClone<SequenceArbitrary<T>>()
         clone.listArbitrary = listArbitrary.uniqueElements(by)
         return clone
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt
index 67d802fd9..0b9ddd1c9 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SizableArbitraryExtensions.kt
@@ -19,4 +19,4 @@ fun <T> IteratorArbitrary<T>.ofSize(range: IntRange): IteratorArbitrary<T> = ofM
 fun <K, V> MapArbitrary<K, V>.ofSize(range: IntRange): MapArbitrary<K, V> = ofMinSize(range.first).ofMaxSize(range.last)
 
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <T, A> ArrayArbitrary<T, A>.ofSize(range: IntRange): ArrayArbitrary<T, A> = ofMinSize(range.first).ofMaxSize(range.last)
+fun <T, A: Any> ArrayArbitrary<T, A>.ofSize(range: IntRange): ArrayArbitrary<T, A> = ofMinSize(range.first).ofMaxSize(range.last)
diff --git a/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt b/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt
index 531372020..b0115584b 100644
--- a/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt
+++ b/kotlin/src/test/kotlin/net/jqwik/kotlin/CollectionExtensionsTests.kt
@@ -1,7 +1,7 @@
 package net.jqwik.kotlin
 
-import net.jqwik.api.Example
-import net.jqwik.api.ForAll
+import net.jqwik.api.*
+import net.jqwik.kotlin.api.any
 import net.jqwik.kotlin.api.anySubset
 import net.jqwik.kotlin.api.anyValue
 import net.jqwik.kotlin.api.ofSize
diff --git a/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt b/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt
index 62d84be0b..bed36587d 100644
--- a/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt
+++ b/kotlin/src/test/kotlin/net/jqwik/kotlin/InternalModifierTests.kt
@@ -1,12 +1,10 @@
 package net.jqwik.kotlin
 
-import net.jqwik.api.ForAll
-import net.jqwik.api.Property
-import net.jqwik.api.PropertyDefaults
-import net.jqwik.api.Provide
+import net.jqwik.api.*
 import net.jqwik.api.lifecycle.AfterContainer
 import net.jqwik.api.lifecycle.AfterProperty
 import net.jqwik.kotlin.api.any
+import kotlin.random.Random
 
 @PropertyDefaults(tries = 10)
 internal class InternalModifierTests {

From 949f47265ca5d7c5d01b76fad6286cdc6fd8d471 Mon Sep 17 00:00:00 2001
From: Vladimir Sitnikov <sitnikov@netcracker.com>
Date: Fri, 7 Jun 2024 15:15:27 +0300
Subject: [PATCH 2/5] Fix more nullability issues

---
 .../main/java/net/jqwik/api/Arbitraries.java  | 20 +++----
 .../main/java/net/jqwik/api/Arbitrary.java    | 44 +++++++-------
 .../java/net/jqwik/api/ArbitrarySupplier.java |  6 +-
 api/src/main/java/net/jqwik/api/Builders.java |  5 +-
 .../main/java/net/jqwik/api/Combinators.java  | 58 +++++++++----------
 .../main/java/net/jqwik/api/EdgeCases.java    | 13 +++--
 .../net/jqwik/api/ExhaustiveGenerator.java    | 15 ++---
 .../main/java/net/jqwik/api/Falsifier.java    |  6 +-
 .../main/java/net/jqwik/api/Functions.java    |  6 +-
 .../java/net/jqwik/api/RandomGenerator.java   | 28 ++++-----
 .../main/java/net/jqwik/api/Shrinkable.java   | 20 +++----
 .../java/net/jqwik/api/ShrinkingDistance.java |  7 +--
 api/src/main/java/net/jqwik/api/Table.java    | 20 ++++---
 api/src/main/java/net/jqwik/api/Tuple.java    | 16 ++---
 .../api/arbitraries/ArbitraryDecorator.java   |  4 +-
 .../api/arbitraries/FunctionArbitrary.java    |  2 +-
 .../jqwik/api/arbitraries/ListArbitrary.java  |  2 +-
 .../jqwik/api/arbitraries/SetArbitrary.java   |  4 +-
 .../api/arbitraries/StreamableArbitrary.java  |  2 +-
 .../jqwik/api/arbitraries/TypeArbitrary.java  |  4 +-
 .../jqwik/api/arbitraries/package-info.java   |  2 +-
 .../configurators/ArbitraryConfigurator.java  |  4 +-
 .../ArbitraryConfiguratorBase.java            |  7 ++-
 .../SelfConfiguringArbitrary.java             |  4 +-
 .../jqwik/api/configurators/package-info.java |  2 +-
 .../jqwik/api/constraints/package-info.java   |  2 +-
 .../net/jqwik/api/domains/package-info.java   |  2 +-
 .../api/facades/ShrinkingSupportFacade.java   | 14 +++--
 .../api/facades/TestingSupportFacade.java     |  4 +-
 .../net/jqwik/api/facades/package-info.java   |  4 +-
 .../net/jqwik/api/footnotes/package-info.java |  2 +-
 .../java/net/jqwik/api/lifecycle/Store.java   | 16 ++---
 .../main/java/net/jqwik/api/package-info.java |  2 +-
 .../net/jqwik/api/providers/package-info.java |  2 +-
 .../net/jqwik/api/sessions/package-info.java  |  2 +-
 .../main/java/net/jqwik/api/state/Action.java | 20 ++++---
 .../net/jqwik/api/state/ActionBuilder.java    | 13 ++---
 .../java/net/jqwik/api/state/ActionChain.java |  8 +--
 .../main/java/net/jqwik/api/state/Chain.java  |  8 ++-
 .../net/jqwik/api/state/ChainArbitrary.java   |  2 +-
 .../net/jqwik/api/state/ChangeDetector.java   | 13 +++--
 .../net/jqwik/api/state/Transformation.java   |  4 +-
 .../java/net/jqwik/api/state/Transformer.java | 12 ++--
 .../net/jqwik/api/state/package-info.java     |  2 +-
 .../java/net/jqwik/api/stateful/Action.java   |  3 +-
 .../jqwik/api/stateful/ActionSequence.java    |  2 +-
 .../api/stateful/ActionSequenceArbitrary.java |  4 +-
 .../net/jqwik/api/stateful/Invariant.java     |  3 +-
 .../net/jqwik/api/stateful/package-info.java  |  2 +-
 .../api/statistics/StatisticsCoverage.java    | 12 ++--
 .../jqwik/api/statistics/package-info.java    |  2 +-
 .../jqwik/api/support/CollectorsSupport.java  |  3 +-
 .../jqwik/api/support/HashCodeSupport.java    |  2 +-
 .../customized-parameter-generation.md        |  2 +-
 .../OddConfigurator.java                      |  4 +-
 .../jqwik/engine/descriptor/package-info.java |  4 ++
 .../jqwik/engine/discovery/package-info.java  |  4 ++
 .../discovery/predicates/package-info.java    |  4 ++
 .../engine/discovery/specs/package-info.java  |  4 ++
 .../execution/ContainerInstancesCreator.java  |  5 +-
 .../engine/execution/GenerationInfo.java      | 14 +++--
 .../lifecycle/CurrentDomainContext.java       |  4 +-
 .../lifecycle/CurrentTestDescriptor.java      |  3 +-
 .../lifecycle/PlainExecutionResult.java       | 10 ++--
 .../execution/lifecycle/ScopedStore.java      |  4 +-
 .../execution/lifecycle/StoreRepository.java  | 13 +++--
 .../execution/lifecycle/package-info.java     |  4 ++
 .../jqwik/engine/execution/package-info.java  |  4 ++
 .../execution/pipeline/package-info.java      |  4 ++
 .../reporting/ParameterChangesDetector.java   | 12 ++--
 .../execution/reporting/package-info.java     |  4 ++
 .../engine/facades/ActionChainFacadeImpl.java |  4 +-
 .../engine/facades/ArbitrariesFacadeImpl.java | 30 +++++-----
 .../engine/facades/ArbitraryFacadeImpl.java   | 38 ++++++------
 .../jqwik/engine/facades/ChainFacadeImpl.java |  4 +-
 .../engine/facades/CombinatorsFacadeImpl.java |  4 +-
 .../engine/facades/EdgeCasesFacadeImpl.java   |  4 +-
 .../ExhaustiveGeneratorFacadeImpl.java        | 10 ++--
 .../engine/facades/FunctionsFacadeImpl.java   |  4 +-
 .../net/jqwik/engine/facades/Memoize.java     |  8 ++-
 .../facades/RandomGeneratorFacadeImpl.java    | 18 +++---
 .../engine/facades/SampleStreamFacade.java    |  9 +--
 .../engine/facades/ShrinkableFacadeImpl.java  | 10 ++--
 .../facades/ShrinkingSupportFacadeImpl.java   | 24 ++++----
 .../jqwik/engine/facades/StoreFacadeImpl.java |  7 ++-
 .../facades/TestingSupportFacadeImpl.java     |  4 +-
 .../jqwik/engine/facades/package-info.java    |  4 ++
 .../engine/hooks/lifecycle/package-info.java  |  4 ++
 .../net/jqwik/engine/hooks/package-info.java  |  4 ++
 .../statistics/StatisticsCollectorImpl.java   | 12 ++--
 .../engine/hooks/statistics/package-info.java |  4 ++
 .../java/net/jqwik/engine/package-info.java   |  4 ++
 .../engine/properties/FeatureExtractor.java   | 10 ++--
 .../properties/PropertyCheckResult.java       | 23 ++++----
 .../RandomizedShrinkablesGenerator.java       |  2 +-
 .../engine/properties/UniquenessChecker.java  | 10 ++--
 .../arbitraries/ArbitraryCollect.java         |  4 +-
 .../arbitraries/ArbitraryFilter.java          |  4 +-
 .../arbitraries/ArbitraryFlatMap.java         |  4 +-
 .../properties/arbitraries/ArbitraryMap.java  |  8 ++-
 .../arbitraries/CreateArbitrary.java          |  4 +-
 .../arbitraries/DefaultArrayArbitrary.java    |  4 +-
 .../arbitraries/DefaultListArbitrary.java     |  4 +-
 .../arbitraries/DefaultSetArbitrary.java      |  4 +-
 .../arbitraries/DefaultTypeArbitrary.java     | 10 ++--
 .../arbitraries/EdgeCasesSupport.java         | 30 +++++-----
 .../arbitraries/FrequencyArbitrary.java       |  6 +-
 .../arbitraries/FrequencyOfArbitrary.java     |  4 +-
 .../GenericEdgeCasesConfiguration.java        |  8 +--
 .../IntegralGeneratingArbitrary.java          |  2 +-
 .../properties/arbitraries/JustArbitrary.java |  6 +-
 .../arbitraries/LazyOfArbitrary.java          |  4 +-
 .../arbitraries/MappedEdgeCasesConsumer.java  |  2 +-
 .../arbitraries/MultivalueArbitraryBase.java  |  4 +-
 .../arbitraries/ReportableStream.java         |  8 ++-
 .../arbitraries/UseGeneratorsArbitrary.java   |  4 +-
 .../combinations/CombineArbitrary.java        | 14 ++---
 .../combinations/DefaultCombinator2.java      | 22 +++----
 .../combinations/DefaultCombinator3.java      | 20 +++----
 .../combinations/DefaultCombinator4.java      | 22 +++----
 .../combinations/DefaultCombinator5.java      | 22 +++----
 .../combinations/DefaultCombinator6.java      | 22 +++----
 .../combinations/DefaultCombinator7.java      | 22 +++----
 .../combinations/DefaultCombinator8.java      | 22 +++----
 .../combinations/DefaultListCombinator.java   | 20 +++----
 .../combinations/package-info.java            |  4 ++
 .../CombinedExhaustiveGenerator.java          |  4 +-
 .../exhaustive/ExhaustiveGenerators.java      | 22 +++----
 .../FilteredExhaustiveGenerator.java          |  4 +-
 .../FlatMappedExhaustiveGenerator.java        |  4 +-
 .../IgnoreExceptionExhaustiveGenerator.java   |  4 +-
 .../IterableBasedExhaustiveGenerator.java     |  4 +-
 .../exhaustive/ListExhaustiveGenerator.java   |  4 +-
 .../exhaustive/MappedExhaustiveGenerator.java |  8 ++-
 .../PermutationExhaustiveGenerator.java       |  6 +-
 .../exhaustive/SetExhaustiveGenerator.java    |  4 +-
 .../WithNullExhaustiveGenerator.java          |  4 +-
 .../arbitraries/exhaustive/package-info.java  |  4 ++
 .../properties/arbitraries/package-info.java  |  4 ++
 .../randomized/CollectGenerator.java          |  4 +-
 .../randomized/ContainerGenerator.java        |  8 +--
 .../randomized/FilteredGenerator.java         |  4 +-
 .../randomized/RandomGenerators.java          | 48 +++++++--------
 .../randomized/WithEdgeCasesGenerator.java    |  4 +-
 .../arbitraries/randomized/package-info.java  |  4 ++
 .../UniqueElementsConfigurator.java           | 16 ++---
 .../configurators/package-info.java           |  4 ++
 .../jqwik/engine/properties/package-info.java |  4 ++
 .../AggressiveSizeOfListShrinker.java         | 12 ++--
 .../shrinking/CollectShrinkable.java          | 12 ++--
 .../shrinking/CombinedShrinkable.java         |  4 +-
 .../shrinking/FilteredShrinkable.java         |  4 +-
 .../FixedValueFlatMappedShrinkable.java       | 10 ++--
 .../shrinking/FlatMappedShrinkable.java       | 12 ++--
 .../shrinking/MappedShrinkable.java           |  4 +-
 .../OneAfterTheOtherParameterShrinker.java    |  4 +-
 .../shrinking/PropertyShrinker.java           |  2 +-
 .../shrinking/ShrinkableContainer.java        | 17 +++---
 .../properties/shrinking/ShrinkableList.java  |  8 +--
 .../properties/shrinking/ShrinkableSet.java   | 12 ++--
 .../shrinking/ShrinkableString.java           | 11 ++--
 .../shrinking/ShrinkingCommons.java           | 12 ++--
 .../shrinking/SizeOfListShrinker.java         | 20 ++++---
 .../properties/shrinking/Unshrinkable.java    |  4 +-
 .../properties/shrinking/package-info.java    |  4 ++
 .../state/DefaultChainArbitrary.java          |  6 +-
 .../state/SequentialActionChain.java          | 14 ++---
 .../properties/state/ShrinkableChain.java     | 20 +++----
 .../state/ShrinkableChainIteration.java       | 10 ++--
 .../engine/properties/state/package-info.java |  4 ++
 .../properties/stateful/ActionGenerator.java  |  4 +-
 .../stateful/RandomActionGenerator.java       |  4 +-
 .../stateful/ShrinkablesActionGenerator.java  |  4 +-
 .../properties/stateful/package-info.java     |  4 ++
 .../jqwik/engine/providers/package-info.java  |  4 ++
 .../jqwik/engine/recording/package-info.java  |  4 ++
 .../jqwik/engine/support/Combinatorics.java   | 26 +++++----
 .../engine/support/JqwikStreamSupport.java    | 10 ++--
 .../engine/support/StreamConcatenation.java   |  4 +-
 .../combinatorics/CombinedIterator.java       | 16 ++---
 .../support/combinatorics/ConcatIterator.java |  8 +--
 .../combinatorics/PermutationIterator.java    |  4 +-
 .../support/combinatorics/SetIterator.java    |  4 +-
 .../support/combinatorics/package-info.java   |  4 ++
 .../jqwik/engine/support/package-info.java    |  4 ++
 .../engine/support/types/package-info.java    |  4 ++
 .../net/jqwik/api/ArbitraryTestHelper.java    |  4 +-
 .../java/net/jqwik/api/ArbitraryTests.java    |  2 +-
 .../net/jqwik/api/ListArbitraryTests.java     |  3 +-
 .../java/net/jqwik/api/SetArbitraryTests.java |  3 +-
 .../constraints/UniqueElementsProperties.java |  4 +-
 .../ContextIsArbitraryConfigurator.java       |  4 +-
 .../shrinking/ArbitraryShrinkingTests.java    |  2 +-
 .../shrinking/CollectShrinkableTests.java     | 14 ++---
 .../shrinking/CombinedShrinkableTests.java    | 10 ++--
 .../jqwik/kotlin/api/AnyForSubtypeOfDsl.kt    | 10 ++--
 .../jqwik/kotlin/api/ArbitraryExtensions.kt   |  2 +-
 .../jqwik/kotlin/api/BuildersExtensions.kt    |  2 +-
 .../jqwik/kotlin/api/CollectionExtensions.kt  |  4 +-
 .../net/jqwik/kotlin/api/JqwikGlobals.kt      | 19 +++---
 .../net/jqwik/kotlin/api/SequenceArbitrary.kt |  2 +-
 .../KotlinUniqueElementsConfigurator.kt       |  2 +-
 .../net/jqwik/testing/CheckReporting.java     |  2 +-
 .../net/jqwik/testing/ShrinkingSupport.java   | 20 ++++---
 .../net/jqwik/testing/TestingFalsifier.java   |  7 ++-
 .../net/jqwik/testing/TestingSupport.java     | 48 +++++++--------
 206 files changed, 1022 insertions(+), 766 deletions(-)
 create mode 100644 engine/src/main/java/net/jqwik/engine/descriptor/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/discovery/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/discovery/predicates/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/discovery/specs/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/execution/lifecycle/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/execution/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/execution/pipeline/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/execution/reporting/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/facades/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/hooks/lifecycle/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/hooks/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/hooks/statistics/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/arbitraries/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/configurators/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/shrinking/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/state/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/properties/stateful/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/providers/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/recording/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/support/combinatorics/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/support/package-info.java
 create mode 100644 engine/src/main/java/net/jqwik/engine/support/types/package-info.java

diff --git a/api/src/main/java/net/jqwik/api/Arbitraries.java b/api/src/main/java/net/jqwik/api/Arbitraries.java
index cefb5d81a..9879ff6e4 100644
--- a/api/src/main/java/net/jqwik/api/Arbitraries.java
+++ b/api/src/main/java/net/jqwik/api/Arbitraries.java
@@ -27,7 +27,7 @@ public static abstract class ArbitrariesFacade {
 
 		public abstract <T extends @Nullable Object> Arbitrary<T> oneOf(Collection<? extends Arbitrary<? extends T>> all);
 
-		public abstract <M> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> actionArbitrary);
+		public abstract <M extends @Nullable Object> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> actionArbitrary);
 
 		public abstract <T extends @Nullable Object> Arbitrary<T> frequencyOf(List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies);
 
@@ -55,7 +55,7 @@ public static abstract class ArbitrariesFacade {
 
 		public abstract <T> Arbitrary<T> defaultFor(Class<T> type, Class<?>[] typeParameters);
 
-		public abstract <T> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<TypeUsage, Arbitrary<Object>> noDefaultResolver);
+		public abstract <T extends @Nullable Object> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<? super TypeUsage, ? extends Arbitrary<T>> noDefaultResolver);
 
 		public abstract <T extends @Nullable Object> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier);
 
@@ -78,7 +78,7 @@ public static abstract class ArbitrariesFacade {
 
 		public abstract Arbitrary<Character> of(char[] chars);
 
-		public abstract <T extends @Nullable Object> Arbitrary<T> of(Collection<T> values);
+		public abstract <T extends @Nullable Object> Arbitrary<T> of(Collection<? extends T> values);
 
 		public abstract <T extends @Nullable Object> Arbitrary<T> create(Supplier<T> supplier);
 
@@ -172,7 +172,7 @@ public static Arbitrary<Random> randoms() {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.1")
-	public static <T extends @Nullable Object> Arbitrary<T> of(Collection<T> values) {
+	public static <T extends @Nullable Object> Arbitrary<T> of(Collection<? extends T> values) {
 		return ArbitrariesFacade.implementation.of(values);
 	}
 
@@ -430,7 +430,7 @@ public static CharacterArbitrary chars() {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.2")
-	public static <@Nullable T> Arbitrary<T> just(@Nullable T value) {
+	public static <T extends @Nullable Object> Arbitrary<T> just(T value) {
 		return ArbitrariesFacade.implementation.just(value);
 	}
 
@@ -515,7 +515,7 @@ public static <T> Arbitrary<T> defaultFor(Class<T> type, Class<?>... typeParamet
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.1")
-	public static <T> Arbitrary<T> defaultFor(TypeUsage typeUsage) {
+	public static <T extends @Nullable Object> Arbitrary<T> defaultFor(TypeUsage typeUsage) {
 		return defaultFor(typeUsage, ignore -> {throw new CannotFindArbitraryException(typeUsage);});
 	}
 
@@ -534,7 +534,7 @@ public static <T> Arbitrary<T> defaultFor(TypeUsage typeUsage) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = EXPERIMENTAL, since = "1.6.1")
-	public static <T> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<TypeUsage, Arbitrary<Object>> noDefaultResolver) {
+	public static <T> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<? super TypeUsage, ? extends Arbitrary<T>> noDefaultResolver) {
 		return ArbitrariesFacade.implementation.defaultFor(typeUsage, noDefaultResolver);
 	}
 
@@ -739,8 +739,8 @@ public static Arbitrary<Void> nothing() {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.6.4")
-	public static <@Nullable T> SetArbitrary<T> subsetOf(Collection<T> values) {
-		return of(values).set();
+	public static <T extends @Nullable Object> SetArbitrary<T> subsetOf(Collection<? extends T> values) {
+		return Arbitraries.<T>of(values).set();
 	}
 
 	/**
@@ -750,7 +750,7 @@ public static Arbitrary<Void> nothing() {
 	 */
 	@SafeVarargs
 	@API(status = MAINTAINED, since = "1.6.4")
-	public static <T> SetArbitrary<T> subsetOf(T... values) {
+	public static <T extends @Nullable Object> SetArbitrary<T> subsetOf(T... values) {
 		return subsetOf(Arrays.asList(values));
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/Arbitrary.java b/api/src/main/java/net/jqwik/api/Arbitrary.java
index 9f0c5ed08..2baafceab 100644
--- a/api/src/main/java/net/jqwik/api/Arbitrary.java
+++ b/api/src/main/java/net/jqwik/api/Arbitrary.java
@@ -30,39 +30,39 @@ abstract class ArbitraryFacade {
 			implementation = FacadeLoader.load(ArbitraryFacade.class);
 		}
 
-		public abstract <T> ListArbitrary<T> list(Arbitrary<T> elementArbitrary);
+		public abstract <T extends @Nullable Object> ListArbitrary<T> list(Arbitrary<T> elementArbitrary);
 
-		public abstract <T> SetArbitrary<T> set(Arbitrary<T> elementArbitrary);
+		public abstract <T extends @Nullable Object> SetArbitrary<T> set(Arbitrary<T> elementArbitrary);
 
-		public abstract <T> StreamArbitrary<T> stream(Arbitrary<T> elementArbitrary);
+		public abstract <T extends @Nullable Object> StreamArbitrary<T> stream(Arbitrary<T> elementArbitrary);
 
-		public abstract <T> IteratorArbitrary<T> iterator(Arbitrary<T> elementArbitrary);
+		public abstract <T extends @Nullable Object> IteratorArbitrary<T> iterator(Arbitrary<T> elementArbitrary);
 
-		public abstract <T, A> ArrayArbitrary<T, A> array(Arbitrary<T> elementArbitrary, Class<A> arrayClass);
+		public abstract <T extends @Nullable Object, A> ArrayArbitrary<T, A> array(Arbitrary<T> elementArbitrary, Class<A> arrayClass);
 
-		public abstract <T> Stream<T> sampleStream(Arbitrary<T> arbitrary);
+		public abstract <T extends @Nullable Object> Stream<T> sampleStream(Arbitrary<T> arbitrary);
 
-		public abstract <T> Arbitrary<T> injectNull(Arbitrary<T> self, double nullProbability);
+		public abstract <T extends @Nullable Object> Arbitrary<@Nullable T> injectNull(Arbitrary<T> self, double nullProbability);
 
-		public abstract <T> Arbitrary<T> filter(Arbitrary<T> self, Predicate<T> filterPredicate, int maxMisses);
+		public abstract <T extends @Nullable Object> Arbitrary<T> filter(Arbitrary<T> self, Predicate<? super T> filterPredicate, int maxMisses);
 
-		public abstract <T, U> Arbitrary<U> map(Arbitrary<T> self, Function<T, U> mapper);
+		public abstract <T, U extends @Nullable Object> Arbitrary<U> map(Arbitrary<T> self, Function<? super T, ? extends U> mapper);
 
-		public abstract <T, U> Arbitrary<U> flatMap(Arbitrary<T> self, Function<T, Arbitrary<U>> mapper);
+		public abstract <T, U extends @Nullable Object> Arbitrary<U> flatMap(Arbitrary<T> self, Function<? super T, ? extends Arbitrary<U>> mapper);
 
-		public abstract <T> Arbitrary<T> ignoreExceptions(Arbitrary<T> self, int maxThrows, Class<? extends Throwable>[] exceptionTypes);
+		public abstract <T extends @Nullable Object> Arbitrary<T> ignoreExceptions(Arbitrary<T> self, int maxThrows, Class<? extends Throwable>[] exceptionTypes);
 
-		public abstract <T> Arbitrary<T> dontShrink(Arbitrary<T> self);
+		public abstract <T extends @Nullable Object> Arbitrary<T> dontShrink(Arbitrary<T> self);
 
-		public abstract <T> Arbitrary<T> configureEdgeCases(Arbitrary<T> self, Consumer<EdgeCases.Config<T>> configurator);
+		public abstract <T extends @Nullable Object> Arbitrary<T> configureEdgeCases(Arbitrary<T> self, Consumer<? super EdgeCases.Config<T>> configurator);
 
-		public abstract <T> Arbitrary<T> withoutEdgeCases(Arbitrary<T> self);
+		public abstract <T extends @Nullable Object> Arbitrary<T> withoutEdgeCases(Arbitrary<T> self);
 
-		public abstract <T> RandomGenerator<T> memoizedGenerator(Arbitrary<T> self, int genSize, boolean withEdgeCases);
+		public abstract <T extends @Nullable Object> RandomGenerator<T> memoizedGenerator(Arbitrary<T> self, int genSize, boolean withEdgeCases);
 
-		public abstract <T> Arbitrary<T> fixGenSize(Arbitrary<T> self, int genSize);
+		public abstract <T extends @Nullable Object> Arbitrary<T> fixGenSize(Arbitrary<T> self, int genSize);
 
-		public abstract <T> Arbitrary<List<T>> collect(Arbitrary<T> self, Predicate<List<T>> until);
+		public abstract <T extends @Nullable Object> Arbitrary<List<T>> collect(Arbitrary<T> self, Predicate<? super List<? extends T>> until);
 	}
 
 	/**
@@ -208,7 +208,7 @@ default void forEachValue(Consumer<? super T> action) {
 	 * @return a new arbitrary instance
 	 * @throws TooManyFilterMissesException if filtering will fail to come up with a value after 10000 tries
 	 */
-	default Arbitrary<T> filter(Predicate<T> filterPredicate) {
+	default Arbitrary<T> filter(Predicate<? super T> filterPredicate) {
 		return filter(10000, filterPredicate);
 	}
 
@@ -222,7 +222,7 @@ default Arbitrary<T> filter(Predicate<T> filterPredicate) {
 	 * @throws TooManyFilterMissesException if filtering will fail to come up with a value after {@code maxMisses} tries
 	 */
 	@API(status = EXPERIMENTAL, since = "1.7.0")
-	default Arbitrary<T> filter(int maxMisses, Predicate<T> filterPredicate) {
+	default Arbitrary<T> filter(int maxMisses, Predicate<? super T> filterPredicate) {
 		return ArbitraryFacade.implementation.filter(this, filterPredicate, maxMisses);
 	}
 
@@ -234,7 +234,7 @@ default Arbitrary<T> filter(int maxMisses, Predicate<T> filterPredicate) {
 	 * @param mapper the function used to map
 	 * @return a new arbitrary instance
 	 */
-	default <U> Arbitrary<U> map(Function<T, U> mapper) {
+	default <U extends @Nullable Object> Arbitrary<U> map(Function<? super T, ? extends U> mapper) {
 		return ArbitraryFacade.implementation.map(this, mapper);
 	}
 
@@ -246,7 +246,7 @@ default <U> Arbitrary<U> map(Function<T, U> mapper) {
 	 * @param mapper the function used to map to arbitrary
 	 * @return a new arbitrary instance
 	 */
-	default <U> Arbitrary<U> flatMap(Function<T, Arbitrary<U>> mapper) {
+	default <U extends @Nullable Object> Arbitrary<U> flatMap(Function<? super T, ? extends Arbitrary<U>> mapper) {
 		return ArbitraryFacade.implementation.flatMap(this, mapper);
 	}
 
@@ -359,7 +359,7 @@ default Arbitrary<Optional<T>> optional(double presenceProbability) {
 	 * @return a new arbitrary instance
 	 */
 	@API(status = MAINTAINED, since = "1.3.0")
-	default Arbitrary<List<T>> collect(Predicate<List<T>> until) {
+	default Arbitrary<List<T>> collect(Predicate<? super List<? extends T>> until) {
 		return ArbitraryFacade.implementation.collect(this, until);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java b/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java
index 0e569fc67..7a604414c 100644
--- a/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java
+++ b/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java
@@ -36,8 +36,7 @@ public interface ArbitrarySupplier<T> extends Supplier<Arbitrary<T>> {
 	 *
 	 * @return A new arbitrary instance
 	 */
-	@Nullable
-	default Arbitrary<T> supplyFor(TypeUsage targetType) {
+	default @Nullable Arbitrary<T> supplyFor(TypeUsage targetType) {
 		return get();
 	}
 
@@ -50,8 +49,7 @@ default Arbitrary<T> supplyFor(TypeUsage targetType) {
 	 *
 	 * @return A new arbitrary instance
 	 */
-	@Nullable
-	default Arbitrary<T> get() {
+	default @Nullable Arbitrary<T> get() {
 		throw new JqwikException("You have to override either ArbitrarySupplier.get() or ArbitrarySupplier.supplyFor()");
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/Builders.java b/api/src/main/java/net/jqwik/api/Builders.java
index b96949449..c6c3ef290 100644
--- a/api/src/main/java/net/jqwik/api/Builders.java
+++ b/api/src/main/java/net/jqwik/api/Builders.java
@@ -64,11 +64,10 @@ private BuilderCombinator(Supplier<B> starter, List<Tuple3<Double, Arbitrary<Obj
 		 * @param <T>           the target object's type
 		 * @return arbitrary of target object
 		 */
-		public <T> Arbitrary<T> build(Function<B, T> buildFunction) {
+		public <T extends @Nullable Object> Arbitrary<T> build(Function<B, T> buildFunction) {
 
 			class Holder {
-				@Nullable
-				final Object value;
+				final @Nullable Object value;
 
 				Holder(@Nullable Object value) {
 					this.value = value;
diff --git a/api/src/main/java/net/jqwik/api/Combinators.java b/api/src/main/java/net/jqwik/api/Combinators.java
index a8a21dfdc..60efb4c89 100644
--- a/api/src/main/java/net/jqwik/api/Combinators.java
+++ b/api/src/main/java/net/jqwik/api/Combinators.java
@@ -68,7 +68,7 @@ public abstract <T1, T2, T3, T4, T5, T6, T7, T8> Combinator8<T1, T2, T3, T4, T5,
 			Arbitrary<T8> a8
 		);
 
-		public abstract <T> ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries);
+		public abstract <T extends @Nullable Object> ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries);
 	}
 
 	private Combinators() {
@@ -173,7 +173,7 @@ public interface Combinator2<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F2<T1, T2, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F2<? super T1, ? super T2, ? extends R> combinator);
 
 		/**
 		 * Filter two values to only let them pass if the predicate is true.
@@ -182,7 +182,7 @@ public interface Combinator2<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator2<T1, T2> filter(F2<T1, T2, Boolean> filter);
+		Combinator2<T1, T2> filter(F2<? super T1, ? super T2, Boolean> filter);
 
 		/**
 		 * Combine two values to create a new arbitrary.
@@ -191,7 +191,7 @@ public interface Combinator2<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F2<T1, T2, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F2<? super T1, ? super T2, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -208,7 +208,7 @@ public interface Combinator3<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F3<T1, T2, T3, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F3<? super T1, ? super T2, ? super T3, ? extends R> combinator);
 
 		/**
 		 * Filter three values to only let them pass if the predicate is true.
@@ -217,7 +217,7 @@ public interface Combinator3<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator3<T1, T2, T3> filter(F3<T1, T2, T3, Boolean> filter);
+		Combinator3<T1, T2, T3> filter(F3<? super T1, ? super T2, ? super T3, Boolean> filter);
 
 		/**
 		 * Combine three values to create a new arbitrary.
@@ -226,7 +226,7 @@ public interface Combinator3<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F3<T1, T2, T3, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F3<? super T1, ? super T2, ? super T3, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -244,7 +244,7 @@ public interface Combinator4<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F4<T1, T2, T3, T4, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F4<? super T1, ? super T2, ? super T3, ? super T4, ? extends R> combinator);
 
 		/**
 		 * Filter four values to only let them pass if the predicate is true.
@@ -253,7 +253,7 @@ public interface Combinator4<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator4<T1, T2, T3, T4> filter(F4<T1, T2, T3, T4, Boolean> filter);
+		Combinator4<T1, T2, T3, T4> filter(F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> filter);
 
 		/**
 		 * Combine four values to create a new arbitrary.
@@ -262,7 +262,7 @@ public interface Combinator4<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F4<T1, T2, T3, T4, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F4<? super T1, ? super T2, ? super T3, ? super T4, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -280,7 +280,7 @@ public interface Combinator5<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F5<T1, T2, T3, T4, T5, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? extends R> combinator);
 
 		/**
 		 * Filter five values to only let them pass if the predicate is true.
@@ -289,7 +289,7 @@ public interface Combinator5<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator5<T1, T2, T3, T4, T5> filter(F5<T1, T2, T3, T4, T5, Boolean> filter);
+		Combinator5<T1, T2, T3, T4, T5> filter(F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> filter);
 
 		/**
 		 * Combine five values to create a new arbitrary.
@@ -298,7 +298,7 @@ public interface Combinator5<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F5<T1, T2, T3, T4, T5, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -315,7 +315,7 @@ public interface Combinator6<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F6<T1, T2, T3, T4, T5, T6, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? extends R> combinator);
 
 		/**
 		 * Filter six values to only let them pass if the predicate is true.
@@ -324,7 +324,7 @@ public interface Combinator6<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator6<T1, T2, T3, T4, T5, T6> filter(F6<T1, T2, T3, T4, T5, T6, Boolean> filter);
+		Combinator6<T1, T2, T3, T4, T5, T6> filter(F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> filter);
 
 		/**
 		 * Combine six values to create a new arbitrary.
@@ -333,7 +333,7 @@ public interface Combinator6<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F6<T1, T2, T3, T4, T5, T6, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -351,7 +351,7 @@ public interface Combinator7<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F7<T1, T2, T3, T4, T5, T6, T7, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? extends R> combinator);
 
 		/**
 		 * Filter seven values to only let them pass if the predicate is true.
@@ -360,7 +360,7 @@ public interface Combinator7<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator7<T1, T2, T3, T4, T5, T6, T7> filter(F7<T1, T2, T3, T4, T5, T6, T7, Boolean> filter);
+		Combinator7<T1, T2, T3, T4, T5, T6, T7> filter(F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> filter);
 
 		/**
 		 * Combine seven values to create a new arbitrary.
@@ -369,7 +369,7 @@ public interface Combinator7<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F7<T1, T2, T3, T4, T5, T6, T7, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F7<T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 
@@ -387,7 +387,7 @@ public interface Combinator8<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, ? extends R> combinator);
 
 		/**
 		 * Filter eight values to only let them pass if the predicate is true.
@@ -396,7 +396,7 @@ public interface Combinator8<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> filter(F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> filter);
+		Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> filter(F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> filter);
 
 		/**
 		 * Combine eight values to create a new arbitrary.
@@ -405,7 +405,7 @@ public interface Combinator8<T1 extends @Nullable Object, T2 extends @Nullable O
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(F8<T1, T2, T3, T4, T5, T6, T7, T8, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(F8<T1, T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
@@ -413,7 +413,7 @@ public interface Combinator8<T1 extends @Nullable Object, T2 extends @Nullable O
 	/**
 	 * Combinator for any number of values.
 	 */
-	public interface ListCombinator<@Nullable T> {
+	public interface ListCombinator<T extends @Nullable Object> {
 
 		/**
 		 * Combine any number of values.
@@ -422,7 +422,7 @@ public interface ListCombinator<@Nullable T> {
 		 * @param <R>        return type
 		 * @return arbitrary instance
 		 */
-		<R extends @Nullable Object> Arbitrary<R> as(Function<List<T>, R> combinator);
+		<R extends @Nullable Object> Arbitrary<R> as(Function<? super List<T>, ? extends R> combinator);
 
 		/**
 		 * Filter list of values to only let them pass if the predicate is true.
@@ -431,7 +431,7 @@ public interface ListCombinator<@Nullable T> {
 		 * @return combinator instance
 		 */
 		@API(status = MAINTAINED, since = "1.8.0")
-		ListCombinator<T> filter(Predicate<List<T>> filter);
+		ListCombinator<T> filter(Predicate<? super List<? extends T>> filter);
 
 		/**
 		 * Combine list of values to create a new arbitrary.
@@ -440,26 +440,26 @@ public interface ListCombinator<@Nullable T> {
 		 * @param <R>            return type of arbitrary
 		 * @return arbitrary instance
 		 */
-		default <R extends @Nullable Object> Arbitrary<R> flatAs(Function<List<T>, Arbitrary<R>> flatCombinator) {
+		default <R extends @Nullable Object> Arbitrary<R> flatAs(Function<? super List<? extends T>, ? extends Arbitrary<R>> flatCombinator) {
 			return as(flatCombinator).flatMap(Function.identity());
 		}
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F2<T1 extends @Nullable Object, T2 extends @Nullable Object, @Nullable R> {
+	public interface F2<T1 extends @Nullable Object, T2 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, @Nullable R> {
+	public interface F3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3);
 	}
 
 	@FunctionalInterface
 	@API(status = INTERNAL)
-	public interface F4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, @Nullable R> {
+	public interface F4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, R extends @Nullable Object> {
 		R apply(T1 t1, T2 t2, T3 t3, T4 t4);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/EdgeCases.java b/api/src/main/java/net/jqwik/api/EdgeCases.java
index 863f75f42..fab4c9a3f 100644
--- a/api/src/main/java/net/jqwik/api/EdgeCases.java
+++ b/api/src/main/java/net/jqwik/api/EdgeCases.java
@@ -4,6 +4,7 @@
 import java.util.function.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
@@ -18,13 +19,13 @@ abstract class EdgeCasesFacade {
 			implementation = FacadeLoader.load(EdgeCases.EdgeCasesFacade.class);
 		}
 
-		public abstract <T> EdgeCases<T> fromSuppliers(List<Supplier<Shrinkable<T>>> suppliers);
+		public abstract <T extends @Nullable Object> EdgeCases<T> fromSuppliers(List<Supplier<Shrinkable<T>>> suppliers);
 	}
 
 	@API(status = MAINTAINED, since = "1.8.0")
 	interface Config<T> {
 
-		static <T> Consumer<Config<T>> noConfig() {
+		static <T extends @Nullable Object> Consumer<Config<T>> noConfig() {
 			return config -> {};
 		}
 
@@ -41,7 +42,7 @@ static <T> Consumer<Config<T>> noConfig() {
 		 * @param filter A predicate
 		 * @return same configuration instance
 		 */
-		Config<T> filter(Predicate<T> filter);
+		Config<T> filter(Predicate<? super T> filter);
 
 		/**
 		 * Add one or more unshrinkable additional values as edge cases.
@@ -86,17 +87,17 @@ default Iterator<Shrinkable<T>> iterator() {
 	}
 
 	@API(status = INTERNAL)
-	static <T> EdgeCases<T> fromSuppliers(List<Supplier<Shrinkable<T>>> suppliers) {
+	static <T extends @Nullable Object> EdgeCases<T> fromSuppliers(List<Supplier<Shrinkable<T>>> suppliers) {
 		return EdgeCasesFacade.implementation.fromSuppliers(suppliers);
 	}
 
 	@API(status = INTERNAL)
-	static <T> EdgeCases<T> none() {
+	static <T extends @Nullable Object> EdgeCases<T> none() {
 		return fromSuppliers(Collections.emptyList());
 	}
 
 	@API(status = INTERNAL)
-	static <T> EdgeCases<T> fromSupplier(Supplier<Shrinkable<T>> supplier) {
+	static <T extends @Nullable Object> EdgeCases<T> fromSupplier(Supplier<Shrinkable<T>> supplier) {
 		return fromSuppliers(Collections.singletonList(supplier));
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/ExhaustiveGenerator.java b/api/src/main/java/net/jqwik/api/ExhaustiveGenerator.java
index b542cd5ec..3547228ee 100644
--- a/api/src/main/java/net/jqwik/api/ExhaustiveGenerator.java
+++ b/api/src/main/java/net/jqwik/api/ExhaustiveGenerator.java
@@ -3,6 +3,7 @@
 import java.util.function.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
@@ -10,7 +11,7 @@
  * Used only internally to run and compute exhaustive generation of parameters
  */
 @API(status = INTERNAL)
-public interface ExhaustiveGenerator<T> extends Iterable<T> {
+public interface ExhaustiveGenerator<T extends @Nullable Object> extends Iterable<T> {
 
 	long MAXIMUM_SAMPLES_TO_GENERATE = Integer.MAX_VALUE;
 
@@ -22,13 +23,13 @@ abstract class ExhaustiveGeneratorFacade {
 			implementation = FacadeLoader.load(ExhaustiveGeneratorFacade.class);
 		}
 
-		public abstract <T, U> ExhaustiveGenerator<U> map(ExhaustiveGenerator<T> self, Function<T, U> mapper);
+		public abstract <T extends @Nullable Object, U extends @Nullable Object> ExhaustiveGenerator<U> map(ExhaustiveGenerator<T> self, Function<? super T, ? extends U> mapper);
 
-		public abstract <T> ExhaustiveGenerator<T> filter(ExhaustiveGenerator<T> self, Predicate<T> filterPredicate, int maxMisses);
+		public abstract <T extends @Nullable Object> ExhaustiveGenerator<T> filter(ExhaustiveGenerator<T> self, Predicate<? super T> filterPredicate, int maxMisses);
 
-		public abstract <T> ExhaustiveGenerator<T> injectNull(ExhaustiveGenerator<T> self);
+		public abstract <T extends @Nullable Object> ExhaustiveGenerator<@Nullable T> injectNull(ExhaustiveGenerator<T> self);
 
-		public abstract <T> ExhaustiveGenerator<T> ignoreExceptions(
+		public abstract <T extends @Nullable Object> ExhaustiveGenerator<T> ignoreExceptions(
 			ExhaustiveGenerator<T> self,
 			Class<? extends Throwable>[] exceptionTypes,
 			int maxThrows
@@ -40,11 +41,11 @@ public abstract <T> ExhaustiveGenerator<T> ignoreExceptions(
 	 */
 	long maxCount();
 
-	default <U> ExhaustiveGenerator<U> map(Function<T, U> mapper) {
+	default <U extends @Nullable Object> ExhaustiveGenerator<U> map(Function<? super T, ? extends U> mapper) {
 		return ExhaustiveGeneratorFacade.implementation.map(this, mapper);
 	}
 
-	default ExhaustiveGenerator<T> filter(Predicate<T> filterPredicate, int maxMisses) {
+	default ExhaustiveGenerator<T> filter(Predicate<? super T> filterPredicate, int maxMisses) {
 		return ExhaustiveGeneratorFacade.implementation.filter(this, filterPredicate, maxMisses);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/Falsifier.java b/api/src/main/java/net/jqwik/api/Falsifier.java
index e1c2d04c7..1afc3fdfc 100644
--- a/api/src/main/java/net/jqwik/api/Falsifier.java
+++ b/api/src/main/java/net/jqwik/api/Falsifier.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.api.lifecycle.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -17,13 +19,13 @@
  */
 @FunctionalInterface
 @API(status = STABLE, since = "1.0")
-public interface Falsifier<T> {
+public interface Falsifier<T extends @Nullable Object> {
 
 	@API(status = INTERNAL)
 	TryExecutionResult execute(T t);
 
 	@API(status = INTERNAL)
-	default Falsifier<T> withFilter(Predicate<T> filter) {
+	default Falsifier<T> withFilter(Predicate<? super T> filter) {
 		return value -> {
 			if (!filter.test(value)) {
 				return TryExecutionResult.invalid();
diff --git a/api/src/main/java/net/jqwik/api/Functions.java b/api/src/main/java/net/jqwik/api/Functions.java
index 368d140fe..7c6c6edcf 100644
--- a/api/src/main/java/net/jqwik/api/Functions.java
+++ b/api/src/main/java/net/jqwik/api/Functions.java
@@ -4,6 +4,8 @@
 
 import net.jqwik.api.arbitraries.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 
@@ -20,7 +22,7 @@ public static abstract class FunctionsFacade {
 
 		public abstract void ensureFunctionalType(Class<?> functionalType);
 
-		public abstract <F, R> FunctionArbitrary<F, R> function(Class<?> functionalType, Arbitrary<R> resultArbitrary);
+		public abstract <F, R extends @Nullable Object> FunctionArbitrary<F, R> function(Class<?> functionalType, Arbitrary<R> resultArbitrary);
 
 	}
 
@@ -65,7 +67,7 @@ private FunctionWrapper(Class<?> functionalType) {
 		 * @return a new arbitrary instance
 		 */
 		@API(status = MAINTAINED, since = "1.6.0")
-		public <F, R> FunctionArbitrary<F, R> returning(Arbitrary<R> resultArbitrary) {
+		public <F, R extends @Nullable Object> FunctionArbitrary<F, R> returning(Arbitrary<R> resultArbitrary) {
 			return FunctionsFacade.implementation.function(functionalType, resultArbitrary);
 		}
 	}
diff --git a/api/src/main/java/net/jqwik/api/RandomGenerator.java b/api/src/main/java/net/jqwik/api/RandomGenerator.java
index b47f3871e..d49e2c18a 100644
--- a/api/src/main/java/net/jqwik/api/RandomGenerator.java
+++ b/api/src/main/java/net/jqwik/api/RandomGenerator.java
@@ -20,25 +20,25 @@ abstract class RandomGeneratorFacade {
 			implementation = FacadeLoader.load(RandomGeneratorFacade.class);
 		}
 
-		public abstract <T, U> Shrinkable<U> flatMap(Shrinkable<T> self, Function<T, RandomGenerator<U>> mapper, long nextLong);
+		public abstract <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> flatMap(Shrinkable<T> self, Function<? super T, ? extends RandomGenerator<U>> mapper, long nextLong);
 
-		public abstract <T, U> Shrinkable<U> flatMap(
+		public abstract <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> flatMap(
 			Shrinkable<T> wrappedShrinkable,
-			Function<T, Arbitrary<U>> mapper,
+			Function<? super T, ? extends Arbitrary<U>> mapper,
 			int genSize,
 			long nextLong,
 			boolean withEmbeddedEdgeCases
 		);
 
-		public abstract <T> RandomGenerator<T> filter(RandomGenerator<T> self, Predicate<T> filterPredicate, int maxMisses);
+		public abstract <T extends @Nullable Object> RandomGenerator<T> filter(RandomGenerator<T> self, Predicate<? super T> filterPredicate, int maxMisses);
 
-		public abstract <T> RandomGenerator<T> withEdgeCases(RandomGenerator<T> self, int genSize, EdgeCases<T> edgeCases);
+		public abstract <T extends @Nullable Object> RandomGenerator<T> withEdgeCases(RandomGenerator<T> self, int genSize, EdgeCases<T> edgeCases);
 
-		public abstract <T> RandomGenerator<List<T>> collect(RandomGenerator<T> self, Predicate<List<T>> until);
+		public abstract <T extends @Nullable Object> RandomGenerator<List<T>> collect(RandomGenerator<T> self, Predicate<? super List<? extends T>> until);
 
-		public abstract <T> RandomGenerator<T> injectDuplicates(RandomGenerator<T> self, double duplicateProbability);
+		public abstract <T extends @Nullable Object> RandomGenerator<T> injectDuplicates(RandomGenerator<T> self, double duplicateProbability);
 
-		public abstract <T> RandomGenerator<T> ignoreExceptions(
+		public abstract <T extends @Nullable Object> RandomGenerator<T> ignoreExceptions(
 			RandomGenerator<T> self,
 			Class<? extends Throwable>[] exceptionTypes,
 			int maxThrows
@@ -52,12 +52,12 @@ public abstract <T> RandomGenerator<T> ignoreExceptions(
 	Shrinkable<T> next(Random random);
 
 	@API(status = INTERNAL)
-	default <U> RandomGenerator<U> map(Function<T, U> mapper) {
+	default <U extends @Nullable Object> RandomGenerator<U> map(Function<? super T, ? extends U> mapper) {
 		return this.mapShrinkable(s -> s.map(mapper));
 	}
 
 	@API(status = INTERNAL)
-	default <U> RandomGenerator<U> mapShrinkable(Function<Shrinkable<T>, Shrinkable<U>> mapper) {
+	default <U extends @Nullable Object> RandomGenerator<U> mapShrinkable(Function<? super Shrinkable<T>, ? extends Shrinkable<U>> mapper) {
 		return random -> {
 			Shrinkable<T> tShrinkable = RandomGenerator.this.next(random);
 			return mapper.apply(tShrinkable);
@@ -65,7 +65,7 @@ default <U> RandomGenerator<U> mapShrinkable(Function<Shrinkable<T>, Shrinkable<
 	}
 
 	@API(status = INTERNAL)
-	default <U> RandomGenerator<U> flatMap(Function<T, RandomGenerator<U>> mapper) {
+	default <U extends @Nullable Object> RandomGenerator<U> flatMap(Function<? super T, ? extends RandomGenerator<U>> mapper) {
 		return random -> {
 			Shrinkable<T> wrappedShrinkable = RandomGenerator.this.next(random);
 			return RandomGeneratorFacade.implementation.flatMap(wrappedShrinkable, mapper, random.nextLong());
@@ -73,7 +73,7 @@ default <U> RandomGenerator<U> flatMap(Function<T, RandomGenerator<U>> mapper) {
 	}
 
 	@API(status = INTERNAL)
-	default <U> RandomGenerator<U> flatMap(Function<T, Arbitrary<U>> mapper, int genSize, boolean withEmbeddedEdgeCases) {
+	default <U extends @Nullable Object> RandomGenerator<U> flatMap(Function<? super T, ? extends Arbitrary<U>> mapper, int genSize, boolean withEmbeddedEdgeCases) {
 		return random -> {
 			Shrinkable<T> wrappedShrinkable = RandomGenerator.this.next(random);
 			return RandomGeneratorFacade.implementation
@@ -82,7 +82,7 @@ default <U> RandomGenerator<U> flatMap(Function<T, Arbitrary<U>> mapper, int gen
 	}
 
 	@API(status = INTERNAL)
-	default RandomGenerator<T> filter(Predicate<T> filterPredicate, int maxMisses) {
+	default RandomGenerator<T> filter(Predicate<? super T> filterPredicate, int maxMisses) {
 		return RandomGeneratorFacade.implementation.filter(this, filterPredicate, maxMisses);
 	}
 
@@ -97,7 +97,7 @@ default Stream<Shrinkable<T>> stream(Random random) {
 	}
 
 	@API(status = INTERNAL)
-	default RandomGenerator<List<T>> collect(Predicate<List<T>> until) {
+	default RandomGenerator<List<T>> collect(Predicate<? super List<? extends T>> until) {
 		return RandomGeneratorFacade.implementation.collect(this, until);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/Shrinkable.java b/api/src/main/java/net/jqwik/api/Shrinkable.java
index c7a4c87db..57b017c5d 100644
--- a/api/src/main/java/net/jqwik/api/Shrinkable.java
+++ b/api/src/main/java/net/jqwik/api/Shrinkable.java
@@ -20,25 +20,25 @@ abstract class ShrinkableFacade {
 			implementation = FacadeLoader.load(ShrinkableFacade.class);
 		}
 
-		public abstract <T> Shrinkable<T> unshrinkable(@Nullable Supplier<T> valueSupplier, ShrinkingDistance distance);
+		public abstract <T extends @Nullable Object> Shrinkable<T> unshrinkable(Supplier<? extends T> valueSupplier, ShrinkingDistance distance);
 
-		public abstract <T, U> Shrinkable<U> map(Shrinkable<T> self, Function<T, U> mapper);
+		public abstract <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> map(Shrinkable<T> self, Function<? super T, ? extends U> mapper);
 
-		public abstract <T> Shrinkable<T> filter(Shrinkable<T> self, Predicate<T> filter);
+		public abstract <T extends @Nullable Object> Shrinkable<T> filter(Shrinkable<T> self, Predicate<? super T> filter);
 
-		public abstract <T, U> Shrinkable<U> flatMap(Shrinkable<T> self, Function<T, Arbitrary<U>> flatMapper, int tries, long randomSeed);
+		public abstract <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> flatMap(Shrinkable<T> self, Function<? super T, ? extends Arbitrary<U>> flatMapper, int tries, long randomSeed);
 	}
 
-	static <T> Shrinkable<T> unshrinkable(@Nullable T value) {
+	static <T extends @Nullable Object> Shrinkable<T> unshrinkable(T value) {
 		return unshrinkable(value, ShrinkingDistance.MIN);
 	}
 
-	static <T> Shrinkable<T> unshrinkable(@Nullable T value, ShrinkingDistance distance) {
+	static <T extends @Nullable Object> Shrinkable<T> unshrinkable(T value, ShrinkingDistance distance) {
 		return ShrinkableFacade.implementation.unshrinkable(() -> value, distance);
 	}
 
 	@API(status = INTERNAL)
-	static <T> Shrinkable<T> supplyUnshrinkable(Supplier<T> supplier) {
+	static <T extends @Nullable Object> Shrinkable<T> supplyUnshrinkable(Supplier<? extends T> supplier) {
 		return ShrinkableFacade.implementation.unshrinkable(supplier, ShrinkingDistance.MIN);
 	}
 
@@ -98,15 +98,15 @@ default Shrinkable<Object> asGeneric() {
 		return (Shrinkable<Object>) this;
 	}
 
-	default <U> Shrinkable<U> map(Function<T, U> mapper) {
+	default <U extends @Nullable Object> Shrinkable<U> map(Function<? super T, ? extends U> mapper) {
 		return ShrinkableFacade.implementation.map(this, mapper);
 	}
 
-	default Shrinkable<T> filter(Predicate<T> filter) {
+	default Shrinkable<T> filter(Predicate<? super T> filter) {
 		return ShrinkableFacade.implementation.filter(this, filter);
 	}
 
-	default <U> Shrinkable<U> flatMap(Function<T, Arbitrary<U>> flatMapper, int tries, long randomSeed) {
+	default <U extends @Nullable Object> Shrinkable<U> flatMap(Function<? super T, ? extends Arbitrary<U>> flatMapper, int tries, long randomSeed) {
 		return ShrinkableFacade.implementation.flatMap(this, flatMapper, tries, randomSeed);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/ShrinkingDistance.java b/api/src/main/java/net/jqwik/api/ShrinkingDistance.java
index 28584ecaf..cbc167582 100644
--- a/api/src/main/java/net/jqwik/api/ShrinkingDistance.java
+++ b/api/src/main/java/net/jqwik/api/ShrinkingDistance.java
@@ -49,7 +49,7 @@ public static ShrinkingDistance of(long... distances) {
 	}
 
 	@API(status = MAINTAINED, since = "1.0")
-	public static <T> ShrinkingDistance forCollection(Collection<Shrinkable<T>> elements) {
+	public static <T extends @Nullable Object> ShrinkingDistance forCollection(Collection<? extends Shrinkable<T>> elements) {
 		// This is an optimization to avoid creating temporary arrays, which the old streams-based implementation did.
 		long[] collectedDistances = sumUp(toDistances(elements));
 		ShrinkingDistance sumDistanceOfElements = new ShrinkingDistance(collectedDistances);
@@ -57,7 +57,7 @@ public static <T> ShrinkingDistance forCollection(Collection<Shrinkable<T>> elem
 	}
 
 	@API(status = MAINTAINED, since = "1.0")
-	public static <T> ShrinkingDistance combine(List<Shrinkable<T>> shrinkables) {
+	public static <T extends @Nullable Object> ShrinkingDistance combine(List<? extends Shrinkable<T>> shrinkables) {
 		// This can happen e.g. when using Combinators.combine() with an empty list of arbitraries.
 		if (shrinkables.isEmpty()) {
 			return ShrinkingDistance.MIN;
@@ -145,8 +145,7 @@ public ShrinkingDistance append(ShrinkingDistance other) {
 		return new ShrinkingDistance(appendedDistances);
 	}
 
-	@NonNull
-	private static <T> List<long[]> toDistances(Collection<Shrinkable<T>> shrinkables) {
+	private static <T extends @Nullable Object> List<long[]> toDistances(Collection<? extends Shrinkable<T>> shrinkables) {
 		List<long[]> listOfDistances = new ArrayList<>(shrinkables.size());
 		for (Shrinkable<?> tShrinkable : shrinkables) {
 			long[] longs = tShrinkable.distance().distances;
diff --git a/api/src/main/java/net/jqwik/api/Table.java b/api/src/main/java/net/jqwik/api/Table.java
index 66a998b2e..d77976107 100644
--- a/api/src/main/java/net/jqwik/api/Table.java
+++ b/api/src/main/java/net/jqwik/api/Table.java
@@ -7,54 +7,56 @@
 
 import net.jqwik.api.Tuple.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = MAINTAINED, since = "1.0")
 public class Table {
 	@SafeVarargs
-	public static <E> Iterable<Tuple1<E>> of(E... elements) {
+	public static <E extends @Nullable Object> Iterable<Tuple1<E>> of(E... elements) {
 		return Arrays.stream(elements)
 				 .map(Tuple::of)
 				 .collect(Collectors.toList());
 	}
 
 	@SafeVarargs
-	public static <T> Iterable<Tuple1<T>> of(Tuple1<T>... tuples) {
+	public static <T extends @Nullable Object> Iterable<Tuple1<T>> of(Tuple1<T>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2> Iterable<Tuple2<T1, T2>> of(Tuple2<T1, T2>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object> Iterable<Tuple2<T1, T2>> of(Tuple2<T1, T2>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2, T3> Iterable<Tuple3<T1, T2, T3>> of(Tuple3<T1, T2, T3>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object> Iterable<Tuple3<T1, T2, T3>> of(Tuple3<T1, T2, T3>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2, T3, T4> Iterable<Tuple4<T1, T2, T3, T4>> of(Tuple4<T1, T2, T3, T4>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object> Iterable<Tuple4<T1, T2, T3, T4>> of(Tuple4<T1, T2, T3, T4>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2, T3, T4, T5> Iterable<Tuple5<T1, T2, T3, T4, T5>> of(Tuple5<T1, T2, T3, T4, T5>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object> Iterable<Tuple5<T1, T2, T3, T4, T5>> of(Tuple5<T1, T2, T3, T4, T5>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2, T3, T4, T5, T6> Iterable<Tuple6<T1, T2, T3, T4, T5, T6>> of(Tuple6<T1, T2, T3, T4, T5, T6>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object> Iterable<Tuple6<T1, T2, T3, T4, T5, T6>> of(Tuple6<T1, T2, T3, T4, T5, T6>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2, T3, T4, T5, T6, T7> Iterable<Tuple7<T1, T2, T3, T4, T5, T6, T7>> of(Tuple7<T1, T2, T3, T4, T5, T6, T7>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object> Iterable<Tuple7<T1, T2, T3, T4, T5, T6, T7>> of(Tuple7<T1, T2, T3, T4, T5, T6, T7>... tuples) {
 		return Arrays.asList(tuples);
 	}
 
 	@SafeVarargs
-	public static <T1, T2, T3, T4, T5, T6, T7, T8> Iterable<Tuple8<T1, T2, T3, T4, T5, T6, T7, T8>> of(Tuple8<T1, T2, T3, T4, T5, T6, T7, T8>... tuples) {
+	public static <T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object> Iterable<Tuple8<T1, T2, T3, T4, T5, T6, T7, T8>> of(Tuple8<T1, T2, T3, T4, T5, T6, T7, T8>... tuples) {
 		return Arrays.asList(tuples);
 	}
 }
diff --git a/api/src/main/java/net/jqwik/api/Tuple.java b/api/src/main/java/net/jqwik/api/Tuple.java
index 6b97d3cc1..226f32c1a 100644
--- a/api/src/main/java/net/jqwik/api/Tuple.java
+++ b/api/src/main/java/net/jqwik/api/Tuple.java
@@ -140,7 +140,7 @@ public T1 get1() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1());
 		}
 
@@ -181,7 +181,7 @@ public T2 get2() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2());
 		}
 
@@ -218,7 +218,7 @@ public T3 get3() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2(), get3());
 		}
 
@@ -256,7 +256,7 @@ public T4 get4() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2(), get3(), get4());
 		}
 
@@ -295,7 +295,7 @@ public T5 get5() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2(), get3(), get4(), get5());
 		}
 
@@ -335,7 +335,7 @@ public T6 get6() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2(), get3(), get4(), get5(), get6());
 		}
 
@@ -384,7 +384,7 @@ public T7 get7() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2(), get3(), get4(), get5(), get6(), get7());
 		}
 
@@ -435,7 +435,7 @@ public T8 get8() {
 		}
 
 		@Override
-		public List<Object> items() {
+		public List<@Nullable Object> items() {
 			return Arrays.asList(get1(), get2(), get3(), get4(), get5(), get6(), get7(), get8());
 		}
 
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/ArbitraryDecorator.java b/api/src/main/java/net/jqwik/api/arbitraries/ArbitraryDecorator.java
index ea8d78bba..7bdd4a314 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/ArbitraryDecorator.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/ArbitraryDecorator.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -21,7 +23,7 @@
 @API(status = MAINTAINED, since = "1.4.0")
 public abstract class ArbitraryDecorator<T>  implements Cloneable, Arbitrary<T> {
 
-	private Arbitrary<T> instance = null;
+	private @Nullable Arbitrary<T> instance = null;
 
 	/**
 	 * Implement by calling jqwik's standard DSL for building arbitraries.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
index 808963692..86dc42e82 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/FunctionArbitrary.java
@@ -17,7 +17,7 @@
  * @param <R> The return type of the functional interface
  */
 @API(status = MAINTAINED, since = "1.3.0")
-public interface FunctionArbitrary<F extends @Nullable Object, R extends @Nullable Object> extends Arbitrary<F> {
+public interface FunctionArbitrary<F, R extends @Nullable Object> extends Arbitrary<F> {
 
 	/**
 	 *
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
index 4c2e69655..5acfae58e 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/ListArbitrary.java
@@ -61,7 +61,7 @@ default ListArbitrary<T> ofSize(int size) {
 	 * @return arbitrary of a list of Us
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	<U> Arbitrary<List<U>> mapEach(BiFunction<List<T>, T, U> mapper);
+	<U extends @Nullable Object> Arbitrary<List<U>> mapEach(BiFunction<? super List<? extends T>, ? super T, ? extends U> mapper);
 
 	/**
 	 * Flat-map over each element of the generated list giving access to all elements of the list.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
index f9a9fe1b5..665c5277c 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/SetArbitrary.java
@@ -61,7 +61,7 @@ default SetArbitrary<T> ofSize(int size) {
 	 * @return arbitrary of a set of Us
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	<U> Arbitrary<Set<U>> mapEach(BiFunction<Set<T>, T, U> mapper);
+	<U extends @Nullable Object> Arbitrary<Set<U>> mapEach(BiFunction<? super Set<? extends T>, ? super T, ? extends U> mapper);
 
 	/**
 	 * Flat-map over each element of the generated set giving access to all elements of the set.
@@ -72,7 +72,7 @@ default SetArbitrary<T> ofSize(int size) {
 	 * @return arbitrary of a set of Us
 	 */
 	@API(status = MAINTAINED, since = "1.4.0")
-	<U> Arbitrary<Set<U>> flatMapEach(BiFunction<Set<T>, T, Arbitrary<U>> flatMapper);
+	<U extends @Nullable Object> Arbitrary<Set<U>> flatMapEach(BiFunction<? super Set<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper);
 
 	/**
 	 * Do not use. Sets have unique elements anyway.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
index d45d63565..f65c40bfa 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/StreamableArbitrary.java
@@ -28,7 +28,7 @@ public interface StreamableArbitrary<T extends @Nullable Object, U> extends Siza
 	 * @param <R>         The result type
 	 * @return The result of accumulating all elements in streamable
 	 */
-	<R> Arbitrary<R> reduce(R initial, BiFunction<R, T, R> accumulator);
+	<R extends @Nullable Object> Arbitrary<R> reduce(R initial, BiFunction<R, ? super T, R> accumulator);
 
 	/**
 	 * Fix the size to {@code size}.
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
index 0ea63796a..57dcf14fa 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
@@ -26,7 +26,7 @@
  * @see net.jqwik.api.domains.DomainContext
  */
 @API(status = MAINTAINED, since = "1.2")
-public interface TypeArbitrary<T extends @Nullable Object> extends Arbitrary<T> {
+public interface TypeArbitrary<T> extends Arbitrary<T> {
 
 	/**
 	 * Add public constructors of class {@code T} to be used
@@ -76,7 +76,7 @@ public interface TypeArbitrary<T extends @Nullable Object> extends Arbitrary<T>
 	 * @param filter Predicate to add only those factory methods for which the predicate returns true
 	 * @return new arbitrary instance
 	 */
-	TypeArbitrary<T> useFactoryMethods(Predicate<Method> filter);
+	TypeArbitrary<T> useFactoryMethods(Predicate<? super Method> filter);
 
 	/**
 	 * Enable recursive use of type arbitrary:
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/package-info.java b/api/src/main/java/net/jqwik/api/arbitraries/package-info.java
index a02d1de34..af09d2ada 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/package-info.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.arbitraries;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfigurator.java b/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfigurator.java
index 4c4aeae81..5c9d7fa9c 100644
--- a/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfigurator.java
+++ b/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfigurator.java
@@ -5,6 +5,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.providers.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -26,7 +28,7 @@ public interface ArbitraryConfigurator extends Comparable<ArbitraryConfigurator>
 	 * @return the newly configured arbitrary instance
 	 */
 	// Type variable T does not make any sense here. Removing it would breaks all implementors, though.
-	<T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType);
+	<T extends @Nullable Object> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType);
 
 	/**
 	 * Determines the order in which a configurator will be applied in regards to other configurators.
diff --git a/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfiguratorBase.java b/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfiguratorBase.java
index f64c3d439..a7c0cda2e 100644
--- a/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfiguratorBase.java
+++ b/api/src/main/java/net/jqwik/api/configurators/ArbitraryConfiguratorBase.java
@@ -6,6 +6,7 @@
 import java.util.stream.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 import org.junit.platform.commons.support.*;
 
 import net.jqwik.api.*;
@@ -38,7 +39,7 @@ public abstract class ArbitraryConfiguratorBase implements ArbitraryConfigurator
 	private final static String CONFIG_METHOD_NAME_PATTERN = "configure.*";
 
 	@Override
-	public <T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
+	public <T extends @Nullable Object> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
 		if (!acceptTargetType(targetType)) {
 			return arbitrary;
 		}
@@ -68,7 +69,7 @@ private List<Annotation> configurationAnnotations(TypeUsage parameter) {
 	}
 
 	@SuppressWarnings("unchecked")
-	private <T> Arbitrary<T> configureWithMethod(Arbitrary<T> arbitrary, Annotation annotation, Method configurationMethod) {
+	private <T extends @Nullable Object> Arbitrary<T> configureWithMethod(Arbitrary<T> arbitrary, Annotation annotation, Method configurationMethod) {
 		Object configurationResult = invokeMethod(configurationMethod, this, arbitrary, annotation);
 		if (configurationResult == null) {
 			return arbitrary;
@@ -80,7 +81,7 @@ private <T> Arbitrary<T> configureWithMethod(Arbitrary<T> arbitrary, Annotation
 	}
 
 	@SuppressWarnings("unchecked")
-	private <T> List<Method> findConfigurationMethods(Arbitrary<T> arbitrary, TypeUsage targetType, Annotation annotation) {
+	private <T extends @Nullable Object> List<Method> findConfigurationMethods(Arbitrary<T> arbitrary, TypeUsage targetType, Annotation annotation) {
 		Class<? extends Arbitrary<T>> arbitraryClass = (Class<? extends Arbitrary<T>>) arbitrary.getClass();
 		return findMethods(
 			getClass(),
diff --git a/api/src/main/java/net/jqwik/api/configurators/SelfConfiguringArbitrary.java b/api/src/main/java/net/jqwik/api/configurators/SelfConfiguringArbitrary.java
index 59c16d0ff..3fe98dda3 100644
--- a/api/src/main/java/net/jqwik/api/configurators/SelfConfiguringArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/configurators/SelfConfiguringArbitrary.java
@@ -5,6 +5,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.providers.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -32,7 +34,7 @@ public interface SelfConfiguringArbitrary<T> {
 	 */
 	@SuppressWarnings("unchecked")
 	@API(status = INTERNAL)
-	static <T> Arbitrary<T> configure(Arbitrary<T> self, ArbitraryConfigurator configurator, TypeUsage targetType) {
+	static <T extends @Nullable Object> Arbitrary<T> configure(Arbitrary<T> self, ArbitraryConfigurator configurator, TypeUsage targetType) {
 		if (self instanceof SelfConfiguringArbitrary) {
 			return ((SelfConfiguringArbitrary<T>) self).configure(configurator, targetType);
 		} else {
diff --git a/api/src/main/java/net/jqwik/api/configurators/package-info.java b/api/src/main/java/net/jqwik/api/configurators/package-info.java
index eb47fc039..3660081f9 100644
--- a/api/src/main/java/net/jqwik/api/configurators/package-info.java
+++ b/api/src/main/java/net/jqwik/api/configurators/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.configurators;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/constraints/package-info.java b/api/src/main/java/net/jqwik/api/constraints/package-info.java
index b988487c8..927d596c0 100644
--- a/api/src/main/java/net/jqwik/api/constraints/package-info.java
+++ b/api/src/main/java/net/jqwik/api/constraints/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.constraints;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/domains/package-info.java b/api/src/main/java/net/jqwik/api/domains/package-info.java
index 4d063658e..834cb3e60 100644
--- a/api/src/main/java/net/jqwik/api/domains/package-info.java
+++ b/api/src/main/java/net/jqwik/api/domains/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.domains;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/facades/ShrinkingSupportFacade.java b/api/src/main/java/net/jqwik/api/facades/ShrinkingSupportFacade.java
index 9d25b660e..abf6c4692 100644
--- a/api/src/main/java/net/jqwik/api/facades/ShrinkingSupportFacade.java
+++ b/api/src/main/java/net/jqwik/api/facades/ShrinkingSupportFacade.java
@@ -7,6 +7,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.lifecycle.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = INTERNAL)
@@ -17,19 +19,19 @@ public abstract class ShrinkingSupportFacade {
 		implementation = FacadeLoader.load(ShrinkingSupportFacade.class);
 	}
 
-	public abstract <T> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random, Falsifier<T> falsifier);
+	public abstract <T extends @Nullable Object> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random, Falsifier<? super T> falsifier);
 
-	public abstract <T> T falsifyThenShrink(RandomGenerator<? extends T> arbitrary, Random random, Falsifier<T> falsifier);
+	public abstract <T extends @Nullable Object> T falsifyThenShrink(RandomGenerator<? extends T> arbitrary, Random random, Falsifier<? super T> falsifier);
 
-	public abstract <T> T shrink(
+	public abstract <T extends @Nullable Object> T shrink(
 			Shrinkable<T> falsifiedShrinkable,
-			Falsifier<T> falsifier,
+			Falsifier<? super T> falsifier,
 			Throwable originalError
 	);
 
-	public abstract <T> ShrunkFalsifiedSample shrinkToSample(
+	public abstract <T extends @Nullable Object> ShrunkFalsifiedSample shrinkToSample(
 			Shrinkable<T> falsifiedShrinkable,
-			Falsifier<T> falsifier,
+			Falsifier<? super T> falsifier,
 			Throwable originalError
 	);
 }
diff --git a/api/src/main/java/net/jqwik/api/facades/TestingSupportFacade.java b/api/src/main/java/net/jqwik/api/facades/TestingSupportFacade.java
index 5684cd428..faef84463 100644
--- a/api/src/main/java/net/jqwik/api/facades/TestingSupportFacade.java
+++ b/api/src/main/java/net/jqwik/api/facades/TestingSupportFacade.java
@@ -5,6 +5,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 public abstract class TestingSupportFacade {
 
 	public static final TestingSupportFacade implementation;
@@ -13,7 +15,7 @@ public abstract class TestingSupportFacade {
 		implementation = FacadeLoader.load(TestingSupportFacade.class);
 	}
 
-	public abstract <T> Shrinkable<T> generateUntil(RandomGenerator<T> generator, Random random, Function<T, Boolean> condition);
+	public abstract <T extends @Nullable Object> Shrinkable<T> generateUntil(RandomGenerator<T> generator, Random random, Function<? super T, Boolean> condition);
 
 	public abstract String singleLineReport(Object any);
 
diff --git a/api/src/main/java/net/jqwik/api/facades/package-info.java b/api/src/main/java/net/jqwik/api/facades/package-info.java
index b722f79bb..f6e7f1050 100644
--- a/api/src/main/java/net/jqwik/api/facades/package-info.java
+++ b/api/src/main/java/net/jqwik/api/facades/package-info.java
@@ -5,5 +5,7 @@
  */
 
 //@API(status = API.Status.INTERNAL) // Currently not allowed by api-guardian
-
+@NullMarked
 package net.jqwik.api.facades;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/api/src/main/java/net/jqwik/api/footnotes/package-info.java b/api/src/main/java/net/jqwik/api/footnotes/package-info.java
index 8fb8b03d1..3eabfaace 100644
--- a/api/src/main/java/net/jqwik/api/footnotes/package-info.java
+++ b/api/src/main/java/net/jqwik/api/footnotes/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.footnotes;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/lifecycle/Store.java b/api/src/main/java/net/jqwik/api/lifecycle/Store.java
index 3d3877a50..61eb2bbbc 100644
--- a/api/src/main/java/net/jqwik/api/lifecycle/Store.java
+++ b/api/src/main/java/net/jqwik/api/lifecycle/Store.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -31,11 +33,11 @@ abstract class StoreFacade {
 			implementation = FacadeLoader.load(Store.StoreFacade.class);
 		}
 
-		public abstract <T> Store<T> create(Object identifier, Lifespan visibility, Supplier<T> initialValueSupplier);
+		public abstract <T extends @Nullable Object> Store<T> create(Object identifier, Lifespan visibility, Supplier<? extends T> initialValueSupplier);
 
-		public abstract <T> Store<T> get(Object identifier);
+		public abstract <T extends @Nullable Object> Store<T> get(Object identifier);
 
-		public abstract <T> Store<T> free(Supplier<T> initialValueSupplier);
+		public abstract <T extends @Nullable Object> Store<T> free(Supplier<? extends T> initialValueSupplier);
 	}
 
 	/**
@@ -63,7 +65,7 @@ interface CloseOnReset {
 	 * @param initialValueSupplier Supplies the value to be used for initializing the store depending on its lifespan
 	 * @return New store instance
 	 */
-	static <T> Store<T> create(Object identifier, Lifespan lifespan, Supplier<T> initialValueSupplier) {
+	static <T extends @Nullable Object> Store<T> create(Object identifier, Lifespan lifespan, Supplier<T> initialValueSupplier) {
 		return StoreFacade.implementation.create(identifier, lifespan, initialValueSupplier);
 	}
 
@@ -82,7 +84,7 @@ static <T> Store<T> create(Object identifier, Lifespan lifespan, Supplier<T> ini
 	 * @param initialValueSupplier Supplies the value to be used for initializing the store depending on its lifespan
 	 * @return New or existing store instance
 	 */
-	static <T> Store<T> getOrCreate(Object identifier, Lifespan lifespan, Supplier<T> initialValueSupplier) {
+	static <T extends @Nullable Object> Store<T> getOrCreate(Object identifier, Lifespan lifespan, Supplier<T> initialValueSupplier) {
 		try {
 			Store<T> store = Store.get(identifier);
 			if (!store.lifespan().equals(lifespan)) {
@@ -107,7 +109,7 @@ static <T> Store<T> getOrCreate(Object identifier, Lifespan lifespan, Supplier<T
 	 * @return Existing store instance
 	 * @throws CannotFindStoreException
 	 */
-	static <T> Store<T> get(Object identifier) {
+	static <T extends @Nullable Object> Store<T> get(Object identifier) {
 		return StoreFacade.implementation.get(identifier);
 	}
 
@@ -119,7 +121,7 @@ static <T> Store<T> get(Object identifier) {
 	 * @return New store instance
 	 */
 	@API(status = EXPERIMENTAL, since = "1.5.0")
-	static <T> Store<T> free(Supplier<T> initializer) {
+	static <T extends @Nullable Object> Store<T> free(Supplier<T> initializer) {
 		return StoreFacade.implementation.free(initializer);
 	}
 }
diff --git a/api/src/main/java/net/jqwik/api/package-info.java b/api/src/main/java/net/jqwik/api/package-info.java
index 4269e07c0..89e01a538 100644
--- a/api/src/main/java/net/jqwik/api/package-info.java
+++ b/api/src/main/java/net/jqwik/api/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/providers/package-info.java b/api/src/main/java/net/jqwik/api/providers/package-info.java
index 5d1718924..b00b6fe58 100644
--- a/api/src/main/java/net/jqwik/api/providers/package-info.java
+++ b/api/src/main/java/net/jqwik/api/providers/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.providers;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/sessions/package-info.java b/api/src/main/java/net/jqwik/api/sessions/package-info.java
index bb70147d0..dc00a88df 100644
--- a/api/src/main/java/net/jqwik/api/sessions/package-info.java
+++ b/api/src/main/java/net/jqwik/api/sessions/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.sessions;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/state/Action.java b/api/src/main/java/net/jqwik/api/state/Action.java
index 73a5909ad..1ab18f4f7 100644
--- a/api/src/main/java/net/jqwik/api/state/Action.java
+++ b/api/src/main/java/net/jqwik/api/state/Action.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -30,33 +32,33 @@
  * @see ActionChainArbitrary
  */
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface Action<S> {
+public interface Action<S extends @Nullable Object> {
 
 	/**
 	 * Create an unconditioned {@linkplain ActionBuilder}.
 	 */
-	static <T> ActionBuilder<T> builder() {
+	static <T extends @Nullable Object> ActionBuilder<T> builder() {
 		return new ActionBuilder<T>();
 	}
 
 	/**
 	 * Create an {@linkplain ActionBuilder} with a precondition.
 	 */
-	static <T> ActionBuilder<T> when(Predicate<T> precondition) {
+	static <T extends @Nullable Object> ActionBuilder<T> when(Predicate<? super T> precondition) {
 		return new ActionBuilder<T>().when(precondition);
 	}
 
 	/**
 	 * Convenience method to create an independent {@linkplain Action} with a constant transformer
 	 */
-	static <T> Action.Independent<T> just(Transformer<T> transformer) {
+	static <T extends @Nullable Object> Action.Independent<T> just(Transformer<T> transformer) {
 		return Action.<T>builder().just(transformer);
 	}
 
 	/**
 	 * Convenience method to create an independent {@linkplain Action} with a description and a constant transformer
 	 */
-	static <T> Action.Independent<T> just(String description, Transformer<T> transformer) {
+	static <T extends @Nullable Object> Action.Independent<T> just(String description, Transformer<T> transformer) {
 		return Action.<T>builder().describeAs(description).just(transformer);
 	}
 
@@ -82,7 +84,7 @@ default boolean precondition(S state) {
 	 * </p>
 	 */
 	@FunctionalInterface
-	interface Dependent<S> extends Action<S> {
+	interface Dependent<S extends @Nullable Object> extends Action<S> {
 		/**
 		 * Return an arbitrary for transformers that depends on the previous state.
 		 *
@@ -109,7 +111,7 @@ interface Dependent<S> extends Action<S> {
 	 * @see JustTransform
 	 */
 	@FunctionalInterface
-	interface Independent<S> extends Action<S> {
+	interface Independent<S extends @Nullable Object> extends Action<S> {
 		/**
 		 * Return an arbitrary for transformers that does not depend on the previous state.
 		 *
@@ -136,7 +138,7 @@ interface Independent<S> extends Action<S> {
 	 *     you have to implement either {@linkplain Action.Dependent} or {@linkplain Action.Independent}.
 	 * </p>
 	 */
-	abstract class JustTransform<S> implements Action.Independent<S> {
+	abstract class JustTransform<S extends @Nullable Object> implements Action.Independent<S> {
 
 		@Override
 		public Arbitrary<Transformer<S>> transformer() {
@@ -166,7 +168,7 @@ String description() {
 	 *     you have to implement either {@linkplain Action.Dependent} or {@linkplain Action.Independent}.
 	 * </p>
 	 */
-	abstract class JustMutate<S> implements Action.Independent<S> {
+	abstract class JustMutate<S extends @Nullable Object> implements Action.Independent<S> {
 
 		@Override
 		public Arbitrary<Transformer<S>> transformer() {
diff --git a/api/src/main/java/net/jqwik/api/state/ActionBuilder.java b/api/src/main/java/net/jqwik/api/state/ActionBuilder.java
index 0adf57740..ceb414597 100644
--- a/api/src/main/java/net/jqwik/api/state/ActionBuilder.java
+++ b/api/src/main/java/net/jqwik/api/state/ActionBuilder.java
@@ -27,24 +27,22 @@
  * @param <S> Type of the object to transform through an action
  */
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public class ActionBuilder<S> {
+public class ActionBuilder<S extends @Nullable Object> {
 
-	@Nullable
-	final private Predicate<S> precondition;
+	final private @Nullable Predicate<? super S> precondition;
 
-	@Nullable
-	final private String description;
+	final private @Nullable String description;
 
 	ActionBuilder() {
 		this(null, null);
 	}
 
-	private ActionBuilder(@Nullable Predicate<S> precondition, @Nullable String description) {
+	private ActionBuilder(@Nullable Predicate<? super S> precondition, @Nullable String description) {
 		this.precondition = precondition;
 		this.description = description;
 	}
 
-	public ActionBuilder<S> when(Predicate<S> precondition) {
+	public ActionBuilder<S> when(Predicate<? super S> precondition) {
 		return new ActionBuilder<>(precondition, description);
 	}
 
@@ -69,7 +67,6 @@ public boolean precondition(S state) {
 		};
 	}
 
-	@NonNull
 	private Arbitrary<Transformer<S>> justTransformer(Transformer<S> transformer) {
 		Transformer<S> withDescription = description == null ? transformer : Transformer.transform(description, transformer);
 		return Arbitraries.just(withDescription);
diff --git a/api/src/main/java/net/jqwik/api/state/ActionChain.java b/api/src/main/java/net/jqwik/api/state/ActionChain.java
index a39ac42b3..e874f31e6 100644
--- a/api/src/main/java/net/jqwik/api/state/ActionChain.java
+++ b/api/src/main/java/net/jqwik/api/state/ActionChain.java
@@ -23,7 +23,7 @@
  * @param <S> The type of the object going through state transformations
  */
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface ActionChain<S> {
+public interface ActionChain<S extends @Nullable Object> {
 
 	@API(status = INTERNAL)
 	abstract class ActionChainFacade {
@@ -33,7 +33,7 @@ abstract class ActionChainFacade {
 			implementation = FacadeLoader.load(ActionChainFacade.class);
 		}
 
-		public abstract <T> ActionChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier);
+		public abstract <T extends @Nullable Object> ActionChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier);
 	}
 
 	/**
@@ -43,7 +43,7 @@ abstract class ActionChainFacade {
 	 * @param <T>             The type of state to be transformed through the chain.
 	 * @return new arbitrary instance
 	 */
-	static <T> ActionChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
+	static <T extends @Nullable Object> ActionChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
 		return ActionChainFacade.implementation.startWith(initialSupplier);
 	}
 
@@ -136,6 +136,6 @@ default ActionChain<S> withInvariant(Consumer<S> invariant) {
 	 * @param peeker A consumer of a state object
 	 * @return the same chain instance
 	 */
-	ActionChain<S> peek(Consumer<S> peeker);
+	ActionChain<S> peek(Consumer<? super S> peeker);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/state/Chain.java b/api/src/main/java/net/jqwik/api/state/Chain.java
index 53b799a45..348393200 100644
--- a/api/src/main/java/net/jqwik/api/state/Chain.java
+++ b/api/src/main/java/net/jqwik/api/state/Chain.java
@@ -7,6 +7,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -28,7 +30,7 @@
  * @param <T> The type of state to be transformed in a chain
  */
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface Chain<T> extends Iterable<T> {
+public interface Chain<T extends @Nullable Object> extends Iterable<T> {
 
 	@API(status = INTERNAL)
 	abstract class ChainFacade {
@@ -38,7 +40,7 @@ abstract class ChainFacade {
 			implementation = FacadeLoader.load(ChainFacade.class);
 		}
 
-		public abstract <T> ChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier);
+		public abstract <T extends @Nullable Object> ChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier);
 	}
 
 	/**
@@ -48,7 +50,7 @@ abstract class ChainFacade {
 	 * @param <T>             The type of state to be transformed through the chain.
 	 * @return new arbitrary instance
 	 */
-	static <T> ChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
+	static <T extends @Nullable Object> ChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
 		return ChainFacade.implementation.startWith(initialSupplier);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java b/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java
index fff13bff8..12d24ca2f 100644
--- a/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java
@@ -61,6 +61,6 @@ default ChainArbitrary<T> infinite() {
 	 *
 	 * @return new instance of arbitrary
 	 */
-	ChainArbitrary<T> improveShrinkingWith(Supplier<ChangeDetector<T>> detectorSupplier);
+	ChainArbitrary<T> improveShrinkingWith(Supplier<? extends ChangeDetector<? super T>> detectorSupplier);
 
 }
diff --git a/api/src/main/java/net/jqwik/api/state/ChangeDetector.java b/api/src/main/java/net/jqwik/api/state/ChangeDetector.java
index 63f3599ee..b66c358d7 100644
--- a/api/src/main/java/net/jqwik/api/state/ChangeDetector.java
+++ b/api/src/main/java/net/jqwik/api/state/ChangeDetector.java
@@ -4,6 +4,7 @@
 import java.util.function.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
@@ -18,14 +19,14 @@
  * @see ActionChainArbitrary#improveShrinkingWith(Supplier)
  */
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface ChangeDetector<T> {
+public interface ChangeDetector<T extends @Nullable Object> {
 
 	/**
 	 * A change detector that can be used for immutable types that implement an equals() method
 	 * @param <T> the type of the stateful object
 	 * @return new instance of change detector
 	 */
-	static <T> ChangeDetector<T> forImmutables() {
+	static <T extends @Nullable Object> ChangeDetector<T> forImmutables() {
 		return new ChangeDetector<T>() {
 			private T before = null;
 
@@ -42,14 +43,14 @@ public boolean hasChanged(T after) {
 	}
 
 	@API(status = INTERNAL)
-	static <T> ChangeDetector<T> alwaysTrue() {
-		return new ChangeDetector<T>() {
+	static ChangeDetector<@Nullable Object> alwaysTrue() {
+		return new ChangeDetector<@Nullable Object>() {
 			@Override
-			public void before(T before) {
+			public void before(@Nullable Object before) {
 			}
 
 			@Override
-			public boolean hasChanged(T after) {
+			public boolean hasChanged(@Nullable Object after) {
 				return true;
 			}
 		};
diff --git a/api/src/main/java/net/jqwik/api/state/Transformation.java b/api/src/main/java/net/jqwik/api/state/Transformation.java
index ae5fe0294..d0efbd2db 100644
--- a/api/src/main/java/net/jqwik/api/state/Transformation.java
+++ b/api/src/main/java/net/jqwik/api/state/Transformation.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 /**
@@ -65,7 +67,7 @@ public Arbitrary<Transformer<T>> apply(Supplier<T> supplier) {
 	/**
 	 * Create a TransformerProvider with a precondition
 	 */
-	static <T> Builder<T> when(Predicate<T> precondition) {
+	static <T extends @Nullable Object> Builder<T> when(Predicate<T> precondition) {
 		return new Builder<T>(precondition);
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/state/Transformer.java b/api/src/main/java/net/jqwik/api/state/Transformer.java
index ba9488415..05f6de95e 100644
--- a/api/src/main/java/net/jqwik/api/state/Transformer.java
+++ b/api/src/main/java/net/jqwik/api/state/Transformer.java
@@ -24,14 +24,14 @@
  */
 @FunctionalInterface
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface Transformer<T> extends Function<@NonNull T, @NonNull T> {
+public interface Transformer<T extends @Nullable Object> extends Function<T, T> {
 
 	/**
 	 * The singleton object used for all calls to {@linkplain #endOfChain()}.
 	 */
 	Transformer<?> END_OF_CHAIN = new Transformer<Object>() {
 		@Override
-		public @NonNull Object apply(@NonNull Object t) {
+		public Object apply(Object t) {
 			return t;
 		}
 
@@ -56,7 +56,7 @@ public String toString() {
 	 */
 	Transformer<?> NOOP = new Transformer<Object>() {
 		@Override
-		public @NonNull Object apply(@NonNull Object t) {
+		public Object apply(Object t) {
 			return t;
 		}
 
@@ -88,7 +88,7 @@ public String toString() {
 	 * @return a transformer instance
 	 */
 	@SuppressWarnings("unchecked")
-	static <T> Transformer<T> endOfChain() {
+	static <T extends @Nullable Object> Transformer<T> endOfChain() {
 		return (Transformer<T>) END_OF_CHAIN;
 	}
 
@@ -117,7 +117,7 @@ static <T> Transformer<T> noop() {
 	 * @param <S>         The type of the state to transform
 	 * @return a new instance of a transformer
 	 */
-	static <S> Transformer<S> transform(String description, Function<S, S> transform) {
+	static <S extends @Nullable Object> Transformer<S> transform(String description, Function<? super S, S> transform) {
 		return new Transformer<S>() {
 			@Override
 			public S apply(S s) {
@@ -140,7 +140,7 @@ public String transformation() {
 	 * @param <S>         The type of the state to mutate
 	 * @return a new instance of a transformer
 	 */
-	static <S> Transformer<S> mutate(String description, Consumer<S> mutate) {
+	static <S extends @Nullable Object> Transformer<S> mutate(String description, Consumer<? super S> mutate) {
 		Function<S, S> transformer = s -> {
 			mutate.accept(s);
 			return s;
diff --git a/api/src/main/java/net/jqwik/api/state/package-info.java b/api/src/main/java/net/jqwik/api/state/package-info.java
index 01b997530..78b0d10a5 100644
--- a/api/src/main/java/net/jqwik/api/state/package-info.java
+++ b/api/src/main/java/net/jqwik/api/state/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.state;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/stateful/Action.java b/api/src/main/java/net/jqwik/api/stateful/Action.java
index 774ba42b2..3fc69e516 100644
--- a/api/src/main/java/net/jqwik/api/stateful/Action.java
+++ b/api/src/main/java/net/jqwik/api/stateful/Action.java
@@ -1,6 +1,7 @@
 package net.jqwik.api.stateful;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
@@ -13,7 +14,7 @@
  * @param <S> Type of the state object
  */
 @API(status = MAINTAINED, since = "1.0")
-public interface Action<S> {
+public interface Action<S extends @Nullable Object> {
 
 	/**
 	 * If this method returns false, the action will not be performed.
diff --git a/api/src/main/java/net/jqwik/api/stateful/ActionSequence.java b/api/src/main/java/net/jqwik/api/stateful/ActionSequence.java
index 7ad606a1e..ab67c5de5 100644
--- a/api/src/main/java/net/jqwik/api/stateful/ActionSequence.java
+++ b/api/src/main/java/net/jqwik/api/stateful/ActionSequence.java
@@ -14,7 +14,7 @@
  * @param <M> The type of the model
  */
 @API(status = MAINTAINED, since = "1.0")
-public interface ActionSequence<M> {
+public interface ActionSequence<M extends @Nullable Object> {
 
 	enum RunState {
 		NOT_RUN, RUNNING, FAILED, SUCCEEDED
diff --git a/api/src/main/java/net/jqwik/api/stateful/ActionSequenceArbitrary.java b/api/src/main/java/net/jqwik/api/stateful/ActionSequenceArbitrary.java
index ab6ba0379..021687c90 100644
--- a/api/src/main/java/net/jqwik/api/stateful/ActionSequenceArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/stateful/ActionSequenceArbitrary.java
@@ -4,10 +4,12 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = MAINTAINED, since = "1.0")
-public interface ActionSequenceArbitrary<M> extends Arbitrary<ActionSequence<M>> {
+public interface ActionSequenceArbitrary<M extends @Nullable Object> extends Arbitrary<ActionSequence<M>> {
 
 	/**
 	 * Set the intended number of steps of this sequence.
diff --git a/api/src/main/java/net/jqwik/api/stateful/Invariant.java b/api/src/main/java/net/jqwik/api/stateful/Invariant.java
index ea56e098a..6ae669657 100644
--- a/api/src/main/java/net/jqwik/api/stateful/Invariant.java
+++ b/api/src/main/java/net/jqwik/api/stateful/Invariant.java
@@ -1,12 +1,13 @@
 package net.jqwik.api.stateful;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
 @FunctionalInterface
 @API(status = MAINTAINED, since = "1.0")
-public interface Invariant<T> {
+public interface Invariant<T extends @Nullable Object> {
 
 	void check(T model);
 }
diff --git a/api/src/main/java/net/jqwik/api/stateful/package-info.java b/api/src/main/java/net/jqwik/api/stateful/package-info.java
index 2c36d5d7b..a187f3992 100644
--- a/api/src/main/java/net/jqwik/api/stateful/package-info.java
+++ b/api/src/main/java/net/jqwik/api/stateful/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.stateful;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java b/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
index 03856d196..975a1f9dc 100644
--- a/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
+++ b/api/src/main/java/net/jqwik/api/statistics/StatisticsCoverage.java
@@ -22,7 +22,7 @@ interface CoverageChecker {
 		 *
 		 * @param countChecker a predicate to accept a selected value set's number of occurrences
 		 */
-		void count(Predicate<Integer> countChecker);
+		void count(Predicate<? super Integer> countChecker);
 
 		/**
 		 * Check the number of occurrences using one or more assertions.
@@ -31,14 +31,14 @@ interface CoverageChecker {
 		 *                     and the count of all submitted value sets to compare with
 		 *                     or make a calculation
 		 */
-		void count(BiPredicate<Integer, Integer> countChecker);
+		void count(BiPredicate<? super Integer, ? super Integer> countChecker);
 
 		/**
 		 * Check the number of occurrences returning true (ok) or false (fail).
 		 *
 		 * @param countChecker a predicate to accept a selected value set's number of occurrences
 		 */
-		void count(Consumer<Integer> countChecker);
+		void count(Consumer<? super Integer> countChecker);
 
 		/**
 		 * Check the number of occurrences using one or more assertions.
@@ -47,7 +47,7 @@ interface CoverageChecker {
 		 *                     and the count of all submitted value sets to compare with
 		 *                     or make a calculation
 		 */
-		void count(BiConsumer<Integer, Integer> countChecker);
+		void count(BiConsumer<? super Integer, ? super Integer> countChecker);
 
 		/**
 		 * Check the percentage of occurrences returning true (ok) or false (fail)
@@ -55,7 +55,7 @@ interface CoverageChecker {
 		 * @param percentageChecker a predicate to accept a selected value set's
 		 *                          percentage (0.0 - 100.0) of occurrences
 		 */
-		void percentage(Predicate<Double> percentageChecker);
+		void percentage(Predicate<? super Double> percentageChecker);
 
 		/**
 		 * Check the number of occurrences returning true (ok) or false (fail).
@@ -63,7 +63,7 @@ interface CoverageChecker {
 		 * @param percentageChecker a predicate to accept a selected value set's
 		 *                          percentage (0.0 - 100.0) of occurrences
 		 */
-		void percentage(Consumer<Double> percentageChecker);
+		void percentage(Consumer<? super Double> percentageChecker);
 
 	}
 
diff --git a/api/src/main/java/net/jqwik/api/statistics/package-info.java b/api/src/main/java/net/jqwik/api/statistics/package-info.java
index af38415d8..2a52f10c1 100644
--- a/api/src/main/java/net/jqwik/api/statistics/package-info.java
+++ b/api/src/main/java/net/jqwik/api/statistics/package-info.java
@@ -1,4 +1,4 @@
 @NullMarked
 package net.jqwik.api.statistics;
 
-import org.jspecify.annotations.*;
\ No newline at end of file
+import org.jspecify.annotations.*;
diff --git a/api/src/main/java/net/jqwik/api/support/CollectorsSupport.java b/api/src/main/java/net/jqwik/api/support/CollectorsSupport.java
index aad1eb313..acdc301a4 100644
--- a/api/src/main/java/net/jqwik/api/support/CollectorsSupport.java
+++ b/api/src/main/java/net/jqwik/api/support/CollectorsSupport.java
@@ -4,6 +4,7 @@
 import java.util.stream.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 
 import static org.apiguardian.api.API.Status.*;
 
@@ -16,7 +17,7 @@ public class CollectorsSupport {
 
 	private CollectorsSupport() {}
 
-	public static <T> Collector<T, ?, Set<T>> toLinkedHashSet() {
+	public static <T extends @Nullable Object> Collector<T, ?, Set<T>> toLinkedHashSet() {
 		return Collectors.toCollection(LinkedHashSet::new);
 	}
 }
diff --git a/api/src/main/java/net/jqwik/api/support/HashCodeSupport.java b/api/src/main/java/net/jqwik/api/support/HashCodeSupport.java
index 41ec8bf90..120a2c386 100644
--- a/api/src/main/java/net/jqwik/api/support/HashCodeSupport.java
+++ b/api/src/main/java/net/jqwik/api/support/HashCodeSupport.java
@@ -19,7 +19,7 @@ public static int hash(@Nullable Object o) {
 		return baseHash(o) + 31;
 	}
 
-	private static int baseHash(Object o) {
+	private static int baseHash(@Nullable Object o) {
 		if (o == null)
 			return 0;
 		return o.hashCode();
diff --git a/documentation/src/docs/include/customized-parameter-generation.md b/documentation/src/docs/include/customized-parameter-generation.md
index 264bf2a8c..b309f76f9 100644
--- a/documentation/src/docs/include/customized-parameter-generation.md
+++ b/documentation/src/docs/include/customized-parameter-generation.md
@@ -801,7 +801,7 @@ for the two String values - using `tuple2()` does that.
 
 If you want to include only part of all the values generated by an arbitrary,
 use
-[`Arbitrary.filter(Predicate<T> filterPredicate)`](/docs/${docsVersion}/javadoc/net/jqwik/api/Arbitrary.html#filter(java.util.function.Predicate)).
+[`Arbitrary.filter(Predicate<? super T> filterPredicate)`](/docs/${docsVersion}/javadoc/net/jqwik/api/Arbitrary.html#filter(java.util.function.Predicate)).
 The following arbitrary will filter out all
 even numbers from the stream of generated integers:
 
diff --git a/documentation/src/test/java/net/jqwik/docs/arbitraryconfigurator/OddConfigurator.java b/documentation/src/test/java/net/jqwik/docs/arbitraryconfigurator/OddConfigurator.java
index 859f849fc..d4ac20b9f 100644
--- a/documentation/src/test/java/net/jqwik/docs/arbitraryconfigurator/OddConfigurator.java
+++ b/documentation/src/test/java/net/jqwik/docs/arbitraryconfigurator/OddConfigurator.java
@@ -6,6 +6,8 @@
 import net.jqwik.api.configurators.*;
 import net.jqwik.api.providers.*;
 
+import org.jspecify.annotations.*;
+
 public class OddConfigurator extends ArbitraryConfiguratorBase {
 
 	public Arbitrary<Integer> configureInteger(Arbitrary<Integer> arbitrary, Odd odd) {
@@ -22,7 +24,7 @@ public Arbitrary<BigInteger> configureBigInteger(Arbitrary<BigInteger> arbitrary
 class PlainOddConfigurator implements ArbitraryConfigurator {
 
 	@Override
-	public <T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
+	public <T extends @Nullable Object> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
 		if (!targetType.isOfType(Integer.class) && !targetType.isOfType(int.class)) {
 			return arbitrary;
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/descriptor/package-info.java b/engine/src/main/java/net/jqwik/engine/descriptor/package-info.java
new file mode 100644
index 000000000..947646bd9
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/descriptor/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.descriptor;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/discovery/package-info.java b/engine/src/main/java/net/jqwik/engine/discovery/package-info.java
new file mode 100644
index 000000000..4743bc28a
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/discovery/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.discovery;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/discovery/predicates/package-info.java b/engine/src/main/java/net/jqwik/engine/discovery/predicates/package-info.java
new file mode 100644
index 000000000..b8a34a95b
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/discovery/predicates/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.discovery.predicates;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/discovery/specs/package-info.java b/engine/src/main/java/net/jqwik/engine/discovery/specs/package-info.java
new file mode 100644
index 000000000..3814bf3fd
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/discovery/specs/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.discovery.specs;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/execution/ContainerInstancesCreator.java b/engine/src/main/java/net/jqwik/engine/execution/ContainerInstancesCreator.java
index 15f429a32..d9265efce 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/ContainerInstancesCreator.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/ContainerInstancesCreator.java
@@ -3,6 +3,7 @@
 import java.lang.reflect.*;
 import java.util.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.engine.*;
 
 import net.jqwik.api.*;
@@ -84,7 +85,7 @@ private ContainerInstances newInstancesOfBaseContainer(Constructor<?> constructo
 		return new ContainerInstances(createNewInstance(constructor, null));
 	}
 
-	private Object createNewInstance(Constructor<?> constructor, Object outerInstance) {
+	private Object createNewInstance(Constructor<?> constructor, @Nullable Object outerInstance) {
 		return JqwikReflectionSupport.newInstance(
 			constructor,
 			resolveConstructorParameters(constructor, outerInstance)
@@ -111,7 +112,7 @@ private List<Constructor<?>> allAccessibleConstructors(Class<?> instanceClass) {
 
 	private Object[] resolveConstructorParameters(
 		Constructor<?> constructor,
-		Object outerInstance
+		@Nullable Object outerInstance
 	) {
 		Object[] args = new Object[constructor.getParameterCount()];
 		for (int i = 0; i < args.length; i++) {
diff --git a/engine/src/main/java/net/jqwik/engine/execution/GenerationInfo.java b/engine/src/main/java/net/jqwik/engine/execution/GenerationInfo.java
index c93cb5ff2..6bee9a491 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/GenerationInfo.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/GenerationInfo.java
@@ -8,26 +8,28 @@
 import net.jqwik.api.lifecycle.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 public class GenerationInfo implements Serializable {
 
 	public final static GenerationInfo NULL = new GenerationInfo(null);
 
-	private final String randomSeed;
+	private final @Nullable String randomSeed;
 	private final int generationIndex;
 
 	// Store ordinals instead of enum objects so that serialization
 	// in jqwik.database uses less disk space
 	private final List<List<Byte>> byteSequences;
 
-	public GenerationInfo(String randomSeed) {
+	public GenerationInfo(@Nullable String randomSeed) {
 		this(randomSeed, 0);
 	}
 
-	public GenerationInfo(String randomSeed, int generationIndex) {
+	public GenerationInfo(@Nullable String randomSeed, int generationIndex) {
 		this(randomSeed, generationIndex, Collections.emptyList());
 	}
 
-	private GenerationInfo(String randomSeed, int generationIndex, List<List<Byte>> byteSequences) {
+	private GenerationInfo(@Nullable String randomSeed, int generationIndex, List<List<Byte>> byteSequences) {
 		this.randomSeed = randomSeed != null ? (randomSeed.isEmpty() ? null : randomSeed) : null;
 		this.generationIndex = generationIndex;
 		this.byteSequences = byteSequences;
@@ -59,7 +61,7 @@ public Optional<List<Shrinkable<Object>>> generateOn(ParametersGenerator generat
 		return useShrinkingSequences(sample);
 	}
 
-	private Optional<List<Shrinkable<Object>>> useShrinkingSequences(List<Shrinkable<Object>> sample) {
+	private Optional<List<Shrinkable<Object>>> useShrinkingSequences(@Nullable List<Shrinkable<Object>> sample) {
 		Optional<List<Shrinkable<Object>>> shrunkSample = Optional.ofNullable(sample);
 		for (List<TryExecutionResult.Status> shrinkingSequence : shrinkingSequences()) {
 			if (!shrunkSample.isPresent()) {
@@ -78,7 +80,7 @@ private Optional<List<Shrinkable<Object>>> shrink(
 		return recreator.recreateFrom(shrinkingSequence);
 	}
 
-	private List<Shrinkable<Object>> useGenerationIndex(ParametersGenerator generator, TryLifecycleContext context) {
+	private @Nullable List<Shrinkable<Object>> useGenerationIndex(ParametersGenerator generator, TryLifecycleContext context) {
 		List<Shrinkable<Object>> sample = null;
 		for (int i = 0; i < generationIndex; i++) {
 			if (generator.hasNext()) {
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentDomainContext.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentDomainContext.java
index ac0b7b2c7..c54d70c75 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentDomainContext.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentDomainContext.java
@@ -9,6 +9,8 @@
 import net.jqwik.engine.properties.configurators.*;
 import net.jqwik.engine.providers.*;
 
+import org.jspecify.annotations.*;
+
 public class CurrentDomainContext {
 
 	public static final DomainContext GLOBAL_DOMAIN_CONTEXT = new GlobalDomainContext();
@@ -22,7 +24,7 @@ public static DomainContext get() {
 		return currentContext.get();
 	}
 
-	public static <T> T runWithContext(DomainContext context, Supplier<T> runnable) {
+	public static <T extends @Nullable Object> T runWithContext(DomainContext context, Supplier<? extends T> runnable) {
 		currentContext.set(context);
 		try {
 			return runnable.get();
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentTestDescriptor.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentTestDescriptor.java
index 608433379..6a80957b2 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentTestDescriptor.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/CurrentTestDescriptor.java
@@ -3,6 +3,7 @@
 import java.util.*;
 import java.util.function.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.engine.*;
 
 public class CurrentTestDescriptor {
@@ -11,7 +12,7 @@ public class CurrentTestDescriptor {
 	// e.g. in JqwikIntegrationTests
 	private static final ThreadLocal<List<TestDescriptor>> descriptors = ThreadLocal.withInitial(ArrayList::new);
 
-	public static <T> T runWithDescriptor(TestDescriptor currentDescriptor, Supplier<T> executable) {
+	public static <T extends @Nullable Object> T runWithDescriptor(TestDescriptor currentDescriptor, Supplier<? extends T> executable) {
 		push(currentDescriptor);
 		try {
 			return executable.get();
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/PlainExecutionResult.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/PlainExecutionResult.java
index 96881c5d3..087dc7ad7 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/PlainExecutionResult.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/PlainExecutionResult.java
@@ -6,13 +6,15 @@
 import net.jqwik.api.lifecycle.*;
 import net.jqwik.engine.execution.*;
 
+import org.jspecify.annotations.*;
+
 public class PlainExecutionResult implements ExtendedPropertyExecutionResult {
 
 	public static ExtendedPropertyExecutionResult successful() {
 		return new PlainExecutionResult(Status.SUCCESSFUL, new GenerationInfo(null), null);
 	}
 
-	public static ExtendedPropertyExecutionResult failed(Throwable throwable, String seed) {
+	public static ExtendedPropertyExecutionResult failed(Throwable throwable, @Nullable String seed) {
 		if (throwable == null) {
 			throw new IllegalArgumentException("throwable must never be null for failed PropertyExecutionResult");
 		}
@@ -32,9 +34,9 @@ public static ExtendedPropertyExecutionResult aborted(Throwable throwable, Gener
 
 	private final Status status;
 	private final GenerationInfo generationInfo;
-	private final Throwable throwable;
+	private final @Nullable Throwable throwable;
 
-	private PlainExecutionResult(Status status, GenerationInfo generationInfo, Throwable throwable) {
+	private PlainExecutionResult(Status status, GenerationInfo generationInfo, @Nullable Throwable throwable) {
 		this.status = status;
 		this.generationInfo = generationInfo;
 		this.throwable = throwable;
@@ -56,7 +58,7 @@ public Optional<Throwable> throwable() {
 	}
 
 	@Override
-	public PropertyExecutionResult mapTo(Status newStatus, Throwable throwable) {
+	public PropertyExecutionResult mapTo(Status newStatus, @Nullable Throwable throwable) {
 		return new PlainExecutionResult(newStatus, generationInfo, throwable);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java
index a37a3c58b..aeeb5e9a3 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java
@@ -17,7 +17,7 @@ public class ScopedStore<T> implements Store<T> {
 	private final Object identifier;
 	private final Lifespan lifespan;
 	private final TestDescriptor scope;
-	private final Supplier<T> initialValueSupplier;
+	private final Supplier<? extends T> initialValueSupplier;
 
 	private T value;
 	private boolean initialized = false;
@@ -26,7 +26,7 @@ public ScopedStore(
 		Object identifier,
 		Lifespan lifespan,
 		TestDescriptor scope,
-		Supplier<T> initialValueSupplier
+		Supplier<? extends T> initialValueSupplier
 	) {
 		this.identifier = identifier;
 		this.lifespan = lifespan;
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/StoreRepository.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/StoreRepository.java
index ecafad478..bf0938db6 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/StoreRepository.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/StoreRepository.java
@@ -4,6 +4,7 @@
 import java.util.function.*;
 import java.util.stream.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.engine.*;
 
 import net.jqwik.api.*;
@@ -29,11 +30,11 @@ private static class IdentifiedStores extends LinkedHashMap<TestDescriptor, Scop
 
 	private final Map<Object, IdentifiedStores> storesByIdentifier = new LinkedHashMap<>();
 
-	public <T> ScopedStore<T> create(
+	public <T extends @Nullable Object> ScopedStore<T> create(
 		TestDescriptor scope,
 		Object identifier,
 		Lifespan lifespan,
-		Supplier<T> initialValueSupplier
+		Supplier<? extends T> initialValueSupplier
 	) {
 		if (scope == null) {
 			throw new IllegalArgumentException("scope must not be null");
@@ -52,7 +53,7 @@ public <T> ScopedStore<T> create(
 		return store;
 	}
 
-	private <T> void addStore(Object identifier, ScopedStore<T> newStore) {
+	private <T extends @Nullable Object> void addStore(Object identifier, ScopedStore<T> newStore) {
 		IdentifiedStores identifiedStores = storesByIdentifier.computeIfAbsent(newStore.getIdentifier(), ignore -> new IdentifiedStores());
 
 		Optional<ScopedStore<?>> conflictingStore =
@@ -76,11 +77,11 @@ private <T> void addStore(Object identifier, ScopedStore<T> newStore) {
 		storesByIdentifier.put(identifier, identifiedStores);
 	}
 
-	private <T> boolean isVisibleInAncestorOrDescendant(ScopedStore<T> newStore, ScopedStore<?> store) {
+	private <T extends @Nullable Object> boolean isVisibleInAncestorOrDescendant(ScopedStore<T> newStore, ScopedStore<?> store) {
 		return store.isVisibleFor(newStore.getScope()) || newStore.isVisibleFor(store.getScope());
 	}
 
-	public <T> Optional<ScopedStore<T>> get(TestDescriptor retriever, Object identifier) {
+	public <T extends @Nullable Object> Optional<ScopedStore<T>> get(TestDescriptor retriever, Object identifier) {
 		if (identifier == null) {
 			throw new IllegalArgumentException("identifier must not be null");
 		}
@@ -94,7 +95,7 @@ public <T> Optional<ScopedStore<T>> get(TestDescriptor retriever, Object identif
 	}
 
 	@SuppressWarnings("unchecked")
-	private <T> Optional<ScopedStore<T>> getFirstVisibleStore(TestDescriptor retriever, IdentifiedStores identifiedStores) {
+	private <T extends @Nullable Object> Optional<ScopedStore<T>> getFirstVisibleStore(TestDescriptor retriever, IdentifiedStores identifiedStores) {
 		return identifiedStores.values()
 							   .stream()
 							   .filter(store -> store.isVisibleFor(retriever))
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/package-info.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/package-info.java
new file mode 100644
index 000000000..5a16582fd
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.execution.lifecycle;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/execution/package-info.java b/engine/src/main/java/net/jqwik/engine/execution/package-info.java
new file mode 100644
index 000000000..f1ba68d50
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/execution/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.execution;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/execution/pipeline/package-info.java b/engine/src/main/java/net/jqwik/engine/execution/pipeline/package-info.java
new file mode 100644
index 000000000..af3be6ad5
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/execution/pipeline/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.execution.pipeline;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/execution/reporting/ParameterChangesDetector.java b/engine/src/main/java/net/jqwik/engine/execution/reporting/ParameterChangesDetector.java
index e30b8797d..7eda43963 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/reporting/ParameterChangesDetector.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/reporting/ParameterChangesDetector.java
@@ -5,13 +5,15 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 class ParameterChangesDetector {
 
-	static boolean haveParametersChanged(List<Object> before, List<Object> after) {
+	static boolean haveParametersChanged(List<? extends @Nullable Object> before, List<? extends @Nullable Object> after) {
 		return atLeastOneParameterHasChanged(before, after);
 	}
 
-	private static boolean atLeastOneParameterHasChanged(List<Object> before, List<Object> after) {
+	private static boolean atLeastOneParameterHasChanged(List<? extends @Nullable Object> before, List<? extends @Nullable Object> after) {
 		if (before.size() != after.size()) {
 			return true;
 		}
@@ -25,11 +27,11 @@ private static boolean atLeastOneParameterHasChanged(List<Object> before, List<O
 		return false;
 	}
 
-	private static boolean valuesDiffer(Object before, Object after) {
-		if (Objects.isNull(before) != Objects.isNull(after)) {
+	private static boolean valuesDiffer(@Nullable Object before, @Nullable Object after) {
+		if ((before == null) != (after == null)) {
 			return true;
 		}
-		if (Objects.isNull(before)) {
+		if (before == null) {
 			return false;
 		}
 		if (before.getClass() != after.getClass()) {
diff --git a/engine/src/main/java/net/jqwik/engine/execution/reporting/package-info.java b/engine/src/main/java/net/jqwik/engine/execution/reporting/package-info.java
new file mode 100644
index 000000000..22ea0c57f
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/execution/reporting/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.execution.reporting;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ActionChainFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ActionChainFacadeImpl.java
index 888abeb91..d2c180c7d 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ActionChainFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ActionChainFacadeImpl.java
@@ -5,12 +5,14 @@
 import net.jqwik.api.state.*;
 import net.jqwik.engine.properties.state.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
 public class ActionChainFacadeImpl extends ActionChain.ActionChainFacade {
 	@Override
-	public <T> ActionChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
+	public <T extends @Nullable Object> ActionChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
 		return new DefaultActionChainArbitrary<>(initialSupplier);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
index f4a318285..4083b2ab2 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ArbitrariesFacadeImpl.java
@@ -41,7 +41,7 @@ public <M> ActionSequenceArbitrary<M> sequences(Arbitrary<? extends Action<M>> a
 	}
 
 	@Override
-	public <T> Arbitrary<T> frequencyOf(List<? extends Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
+	public <T extends @Nullable Object> Arbitrary<T> frequencyOf(List<? extends Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> frequencies) {
 		List<Tuple.Tuple2<Integer, ? extends Arbitrary<T>>> aboveZeroFrequencies =
 			frequencies.stream().filter(f -> f.get1() > 0).collect(Collectors.toList());
 
@@ -107,12 +107,12 @@ public CharacterArbitrary chars() {
 	}
 
 	@Override
-	public <T> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
+	public <T extends @Nullable Object> Arbitrary<T> lazy(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		return new LazyArbitrary<>(arbitrarySupplier);
 	}
 
 	@Override
-	public <T> Arbitrary<T> lazyOf(List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
+	public <T extends @Nullable Object> Arbitrary<T> lazyOf(List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		int hashIdentifier = calculateIdentifier(suppliers.size());
 		return LazyOfArbitrary.of(hashIdentifier, suppliers);
 	}
@@ -128,28 +128,28 @@ public Arbitrary<Character> of(char[] chars) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> of(Collection<T> values) {
+	public <T extends @Nullable Object> Arbitrary<T> of(Collection<? extends T> values) {
 		List<T> valueList = values instanceof List ? (List<T>) values : new ArrayList<>(values);
 		return new ChooseValueArbitrary<>(valueList);
 	}
 
 	@Override
-	public <T> Arbitrary<T> create(Supplier<T> supplier) {
+	public <T extends @Nullable Object> Arbitrary<T> create(Supplier<T> supplier) {
 		return new CreateArbitrary<>(supplier);
 	}
 
 	@Override
-	public <T> Arbitrary<List<T>> shuffle(List<T> values) {
+	public <T extends @Nullable Object> Arbitrary<List<T>> shuffle(List<T> values) {
 		return new ShuffleArbitrary<>(values);
 	}
 
 	@Override
-	public <T> Arbitrary<T> fromGenerator(IntFunction<? extends RandomGenerator<T>> generatorSupplier) {
+	public <T extends @Nullable Object> Arbitrary<T> fromGenerator(IntFunction<? extends RandomGenerator<T>> generatorSupplier) {
 		return new FromGeneratorWithSizeArbitrary<>(generatorSupplier);
 	}
 
 	@Override
-	public <T> Arbitrary<T> frequency(List<? extends Tuple.Tuple2<Integer, T>> frequencies) {
+	public <T extends @Nullable Object> Arbitrary<T> frequency(List<? extends Tuple.Tuple2<Integer, T>> frequencies) {
 		List<Tuple.Tuple2<Integer, T>> frequenciesAbove0 = frequencies.stream()
 																	  .filter(f -> f.get1() > 0)
 																	  .collect(Collectors.toList());
@@ -199,10 +199,10 @@ public <T> Arbitrary<T> defaultFor(Class<T> type, Class<?>[] typeParameters) {
 
 	@SuppressWarnings("unchecked")
 	@Override
-	public <T> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<TypeUsage, Arbitrary<Object>> noDefaultResolver) {
+	public <T extends @Nullable Object> Arbitrary<T> defaultFor(TypeUsage typeUsage, Function<? super TypeUsage, ? extends Arbitrary<T>> noDefaultResolver) {
 		Set<Arbitrary<?>> arbitraries = allDefaultsFor(typeUsage, noDefaultResolver);
 		if (arbitraries.isEmpty()) {
-			return (Arbitrary<T>) noDefaultResolver.apply(typeUsage);
+			return noDefaultResolver.apply(typeUsage);
 		}
 
 		List<Arbitrary<? extends T>> arbitrariesList = new ArrayList<>();
@@ -216,18 +216,18 @@ public <T> TypeArbitrary<T> forType(Class<T> targetType) {
 	}
 
 	@Override
-	public <K, V> MapArbitrary<K, V> maps(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary) {
+	public <K extends @Nullable Object, V extends @Nullable Object> MapArbitrary<K, V> maps(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary) {
 		// The map cannot be larger than the max number of possible keys
 		return new DefaultMapArbitrary<>(keysArbitrary, valuesArbitrary)
 				   .ofMaxSize(maxNumberOfElements(keysArbitrary, RandomGenerators.DEFAULT_COLLECTION_SIZE));
 	}
 
 	@Override
-	public <K, V> Arbitrary<Map.Entry<K, V>> entries(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary) {
+	public <K extends @Nullable Object, V extends @Nullable Object> Arbitrary<Map.Entry<K, V>> entries(Arbitrary<K> keysArbitrary, Arbitrary<V> valuesArbitrary) {
 		return Combinators.combine(keysArbitrary, valuesArbitrary).as(AbstractMap.SimpleEntry::new);
 	}
 
-	private static Set<Arbitrary<?>> allDefaultsFor(TypeUsage typeUsage, Function<TypeUsage, Arbitrary<Object>> noDefaultResolver) {
+	private static Set<Arbitrary<?>> allDefaultsFor(TypeUsage typeUsage, Function<? super TypeUsage, ? extends Arbitrary<?>> noDefaultResolver) {
 		DomainContext domainContext = CurrentDomainContext.get();
 		Set<Arbitrary<?>> unconfiguredArbitraries = createDefaultArbitraries(typeUsage, noDefaultResolver, domainContext);
 		return configureDefaultArbitraries(typeUsage, domainContext, unconfiguredArbitraries);
@@ -246,7 +246,7 @@ private static Set<Arbitrary<?>> configureDefaultArbitraries(
 
 	private static Set<Arbitrary<?>> createDefaultArbitraries(
 		TypeUsage typeUsage,
-		Function<TypeUsage, Arbitrary<Object>> noDefaultResolver,
+		Function<? super TypeUsage, ? extends Arbitrary<?>> noDefaultResolver,
 		DomainContext domainContext
 	) {
 		RegisteredArbitraryResolver defaultArbitraryResolver = new RegisteredArbitraryResolver(domainContext.getArbitraryProviders());
@@ -286,7 +286,7 @@ private static Set<Arbitrary<?>> createDefaultArbitraries(
 		}
 	}
 
-	private <T> Arbitrary<T> recursive(
+	private <T extends @Nullable Object> Arbitrary<T> recursive(
 		Supplier<? extends Arbitrary<T>> base,
 		Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur,
 		int depth
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ArbitraryFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ArbitraryFacadeImpl.java
index 5bf580f7d..5e563b00f 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ArbitraryFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ArbitraryFacadeImpl.java
@@ -10,6 +10,8 @@
 import net.jqwik.engine.properties.arbitraries.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.properties.arbitraries.ArbitrariesSupport.*;
 
 /**
@@ -18,54 +20,54 @@
 public class ArbitraryFacadeImpl extends Arbitrary.ArbitraryFacade {
 
 	@Override
-	public <T> ListArbitrary<T> list(Arbitrary<T> elementArbitrary) {
+	public <T extends @Nullable Object> ListArbitrary<T> list(Arbitrary<T> elementArbitrary) {
 		return new DefaultListArbitrary<>(elementArbitrary);
 	}
 
 	@Override
-	public <T> SetArbitrary<T> set(Arbitrary<T> elementArbitrary) {
+	public <T extends @Nullable Object> SetArbitrary<T> set(Arbitrary<T> elementArbitrary) {
 		// The set can never be larger than the max number of possible elements
 		return new DefaultSetArbitrary<>(elementArbitrary)
 				   .ofMaxSize(maxNumberOfElements(elementArbitrary, RandomGenerators.DEFAULT_COLLECTION_SIZE));
 	}
 
 	@Override
-	public <T> StreamArbitrary<T> stream(Arbitrary<T> elementArbitrary) {
+	public <T extends @Nullable Object> StreamArbitrary<T> stream(Arbitrary<T> elementArbitrary) {
 		return new DefaultStreamArbitrary<>(elementArbitrary);
 	}
 
 	@Override
-	public <T> IteratorArbitrary<T> iterator(Arbitrary<T> elementArbitrary) {
+	public <T extends @Nullable Object> IteratorArbitrary<T> iterator(Arbitrary<T> elementArbitrary) {
 		return new DefaultIteratorArbitrary<>(elementArbitrary);
 	}
 
 	@Override
-	public <T, A> ArrayArbitrary<T, A> array(Arbitrary<T> elementArbitrary, Class<A> arrayClass) {
+	public <T extends @Nullable Object, A> ArrayArbitrary<T, A> array(Arbitrary<T> elementArbitrary, Class<A> arrayClass) {
 		return DefaultArrayArbitrary.forArrayType(elementArbitrary, arrayClass);
 	}
 
 	@Override
-	public <T> Arbitrary<T> filter(Arbitrary<T> self, Predicate<T> filterPredicate, int maxMisses) {
+	public <T extends @Nullable Object> Arbitrary<T> filter(Arbitrary<T> self, Predicate<? super T> filterPredicate, int maxMisses) {
 		return new ArbitraryFilter<>(self, filterPredicate, maxMisses);
 	}
 
 	@Override
-	public <T, U> Arbitrary<U> map(Arbitrary<T> self, Function<T, U> mapper) {
+	public <T extends @Nullable Object, U extends @Nullable Object> Arbitrary<U> map(Arbitrary<T> self, Function<? super T, ? extends U> mapper) {
 		return new ArbitraryMap<>(self, mapper);
 	}
 
 	@Override
-	public <T, U> Arbitrary<U> flatMap(Arbitrary<T> self, Function<T, Arbitrary<U>> mapper) {
+	public <T extends @Nullable Object, U extends @Nullable Object> Arbitrary<U> flatMap(Arbitrary<T> self, Function<? super T, ? extends Arbitrary<U>> mapper) {
 		return new ArbitraryFlatMap<>(self, mapper);
 	}
 
 	@Override
-	public <T> Stream<T> sampleStream(Arbitrary<T> arbitrary) {
+	public <T extends @Nullable Object> Stream<T> sampleStream(Arbitrary<T> arbitrary) {
 		return new SampleStreamFacade().sampleStream(arbitrary);
 	}
 
 	@Override
-	public <T> Arbitrary<T> injectNull(Arbitrary<T> self, double nullProbability) {
+	public <T extends @Nullable Object> Arbitrary<@Nullable T> injectNull(Arbitrary<T> self, double nullProbability) {
 		int frequencyNull = (int) Math.round(nullProbability * 1000);
 		int frequencyNotNull = 1000 - frequencyNull;
 		if (frequencyNull <= 0) {
@@ -81,7 +83,7 @@ public <T> Arbitrary<T> injectNull(Arbitrary<T> self, double nullProbability) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> ignoreExceptions(Arbitrary<T> self, int maxThrows, Class<? extends Throwable>[] exceptionTypes) {
+	public <T extends @Nullable Object> Arbitrary<T> ignoreExceptions(Arbitrary<T> self, int maxThrows, Class<? extends Throwable>[] exceptionTypes) {
 		if (exceptionTypes.length == 0) {
 			return self;
 		}
@@ -110,7 +112,7 @@ public EdgeCases<T> edgeCases(int maxEdgeCases) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> dontShrink(Arbitrary<T> self) {
+	public <T extends @Nullable Object> Arbitrary<T> dontShrink(Arbitrary<T> self) {
 		return new ArbitraryDelegator<T>(self) {
 			@Override
 			public RandomGenerator<T> generator(int genSize) {
@@ -130,7 +132,7 @@ public EdgeCases<T> edgeCases(int maxEdgeCases) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> configureEdgeCases(Arbitrary<T> self, Consumer<EdgeCases.Config<T>> configurator) {
+	public <T extends @Nullable Object> Arbitrary<T> configureEdgeCases(Arbitrary<T> self, Consumer<? super EdgeCases.Config<T>> configurator) {
 		return new ArbitraryDelegator<T>(self) {
 			@Override
 			public EdgeCases<T> edgeCases(int maxEdgeCases) {
@@ -141,7 +143,7 @@ public EdgeCases<T> edgeCases(int maxEdgeCases) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> withoutEdgeCases(Arbitrary<T> self) {
+	public <T extends @Nullable Object> Arbitrary<T> withoutEdgeCases(Arbitrary<T> self) {
 		return new ArbitraryDelegator<T>(self) {
 			@Override
 			public RandomGenerator<T> generator(int genSize, boolean withEdgeCases) {
@@ -161,7 +163,7 @@ public RandomGenerator<T> generatorWithEmbeddedEdgeCases(int genSize) {
 	}
 
 	@Override
-	public <T> Arbitrary<T> fixGenSize(Arbitrary<T> self, int genSize) {
+	public <T extends @Nullable Object> Arbitrary<T> fixGenSize(Arbitrary<T> self, int genSize) {
 		return new ArbitraryDelegator<T>(self) {
 			@Override
 			public RandomGenerator<T> generator(int ignoredGenSize) {
@@ -176,16 +178,16 @@ public RandomGenerator<T> generatorWithEmbeddedEdgeCases(int ignoredGenSize) {
 	}
 
 	@Override
-	public <T> Arbitrary<List<T>> collect(Arbitrary<T> self, Predicate<List<T>> until) {
+	public <T extends @Nullable Object> Arbitrary<List<T>> collect(Arbitrary<T> self, Predicate<? super List<? extends T>> until) {
 		return new ArbitraryCollect<>(self, until);
 	}
 
 	@Override
-	public <T> RandomGenerator<T> memoizedGenerator(Arbitrary<T> self, int genSize, boolean withEdgeCases) {
+	public <T extends @Nullable Object> RandomGenerator<T> memoizedGenerator(Arbitrary<T> self, int genSize, boolean withEdgeCases) {
 		return Memoize.memoizedGenerator(self, genSize, withEdgeCases, () -> generator(self, genSize, withEdgeCases));
 	}
 
-	private <U> RandomGenerator<U> generator(Arbitrary<U> arbitrary, int genSize, boolean withEdgeCases) {
+	private <U extends @Nullable Object> RandomGenerator<U> generator(Arbitrary<U> arbitrary, int genSize, boolean withEdgeCases) {
 		if (withEdgeCases) {
 			int maxEdgeCases = Math.max(genSize, 10);
 			return arbitrary.generatorWithEmbeddedEdgeCases(genSize).withEdgeCases(genSize, arbitrary.edgeCases(maxEdgeCases));
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ChainFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ChainFacadeImpl.java
index e9d5ccbae..28e4d72f0 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ChainFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ChainFacadeImpl.java
@@ -7,13 +7,15 @@
 import net.jqwik.api.state.*;
 import net.jqwik.engine.properties.state.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
 public class ChainFacadeImpl extends Chain.ChainFacade {
 
 	@Override
-	public <T> ChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
+	public <T extends @Nullable Object> ChainArbitrary<T> startWith(Supplier<? extends T> initialSupplier) {
 		return new DefaultChainArbitrary<>(initialSupplier);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
index 27b8dfaa0..54acea4dc 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/CombinatorsFacadeImpl.java
@@ -5,6 +5,8 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.arbitraries.combinations.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
@@ -81,7 +83,7 @@ public <T1, T2, T3, T4, T5, T6, T7, T8> Combinators.Combinator8<T1, T2, T3, T4,
 	}
 
 	@Override
-	public <T> Combinators.ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries) {
+	public <T extends @Nullable Object> Combinators.ListCombinator<T> combineList(List<? extends Arbitrary<T>> listOfArbitraries) {
 		return new DefaultListCombinator<>(listOfArbitraries);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/facades/EdgeCasesFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/EdgeCasesFacadeImpl.java
index b4b2519c4..4b1b8d96d 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/EdgeCasesFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/EdgeCasesFacadeImpl.java
@@ -6,13 +6,15 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.arbitraries.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
 public class EdgeCasesFacadeImpl extends EdgeCases.EdgeCasesFacade {
 
 	@Override
-	public <T> EdgeCases<T> fromSuppliers(final List<Supplier<Shrinkable<T>>> suppliers) {
+	public <T extends @Nullable Object> EdgeCases<T> fromSuppliers(final List<Supplier<Shrinkable<T>>> suppliers) {
 		return EdgeCasesSupport.fromSuppliers(suppliers);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ExhaustiveGeneratorFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ExhaustiveGeneratorFacadeImpl.java
index ef1ae38c4..40ebf0d31 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ExhaustiveGeneratorFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ExhaustiveGeneratorFacadeImpl.java
@@ -5,27 +5,29 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
 public class ExhaustiveGeneratorFacadeImpl extends ExhaustiveGenerator.ExhaustiveGeneratorFacade {
 	@Override
-	public <T, U> ExhaustiveGenerator<U> map(ExhaustiveGenerator<T> self, Function<T, U> mapper) {
+	public <T extends @Nullable Object, U extends @Nullable Object> ExhaustiveGenerator<U> map(ExhaustiveGenerator<T> self, Function<? super T, ? extends U> mapper) {
 		return new MappedExhaustiveGenerator<>(self, mapper);
 	}
 
 	@Override
-	public <T> ExhaustiveGenerator<T> filter(ExhaustiveGenerator<T> self, Predicate<T> filterPredicate, int maxMisses) {
+	public <T extends @Nullable Object> ExhaustiveGenerator<T> filter(ExhaustiveGenerator<T> self, Predicate<? super T> filterPredicate, int maxMisses) {
 		return new FilteredExhaustiveGenerator<>(self, filterPredicate, maxMisses);
 	}
 
 	@Override
-	public <T> ExhaustiveGenerator<T> injectNull(ExhaustiveGenerator<T> self) {
+	public <T extends @Nullable Object> ExhaustiveGenerator<@Nullable T> injectNull(ExhaustiveGenerator<T> self) {
 		return new WithNullExhaustiveGenerator<>(self);
 	}
 
 	@Override
-	public <T> ExhaustiveGenerator<T> ignoreExceptions(
+	public <T extends @Nullable Object> ExhaustiveGenerator<T> ignoreExceptions(
 		final ExhaustiveGenerator<T> self,
 		final Class<? extends Throwable>[] exceptionTypes,
 		int maxThrows
diff --git a/engine/src/main/java/net/jqwik/engine/facades/FunctionsFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/FunctionsFacadeImpl.java
index a3f9c79ba..0ec7fc9e3 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/FunctionsFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/FunctionsFacadeImpl.java
@@ -4,6 +4,8 @@
 import net.jqwik.api.arbitraries.*;
 import net.jqwik.engine.properties.arbitraries.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.support.JqwikReflectionSupport.*;
 
 /**
@@ -20,7 +22,7 @@ public void ensureFunctionalType(Class<?> functionalType) {
 
 	@SuppressWarnings("unchecked")
 	@Override
-	public <F, R> FunctionArbitrary<F, R> function(Class<?> functionalType, Arbitrary<R> resultArbitrary) {
+	public <F, R extends @Nullable Object> FunctionArbitrary<F, R> function(Class<?> functionalType, Arbitrary<R> resultArbitrary) {
 		return (FunctionArbitrary<F, R>) new DefaultFunctionArbitrary<>(functionalType, resultArbitrary);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/facades/Memoize.java b/engine/src/main/java/net/jqwik/engine/facades/Memoize.java
index 75b273461..1dc63af1b 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/Memoize.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/Memoize.java
@@ -8,6 +8,8 @@
 import net.jqwik.api.lifecycle.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 public class Memoize {
 
 	private static Store<Map<Tuple3<Arbitrary<?>, Integer, Boolean>, RandomGenerator<?>>> generatorStore() {
@@ -15,11 +17,11 @@ private static Store<Map<Tuple3<Arbitrary<?>, Integer, Boolean>, RandomGenerator
 	}
 
 	@SuppressWarnings("unchecked")
-	public static <U> RandomGenerator<U> memoizedGenerator(
+	public static <U extends @Nullable Object> RandomGenerator<U> memoizedGenerator(
 			Arbitrary<? extends U> arbitrary,
 			int genSize,
 			boolean withEdgeCases,
-			Supplier<RandomGenerator<? extends U>> generatorSupplier
+			Supplier<? extends RandomGenerator<? extends U>> generatorSupplier
 	) {
 		if (!arbitrary.isGeneratorMemoizable()){
 			return (RandomGenerator<U>) generatorSupplier.get();
@@ -36,7 +38,7 @@ public static <U> RandomGenerator<U> memoizedGenerator(
 
 	// Had to roll my on computeIfAbsent because HashMap.computeIfAbsent()
 	// does not allow modifications of the map within the mapping function
-	private static <K, V> V computeIfAbsent(
+	private static <K extends @Nullable Object, V extends @Nullable Object> V computeIfAbsent(
 			Map<K, V> cache,
 			K key,
 			Function<? super K, ? extends V> mappingFunction
diff --git a/engine/src/main/java/net/jqwik/engine/facades/RandomGeneratorFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/RandomGeneratorFacadeImpl.java
index c91ff7589..3ce6507cc 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/RandomGeneratorFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/RandomGeneratorFacadeImpl.java
@@ -7,19 +7,21 @@
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
 public class RandomGeneratorFacadeImpl extends RandomGenerator.RandomGeneratorFacade {
 	@Override
-	public <T, U> Shrinkable<U> flatMap(Shrinkable<T> self, Function<T, RandomGenerator<U>> mapper, long nextLong) {
+	public <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> flatMap(Shrinkable<T> self, Function<? super T, ? extends RandomGenerator<U>> mapper, long nextLong) {
 		return new FlatMappedShrinkable<>(self, mapper, nextLong);
 	}
 
 	@Override
-	public <T, U> Shrinkable<U> flatMap(
+	public <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> flatMap(
 		Shrinkable<T> self,
-		Function<T, Arbitrary<U>> mapper,
+		Function<? super T, ? extends Arbitrary<U>> mapper,
 		int genSize,
 		long nextLong,
 		boolean withEmbeddedEdgeCases
@@ -28,27 +30,27 @@ public <T, U> Shrinkable<U> flatMap(
 	}
 
 	@Override
-	public <T> RandomGenerator<T> filter(RandomGenerator<T> self, Predicate<T> filterPredicate, int maxMisses) {
+	public <T extends @Nullable Object> RandomGenerator<T> filter(RandomGenerator<T> self, Predicate<? super T> filterPredicate, int maxMisses) {
 		return new FilteredGenerator<>(self, filterPredicate, maxMisses);
 	}
 
 	@Override
-	public <T> RandomGenerator<T> withEdgeCases(RandomGenerator<T> self, int genSize, EdgeCases<T> edgeCases) {
+	public <T extends @Nullable Object> RandomGenerator<T> withEdgeCases(RandomGenerator<T> self, int genSize, EdgeCases<T> edgeCases) {
 		return RandomGenerators.withEdgeCases(self, genSize, edgeCases);
 	}
 
 	@Override
-	public <T> RandomGenerator<List<T>> collect(RandomGenerator<T> self, Predicate<List<T>> until) {
+	public <T extends @Nullable Object> RandomGenerator<List<T>> collect(RandomGenerator<T> self, Predicate<? super List<? extends T>> until) {
 		return new CollectGenerator<>(self, until);
 	}
 
 	@Override
-	public <T> RandomGenerator<T> injectDuplicates(RandomGenerator<T> self, double duplicateProbability) {
+	public <T extends @Nullable Object> RandomGenerator<T> injectDuplicates(RandomGenerator<T> self, double duplicateProbability) {
 		return new InjectDuplicatesGenerator<>(self, duplicateProbability);
 	}
 
 	@Override
-	public <T> RandomGenerator<T> ignoreExceptions(RandomGenerator<T> self, Class<? extends Throwable>[] exceptionTypes, int maxThrows) {
+	public <T extends @Nullable Object> RandomGenerator<T> ignoreExceptions(RandomGenerator<T> self, Class<? extends Throwable>[] exceptionTypes, int maxThrows) {
 		return new IgnoreExceptionGenerator<>(self, exceptionTypes, maxThrows);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/facades/SampleStreamFacade.java b/engine/src/main/java/net/jqwik/engine/facades/SampleStreamFacade.java
index f2c4156bf..8d30de0b2 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/SampleStreamFacade.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/SampleStreamFacade.java
@@ -4,6 +4,7 @@
 import java.util.function.*;
 import java.util.stream.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.engine.*;
 import org.junit.platform.engine.support.descriptor.*;
 
@@ -29,7 +30,7 @@ public Type getType() {
 	private static final Map<Arbitrary<Object>, RandomGenerator<Object>> generators = new LruCache<>(500);
 
 	@SuppressWarnings("unchecked")
-	private static <T> RandomGenerator<T> getGeneratorForSampling(Arbitrary<T> arbitrary) {
+	private static <T extends @Nullable Object> RandomGenerator<T> getGeneratorForSampling(Arbitrary<T> arbitrary) {
 		return runInDescriptor(() -> getGenerator((Arbitrary<Object>) arbitrary));
 	}
 
@@ -51,11 +52,11 @@ private static <T> RandomGenerator<T> getGenerator(Arbitrary<Object> arbitrary)
 		// );
 	}
 
-	private static <T> Supplier<T> wrapInDescriptor(Supplier<T> code) {
+	private static <T extends @Nullable Object> Supplier<T> wrapInDescriptor(Supplier<T> code) {
 		return () -> runInDescriptor(code);
 	}
 
-	private static <T> T runInDescriptor(Supplier<T> code) {
+	private static <T extends @Nullable Object> T runInDescriptor(Supplier<T> code) {
 		if (CurrentTestDescriptor.isEmpty()) {
 			return CurrentTestDescriptor.runWithDescriptor(SAMPLE_STREAM_DESCRIPTOR, code);
 		} else {
@@ -63,7 +64,7 @@ private static <T> T runInDescriptor(Supplier<T> code) {
 		}
 	}
 
-	<T> Stream<T> sampleStream(Arbitrary<T> arbitrary) {
+	<T extends @Nullable Object> Stream<T> sampleStream(Arbitrary<T> arbitrary) {
 		RandomGenerator<T> generator = getGeneratorForSampling(arbitrary);
 		return Stream.generate(wrapInDescriptor(() -> generator.next(SourceOfRandomness.current())))
 					 .map(shrinkable -> runInDescriptor(shrinkable::value));
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ShrinkableFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ShrinkableFacadeImpl.java
index 0d6ce28a4..6c7942281 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ShrinkableFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ShrinkableFacadeImpl.java
@@ -5,27 +5,29 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 /**
  * Is loaded through reflection in api module
  */
 public class ShrinkableFacadeImpl extends Shrinkable.ShrinkableFacade {
 	@Override
-	public <T> Shrinkable<T> unshrinkable(Supplier<T> valueSupplier, ShrinkingDistance distance) {
+	public <T extends @Nullable Object> Shrinkable<T> unshrinkable(Supplier<? extends T> valueSupplier, ShrinkingDistance distance) {
 		return new Unshrinkable<>(valueSupplier, distance);
 	}
 
 	@Override
-	public <T, U> Shrinkable<U> map(Shrinkable<T> self, Function<T, U> mapper) {
+	public <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> map(Shrinkable<T> self, Function<? super T, ? extends U> mapper) {
 		return new MappedShrinkable<>(self, mapper);
 	}
 
 	@Override
-	public <T> Shrinkable<T> filter(Shrinkable<T> self, Predicate<T> filter) {
+	public <T extends @Nullable Object> Shrinkable<T> filter(Shrinkable<T> self, Predicate<? super T> filter) {
 		return new FilteredShrinkable<>(self, filter);
 	}
 
 	@Override
-	public <T, U> Shrinkable<U> flatMap(Shrinkable<T> self, Function<T, Arbitrary<U>> flatMapper, int tries, long randomSeed) {
+	public <T extends @Nullable Object, U extends @Nullable Object> Shrinkable<U> flatMap(Shrinkable<T> self, Function<? super T, ? extends Arbitrary<U>> flatMapper, int tries, long randomSeed) {
 		return new FlatMappedShrinkable<>(self, flatMapper, tries, randomSeed, false);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/facades/ShrinkingSupportFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/ShrinkingSupportFacadeImpl.java
index 4e0027fa1..0b2dbe81c 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/ShrinkingSupportFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/ShrinkingSupportFacadeImpl.java
@@ -10,22 +10,24 @@
 import net.jqwik.engine.properties.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 public class ShrinkingSupportFacadeImpl extends ShrinkingSupportFacade {
 
 	private final TestingSupportFacadeImpl testingSupportFacade = new TestingSupportFacadeImpl();
 
 	@Override
-	public <T> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random, Falsifier<T> falsifier) {
+	public <T extends @Nullable Object> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random, Falsifier<? super T> falsifier) {
 		RandomGenerator<? extends T> generator = arbitrary.generator(10, true);
 		return falsifyThenShrink(generator, random, falsifier);
 	}
 
 	@Override
 	@SuppressWarnings("unchecked")
-	public <T> T falsifyThenShrink(
+	public <T extends @Nullable Object> T falsifyThenShrink(
 			RandomGenerator<? extends T> generator,
 			Random random,
-			Falsifier<T> falsifier
+			Falsifier<? super T> falsifier
 	) {
 		Throwable[] originalError = new Throwable[1];
 		Shrinkable<T> falsifiedShrinkable =
@@ -42,9 +44,9 @@ public <T> T falsifyThenShrink(
 
 	@Override
 	@SuppressWarnings("unchecked")
-	public <T> T shrink(
+	public <T extends @Nullable Object> T shrink(
 			Shrinkable<T> falsifiedShrinkable,
-			Falsifier<T> falsifier,
+			Falsifier<? super T> falsifier,
 			Throwable originalError
 	) {
 		ShrunkFalsifiedSample sample = shrinkToSample(falsifiedShrinkable, falsifier, originalError);
@@ -52,10 +54,10 @@ public <T> T shrink(
 	}
 
 	@Override
-	public <T> ShrunkFalsifiedSample shrinkToSample(
-			Shrinkable<T> falsifiedShrinkable,
-			Falsifier<T> falsifier,
-			Throwable originalError
+	public <T extends @Nullable Object> ShrunkFalsifiedSample shrinkToSample(
+		Shrinkable<T> falsifiedShrinkable,
+		Falsifier<? super T> falsifier,
+		Throwable originalError
 	) {
 		FalsifiedSample sample = toFalsifiedSample(falsifiedShrinkable, originalError);
 		Consumer<FalsifiedSample> parametersReporter = ignore -> {};
@@ -65,8 +67,10 @@ public <T> ShrunkFalsifiedSample shrinkToSample(
 	}
 
 	@SuppressWarnings("unchecked")
-	private static <T> Falsifier<List<Object>> toParamFalsifier(Falsifier<T> tFalsifier) {
+	private static <T extends @Nullable Object> Falsifier<List<Object>> toParamFalsifier(Falsifier<T> tFalsifier) {
 		return params -> {
+			// At best PropertyShrinker should be parameterized, however, currently it is not
+			// Even though PropertyShrinker passes List<Object>, the elements are of type T
 			T t = (T) params.get(0);
 			return tFalsifier.execute(t);
 		};
diff --git a/engine/src/main/java/net/jqwik/engine/facades/StoreFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/StoreFacadeImpl.java
index 099c978c3..734b3fd90 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/StoreFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/StoreFacadeImpl.java
@@ -3,6 +3,7 @@
 import java.util.*;
 import java.util.function.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.engine.*;
 
 import net.jqwik.api.lifecycle.*;
@@ -14,20 +15,20 @@
 public class StoreFacadeImpl extends Store.StoreFacade {
 
 	@Override
-	public <T> Store<T> create(Object identifier, Lifespan lifespan, Supplier<T> initialValueSupplier) {
+	public <T extends @Nullable Object> Store<T> create(Object identifier, Lifespan lifespan, Supplier<? extends T> initialValueSupplier) {
 		TestDescriptor scope = CurrentTestDescriptor.get();
 		return StoreRepository.getCurrent().create(scope, identifier, lifespan, initialValueSupplier);
 	}
 
 	@Override
-	public <T> Store<T> get(Object identifier) {
+	public <T extends @Nullable Object> Store<T> get(Object identifier) {
 		TestDescriptor retriever = CurrentTestDescriptor.get();
 		Optional<? extends Store<T>> store = StoreRepository.getCurrent().get(retriever, identifier);
 		return store.orElseThrow(() -> new CannotFindStoreException(identifier, retriever.getUniqueId().toString()));
 	}
 
 	@Override
-	public <T> Store<T> free(Supplier<T> initialValueSupplier) {
+	public <T extends @Nullable Object> Store<T> free(Supplier<? extends T> initialValueSupplier) {
 		return new Store<T>() {
 			T t = initialValueSupplier.get();
 
diff --git a/engine/src/main/java/net/jqwik/engine/facades/TestingSupportFacadeImpl.java b/engine/src/main/java/net/jqwik/engine/facades/TestingSupportFacadeImpl.java
index d8bf67277..35c3057e2 100644
--- a/engine/src/main/java/net/jqwik/engine/facades/TestingSupportFacadeImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/facades/TestingSupportFacadeImpl.java
@@ -7,10 +7,12 @@
 import net.jqwik.api.facades.*;
 import net.jqwik.engine.execution.reporting.*;
 
+import org.jspecify.annotations.*;
+
 public class TestingSupportFacadeImpl extends TestingSupportFacade {
 
 	@Override
-	public  <T> Shrinkable<T> generateUntil(RandomGenerator<T> generator, Random random, Function<T, Boolean> condition) {
+	public  <T extends @Nullable Object> Shrinkable<T> generateUntil(RandomGenerator<T> generator, Random random, Function<? super T, Boolean> condition) {
 		long maxTries = 1000;
 		return generator
 					   .stream(random)
diff --git a/engine/src/main/java/net/jqwik/engine/facades/package-info.java b/engine/src/main/java/net/jqwik/engine/facades/package-info.java
new file mode 100644
index 000000000..4e7a8bc3c
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/facades/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.facades;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/hooks/lifecycle/package-info.java b/engine/src/main/java/net/jqwik/engine/hooks/lifecycle/package-info.java
new file mode 100644
index 000000000..3929dd572
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/hooks/lifecycle/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.hooks.lifecycle;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/hooks/package-info.java b/engine/src/main/java/net/jqwik/engine/hooks/package-info.java
new file mode 100644
index 000000000..f8cdd6b3a
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/hooks/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.hooks;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java b/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
index cee4275a6..c4ce8c51e 100644
--- a/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
+++ b/engine/src/main/java/net/jqwik/engine/hooks/statistics/StatisticsCollectorImpl.java
@@ -226,7 +226,7 @@ public CoverageCheckerImpl(String label, StatisticsEntry entry, int countAll) {
 		}
 
 		@Override
-		public void count(Predicate<Integer> countChecker) {
+		public void count(Predicate<? super Integer> countChecker) {
 			if (!countChecker.test(entry.count())) {
 				String condition = String.format("Count of %s", entry.count());
 				failCondition(condition);
@@ -234,7 +234,7 @@ public void count(Predicate<Integer> countChecker) {
 		}
 
 		@Override
-		public void count(BiPredicate<Integer, Integer> countChecker) {
+		public void count(BiPredicate<? super Integer, ? super Integer> countChecker) {
 			if (!countChecker.test(entry.count(), countAll)) {
 				String condition = String.format("Count of (%s, %s)", entry.count(), countAll);
 				failCondition(condition);
@@ -242,7 +242,7 @@ public void count(BiPredicate<Integer, Integer> countChecker) {
 		}
 
 		@Override
-		public void count(Consumer<Integer> countChecker) {
+		public void count(Consumer<? super Integer> countChecker) {
 			count(c -> {
 				countChecker.accept(c);
 				return true;
@@ -250,7 +250,7 @@ public void count(Consumer<Integer> countChecker) {
 		}
 
 		@Override
-		public void count(BiConsumer<Integer, Integer> countChecker) {
+		public void count(BiConsumer<? super Integer, ? super Integer> countChecker) {
 			count((c, a) -> {
 				countChecker.accept(c, a);
 				return true;
@@ -258,7 +258,7 @@ public void count(BiConsumer<Integer, Integer> countChecker) {
 		}
 
 		@Override
-		public void percentage(Predicate<Double> percentageChecker) {
+		public void percentage(Predicate<? super Double> percentageChecker) {
 			if (!percentageChecker.test(entry.percentage())) {
 				String condition = String.format("Percentage of %s", entry.percentage());
 				failCondition(condition);
@@ -266,7 +266,7 @@ public void percentage(Predicate<Double> percentageChecker) {
 		}
 
 		@Override
-		public void percentage(Consumer<Double> percentageChecker) {
+		public void percentage(Consumer<? super Double> percentageChecker) {
 			percentage(p -> {
 				percentageChecker.accept(p);
 				return true;
diff --git a/engine/src/main/java/net/jqwik/engine/hooks/statistics/package-info.java b/engine/src/main/java/net/jqwik/engine/hooks/statistics/package-info.java
new file mode 100644
index 000000000..6e526e264
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/hooks/statistics/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.hooks.statistics;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/package-info.java b/engine/src/main/java/net/jqwik/engine/package-info.java
new file mode 100644
index 000000000..a58a6316e
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/FeatureExtractor.java b/engine/src/main/java/net/jqwik/engine/properties/FeatureExtractor.java
index 7f05a8a90..1a50d444e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/FeatureExtractor.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/FeatureExtractor.java
@@ -1,12 +1,14 @@
 package net.jqwik.engine.properties;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 import java.util.function.*;
 
 @FunctionalInterface
-public interface FeatureExtractor<T> extends Function<T, Object> {
+public interface FeatureExtractor<T extends @Nullable Object> extends Function<T, Object> {
 
-	static <T> FeatureExtractor<T> identity() {
+	static <T extends @Nullable Object> FeatureExtractor<T> identity() {
 		return t -> t;
 	}
 
@@ -18,7 +20,7 @@ default Object applySafe(T t) {
 		}
 	}
 
-	default boolean isUniqueIn(T value, Collection<T> elements) {
+	default boolean isUniqueIn(T value, Collection<? extends T> elements) {
 		if (this == identity()) {
 			return !elements.contains(value);
 		}
@@ -28,7 +30,7 @@ default boolean isUniqueIn(T value, Collection<T> elements) {
 					   .noneMatch(x -> Objects.equals(x, feature));
 	}
 
-	default boolean areUnique(Collection<T> elements) {
+	default boolean areUnique(Collection<? extends T> elements) {
 		Set<Object> set = new HashSet<>();
 		for (T x : elements) {
 			if (!set.add(applySafe(x))) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/PropertyCheckResult.java b/engine/src/main/java/net/jqwik/engine/properties/PropertyCheckResult.java
index 0d2a071d0..7e911c8c5 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/PropertyCheckResult.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/PropertyCheckResult.java
@@ -2,6 +2,7 @@
 
 import java.util.*;
 
+import org.jspecify.annotations.*;
 import org.opentest4j.*;
 
 import net.jqwik.api.*;
@@ -56,9 +57,9 @@ public static PropertyCheckResult failed(
 		EdgeCasesMode edgeCasesMode,
 		int edgeCasesTotal,
 		int edgeCasesTried,
-		FalsifiedSample originalSample,
-		ShrunkFalsifiedSample shrunkSample,
-		Throwable throwable
+		@Nullable FalsifiedSample originalSample,
+		@Nullable ShrunkFalsifiedSample shrunkSample,
+		@Nullable Throwable throwable
 	) {
 		// If no shrinking was possible, report only original sample
 		if (shrunkSample != null && areEquivalent(originalSample, shrunkSample)) {
@@ -151,9 +152,9 @@ public static PropertyCheckResult exhausted(
 	private final EdgeCasesMode edgeCasesMode;
 	private final int edgeCasesTotal;
 	private final int edgeCasesTried;
-	private final FalsifiedSample originalSample;
-	private final ShrunkFalsifiedSample shrunkSample;
-	private final Throwable throwable;
+	private final @Nullable FalsifiedSample originalSample;
+	private final @Nullable ShrunkFalsifiedSample shrunkSample;
+	private final @Nullable Throwable throwable;
 
 	private PropertyCheckResult(
 		CheckStatus status, String stereotype,
@@ -165,9 +166,9 @@ private PropertyCheckResult(
 		EdgeCasesMode edgeCasesMode,
 		int edgeCasesTotal,
 		int edgeCasesTried,
-		FalsifiedSample originalSample,
-		ShrunkFalsifiedSample shrunkSample,
-		Throwable throwable
+		@Nullable FalsifiedSample originalSample,
+		@Nullable ShrunkFalsifiedSample shrunkSample,
+		@Nullable Throwable throwable
 	) {
 		this.stereotype = stereotype;
 		this.status = status;
@@ -184,7 +185,7 @@ private PropertyCheckResult(
 		this.throwable = determineThrowable(status, throwable);
 	}
 
-	private Throwable determineThrowable(CheckStatus status, Throwable throwable) {
+	private @Nullable Throwable determineThrowable(CheckStatus status, @Nullable Throwable throwable) {
 		if (status == CheckStatus.SUCCESSFUL) {
 			return null;
 		} else {
@@ -218,7 +219,7 @@ public Optional<Throwable> throwable() {
 	}
 
 	@Override
-	public PropertyExecutionResult mapTo(Status newStatus, Throwable throwable) {
+	public PropertyExecutionResult mapTo(Status newStatus, @Nullable Throwable throwable) {
 		switch (newStatus) {
 			case ABORTED:
 				return PlainExecutionResult.aborted(throwable, generationInfo);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/RandomizedShrinkablesGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/RandomizedShrinkablesGenerator.java
index 956e6597f..17956ddd4 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/RandomizedShrinkablesGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/RandomizedShrinkablesGenerator.java
@@ -102,7 +102,7 @@ private static List<EdgeCases<Object>> listOfEdgeCases(
 		return listOfEdgeCases;
 	}
 
-	private static <T> int calculateNextParamMaxEdgeCases(int maxEdgeCases, int baseCasesSize) {
+	private static int calculateNextParamMaxEdgeCases(int maxEdgeCases, int baseCasesSize) {
 		int maxDerivedEdgeCases = Math.max(1, maxEdgeCases / baseCasesSize);
 		// When in doubt generate a few more edge cases
 		if (maxEdgeCases % baseCasesSize > 0) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/UniquenessChecker.java b/engine/src/main/java/net/jqwik/engine/properties/UniquenessChecker.java
index 8296be649..84d3685ea 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/UniquenessChecker.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/UniquenessChecker.java
@@ -5,9 +5,11 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 public class UniquenessChecker {
 
-	public static <T> boolean checkShrinkableUniqueIn(Collection<FeatureExtractor<T>> extractors, Shrinkable<T> shrinkable, List<Shrinkable<T>> shrinkables) {
+	public static <T extends @Nullable Object> boolean checkShrinkableUniqueIn(Collection<? extends FeatureExtractor<T>> extractors, Shrinkable<T> shrinkable, List<? extends Shrinkable<T>> shrinkables) {
 		if (extractors.isEmpty()) {
 			return true;
 		}
@@ -16,7 +18,7 @@ public static <T> boolean checkShrinkableUniqueIn(Collection<FeatureExtractor<T>
 		return checkValueUniqueIn(extractors, value, elements);
 	}
 
-	public static <T> boolean checkValueUniqueIn(Collection<FeatureExtractor<T>> extractors, T value, Collection<T> elements) {
+	public static <T> boolean checkValueUniqueIn(Collection<? extends FeatureExtractor<T>> extractors, T value, Collection<? extends T> elements) {
 		for (FeatureExtractor<T> extractor : extractors) {
 			if (!extractor.isUniqueIn(value, elements)) {
 				return false;
@@ -25,7 +27,7 @@ public static <T> boolean checkValueUniqueIn(Collection<FeatureExtractor<T>> ext
 		return true;
 	}
 
-	public static <T> boolean checkUniquenessOfShrinkables(Collection<FeatureExtractor<T>> extractors, List<Shrinkable<T>> shrinkables) {
+	public static <T extends @Nullable Object> boolean checkUniquenessOfShrinkables(Collection<? extends FeatureExtractor<T>> extractors, List<? extends Shrinkable<T>> shrinkables) {
 		if (extractors.isEmpty()) {
 			return true;
 		}
@@ -33,7 +35,7 @@ public static <T> boolean checkUniquenessOfShrinkables(Collection<FeatureExtract
 		return checkUniquenessOfValues(extractors, elements);
 	}
 
-	public static <T> boolean checkUniquenessOfValues(Collection<FeatureExtractor<T>> extractors, Collection<T> elements) {
+	public static <T extends @Nullable Object> boolean checkUniquenessOfValues(Collection<? extends FeatureExtractor<T>> extractors, Collection<? extends T> elements) {
 		for (FeatureExtractor<T> extractor : extractors) {
 			if (!extractor.areUnique(elements)) {
 				return false;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java
index 248d961a8..517c47a74 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java
@@ -9,9 +9,9 @@
 public class ArbitraryCollect<T> implements Arbitrary<List<T>> {
 
 	private final Arbitrary<T> elementArbitrary;
-	private final Predicate<List<T>> until;
+	private final Predicate<? super List<? extends T>> until;
 
-	public ArbitraryCollect(Arbitrary<T> elementArbitrary, Predicate<List<T>> until) {
+	public ArbitraryCollect(Arbitrary<T> elementArbitrary, Predicate<? super List<? extends T>> until) {
 		this.elementArbitrary = elementArbitrary;
 		this.until = until;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java
index 278d41d96..4128cbf96 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java
@@ -8,10 +8,10 @@
 import net.jqwik.engine.*;
 
 public class ArbitraryFilter<T> extends ArbitraryDelegator<T> {
-	private final Predicate<T> filterPredicate;
+	private final Predicate<? super T> filterPredicate;
 	private final int maxMisses;
 
-	public ArbitraryFilter(Arbitrary<T> self, Predicate<T> filterPredicate, int maxMisses) {
+	public ArbitraryFilter(Arbitrary<T> self, Predicate<? super T> filterPredicate, int maxMisses) {
 		super(self);
 		this.filterPredicate = filterPredicate;
 		this.maxMisses = maxMisses;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFlatMap.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFlatMap.java
index 0cac5d474..c89dd09a3 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFlatMap.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFlatMap.java
@@ -9,9 +9,9 @@
 
 public class ArbitraryFlatMap<T, U> implements Arbitrary<U> {
 	private final Arbitrary<T> self;
-	private final Function<T, Arbitrary<U>> mapper;
+	private final Function<? super T, ? extends Arbitrary<U>> mapper;
 
-	public ArbitraryFlatMap(Arbitrary<T> self, Function<T, Arbitrary<U>> mapper) {
+	public ArbitraryFlatMap(Arbitrary<T> self, Function<? super T, ? extends Arbitrary<U>> mapper) {
 		this.self = self;
 		this.mapper = mapper;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryMap.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryMap.java
index 5ae026401..18f62bbdd 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryMap.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryMap.java
@@ -6,11 +6,13 @@
 import net.jqwik.api.*;
 import net.jqwik.api.support.*;
 
-public class ArbitraryMap<T, U> implements Arbitrary<U> {
+import org.jspecify.annotations.*;
+
+public class ArbitraryMap<T extends @Nullable Object, U extends @Nullable Object> implements Arbitrary<U> {
 	private final Arbitrary<T> self;
-	private final Function<T, U> mapper;
+	private final Function<? super T, ? extends U> mapper;
 
-	public ArbitraryMap(Arbitrary<T> self, Function<T, U> mapper) {
+	public ArbitraryMap(Arbitrary<T> self, Function<? super T, ? extends U> mapper) {
 		this.self = self;
 		this.mapper = mapper;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/CreateArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/CreateArbitrary.java
index 2596a98e2..3feadc7fe 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/CreateArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/CreateArbitrary.java
@@ -6,7 +6,9 @@
 import net.jqwik.api.support.*;
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 
-public class CreateArbitrary<T> extends UseGeneratorsArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class CreateArbitrary<T extends @Nullable Object> extends UseGeneratorsArbitrary<T> {
 
 	private final Supplier<T> supplier;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
index 4b7fadece..6b535a5e1 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultArrayArbitrary.java
@@ -10,6 +10,8 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 public class DefaultArrayArbitrary<T, A> extends MultivalueArbitraryBase<T, A> implements ArrayArbitrary<T, A> {
 
 	public static <T, A> ArrayArbitrary<T, A> forArrayType(Arbitrary<T> elementArbitrary, Class<A> arrayClass) {
@@ -20,7 +22,7 @@ public static <T, A> ArrayArbitrary<T, A> forArrayType(Arbitrary<T> elementArbit
 		return new DefaultArrayArbitrary<>(elementArbitrary, arrayClass.getComponentType());
 	}
 
-	public static <T> ArrayArbitrary<T, T[]> forComponentType(Arbitrary<T> elementArbitrary, Class<?> componentType) {
+	public static <T extends @Nullable Object> ArrayArbitrary<T, T[]> forComponentType(Arbitrary<T> elementArbitrary, Class<?> componentType) {
 		return new DefaultArrayArbitrary<>(elementArbitrary, componentType);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
index 389031013..895b28dd0 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultListArbitrary.java
@@ -60,7 +60,7 @@ public ListArbitrary<T> withSizeDistribution(RandomDistribution distribution) {
 
 	// TODO: Remove duplication with DefaultSetArbitrary.mapEach()
 	@Override
-	public <U> Arbitrary<List<U>> mapEach(BiFunction<List<T>, T, U> mapper) {
+	public <U extends @Nullable Object> Arbitrary<List<U>> mapEach(BiFunction<? super List<? extends T>, ? super T, ? extends U> mapper) {
 		return this.map(elements -> elements.stream()
 											.map(e -> mapper.apply(elements, e))
 											.collect(Collectors.toList()));
@@ -68,7 +68,7 @@ public <U> Arbitrary<List<U>> mapEach(BiFunction<List<T>, T, U> mapper) {
 
 	// TODO: Remove duplication with DefaultSetArbitrary.flatMapEach()
 	@Override
-	public <U> Arbitrary<List<U>> flatMapEach(BiFunction<? super List<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper) {
+	public <U extends @Nullable Object> Arbitrary<List<U>> flatMapEach(BiFunction<? super List<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper) {
 		return this.flatMap(elements -> {
 			List<? extends Arbitrary<U>> arbitraries =
 				elements.stream()
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
index 5ede6e77f..b9138641f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
@@ -69,7 +69,7 @@ public SetArbitrary<T> withSizeDistribution(RandomDistribution distribution) {
 
 	// TODO: Remove duplication with DefaultListArbitrary.mapEach()
 	@Override
-	public <U> Arbitrary<Set<U>> mapEach(BiFunction<Set<T>, T, U> mapper) {
+	public <U> Arbitrary<Set<U>> mapEach(BiFunction<? super Set<? extends T>, ? super T, ? extends U> mapper) {
 		return this.map(elements -> elements.stream()
 											.map(e -> mapper.apply(elements, e))
 											.collect(CollectorsSupport.toLinkedHashSet()));
@@ -77,7 +77,7 @@ public <U> Arbitrary<Set<U>> mapEach(BiFunction<Set<T>, T, U> mapper) {
 
 	// TODO: Remove duplication with DefaultListArbitrary.flatMapEach()
 	@Override
-	public <U> Arbitrary<Set<U>> flatMapEach(BiFunction<Set<T>, T, Arbitrary<U>> flatMapper) {
+	public <U> Arbitrary<Set<U>> flatMapEach(BiFunction<? super Set<? extends T>, ? super T, ? extends Arbitrary<U>> flatMapper) {
 		return this.flatMap(elements -> {
 			List<Arbitrary<U>> arbitraries =
 				elements.stream()
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java
index bb5da44ce..490966a8d 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java
@@ -17,7 +17,7 @@ public class DefaultTypeArbitrary<T> extends ArbitraryDecorator<T> implements Ty
 	private final Class<T> targetType;
 	private final Set<Executable> explicitCreators = new LinkedHashSet<>();
 	private final Set<Predicate<? super Constructor<?>>> constructorFilters = new LinkedHashSet<>();
-	private final Set<Predicate<Method>> factoryMethodFilters = new LinkedHashSet<>();
+	private final Set<Predicate<? super Method>> factoryMethodFilters = new LinkedHashSet<>();
 	private boolean defaultsSet = true;
 
 	private TraverseArbitrary<T> traverseArbitrary;
@@ -81,13 +81,13 @@ private void addAllConstructors() {
 	}
 
 	@Override
-	public TypeArbitrary<T> useFactoryMethods(Predicate<Method> filter) {
+	public TypeArbitrary<T> useFactoryMethods(Predicate<? super Method> filter) {
 		DefaultTypeArbitrary<T> clone = cloneWithoutDefaultsSet();
 		clone.addFactoryMethods(filter);
 		return clone;
 	}
 
-	private void addFactoryMethods(Predicate<Method> filter) {
+	private void addFactoryMethods(Predicate<? super Method> filter) {
 		factoryMethodFilters.add(filter);
 	}
 
@@ -134,7 +134,7 @@ public Set<Executable> findCreators(TypeUsage target) {
 			if (target.isOfType(targetType)) {
 				creators.addAll(explicitCreators);
 			}
-			for (Predicate<Method> filter : factoryMethodFilters) {
+			for (Predicate<? super Method> filter : factoryMethodFilters) {
 				appendFactoryMethods(creators, target, filter);
 			}
 			for (Predicate<? super Constructor<?>> filter : constructorFilters) {
@@ -153,7 +153,7 @@ private void appendConstructors(Set<Executable> creators, TypeUsage target, Pred
 
 		}
 
-		private void appendFactoryMethods(Set<Executable> creators, TypeUsage target, Predicate<Method> filter) {
+		private void appendFactoryMethods(Set<Executable> creators, TypeUsage target, Predicate<? super Method> filter) {
 			Arrays.stream(target.getRawType().getDeclaredMethods())
 				  .filter(ModifierSupport::isStatic)
 				  .filter(creator -> hasFittingReturnType(creator, target))
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
index 1f0ad50c9..907509440 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/EdgeCasesSupport.java
@@ -9,11 +9,13 @@
 import net.jqwik.engine.properties.shrinking.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.support.JqwikExceptionSupport.*;
 
 public class EdgeCasesSupport {
 
-	public static <T> EdgeCases<T> fromSuppliers(final List<Supplier<Shrinkable<T>>> suppliers) {
+	public static <T extends @Nullable Object> EdgeCases<T> fromSuppliers(final List<Supplier<Shrinkable<T>>> suppliers) {
 		return new EdgeCases<T>() {
 			@Override
 			public List<Supplier<Shrinkable<T>>> suppliers() {
@@ -34,7 +36,7 @@ public String toString() {
 		};
 	}
 
-	public static <T> EdgeCases<T> choose(final List<T> values, int maxEdgeCases) {
+	public static <T extends @Nullable Object> EdgeCases<T> choose(final List<T> values, int maxEdgeCases) {
 		List<Shrinkable<T>> shrinkables = new ArrayList<>();
 		if (values.size() > 0) {
 			shrinkables.add(new ChooseValueShrinkable<>(values.get(0), values));
@@ -52,7 +54,7 @@ public static <T> EdgeCases<T> choose(final List<T> values, int maxEdgeCases) {
 		return EdgeCasesSupport.fromShrinkables(shrinkables);
 	}
 
-	public static <T> EdgeCases<T> concatFrom(final List<? extends Arbitrary<T>> arbitraries, int maxEdgeCases) {
+	public static <T extends @Nullable Object> EdgeCases<T> concatFrom(final List<? extends Arbitrary<T>> arbitraries, int maxEdgeCases) {
 		List<Shrinkable<Arbitrary<T>>> shrinkables = new ArrayList<>();
 		for (Arbitrary<T> arbitrary : arbitraries) {
 			shrinkables.add(new ChooseValueShrinkable<>(arbitrary, arbitraries));
@@ -60,7 +62,7 @@ public static <T> EdgeCases<T> concatFrom(final List<? extends Arbitrary<T>> arb
 		return flatMapArbitrary(fromShrinkables(shrinkables), Function.identity(), maxEdgeCases);
 	}
 
-	public static <T> EdgeCases<T> concat(List<EdgeCases<T>> edgeCases, int maxEdgeCases) {
+	public static <T extends @Nullable Object> EdgeCases<T> concat(List<EdgeCases<T>> edgeCases, int maxEdgeCases) {
 		if (edgeCases.isEmpty() || maxEdgeCases <= 0) {
 			return EdgeCases.none();
 		}
@@ -84,18 +86,18 @@ public static <T> EdgeCases<T> concat(List<EdgeCases<T>> edgeCases, int maxEdgeC
 		return EdgeCasesSupport.fromSuppliers(concatenatedSuppliers);
 	}
 
-	public static <T> EdgeCases<T> fromShrinkables(List<Shrinkable<T>> shrinkables) {
+	public static <T extends @Nullable Object> EdgeCases<T> fromShrinkables(List<Shrinkable<T>> shrinkables) {
 		return () -> shrinkables
 						 .stream()
 						 .map(shrinkable -> (Supplier<Shrinkable<T>>) () -> shrinkable)
 						 .collect(Collectors.toList());
 	}
 
-	public static <T, U> EdgeCases<U> map(EdgeCases<T> self, Function<T, U> mapper) {
+	public static <T extends @Nullable Object, U extends @Nullable Object> EdgeCases<U> map(EdgeCases<T> self, Function<? super T, ? extends U> mapper) {
 		return mapShrinkable(self, tShrinkable -> tShrinkable.map(mapper));
 	}
 
-	public static <T, U> EdgeCases<U> mapShrinkable(EdgeCases<T> self, Function<Shrinkable<T>, Shrinkable<U>> mapper) {
+	public static <T extends @Nullable Object, U extends @Nullable Object> EdgeCases<U> mapShrinkable(EdgeCases<T> self, Function<? super Shrinkable<T>, ? extends Shrinkable<U>> mapper) {
 		List<Supplier<Shrinkable<U>>> mappedSuppliers =
 			self.suppliers().stream()
 				.map(tSupplier -> mapper.apply(tSupplier.get()))
@@ -105,7 +107,7 @@ public static <T, U> EdgeCases<U> mapShrinkable(EdgeCases<T> self, Function<Shri
 		return EdgeCases.fromSuppliers(mappedSuppliers);
 	}
 
-	public static <T> EdgeCases<T> filter(EdgeCases<T> self, Predicate<T> filterPredicate) {
+	public static <T extends @Nullable Object> EdgeCases<T> filter(EdgeCases<T> self, Predicate<? super T> filterPredicate) {
 		List<Supplier<Shrinkable<T>>> filteredSuppliers =
 			self.suppliers().stream()
 				.filter(supplier -> filterPredicate.test(supplier.get().value()))
@@ -114,7 +116,7 @@ public static <T> EdgeCases<T> filter(EdgeCases<T> self, Predicate<T> filterPred
 		return EdgeCases.fromSuppliers(filteredSuppliers);
 	}
 
-	public static <T> EdgeCases<T> ignoreExceptions(final EdgeCases<T> self, final Class<? extends Throwable>[] exceptionTypes) {
+	public static <T extends @Nullable Object> EdgeCases<T> ignoreExceptions(final EdgeCases<T> self, final Class<? extends Throwable>[] exceptionTypes) {
 		List<Supplier<Shrinkable<T>>> filteredSuppliers =
 			self.suppliers().stream()
 				.filter(supplier -> {
@@ -136,16 +138,16 @@ public static <T> EdgeCases<T> ignoreExceptions(final EdgeCases<T> self, final C
 		return EdgeCases.fromSuppliers(filteredSuppliers);
 	}
 
-	public static <T> EdgeCases<T> dontShrink(EdgeCases<T> self) {
+	public static <T extends @Nullable Object> EdgeCases<T> dontShrink(EdgeCases<T> self) {
 		return () -> self.suppliers()
 						 .stream()
 						 .map(supplier -> (Supplier<Shrinkable<T>>) () -> supplier.get().makeUnshrinkable())
 						 .collect(Collectors.toList());
 	}
 
-	public static <T, U> EdgeCases<U> flatMapArbitrary(
+	public static <T extends @Nullable Object, U extends @Nullable Object> EdgeCases<U> flatMapArbitrary(
 		EdgeCases<T> self,
-		Function<T, Arbitrary<U>> mapper,
+		Function<? super T, ? extends Arbitrary<U>> mapper,
 		int maxEdgeCases
 	) {
 		List<Supplier<Shrinkable<U>>> flatMappedSuppliers =
@@ -170,9 +172,9 @@ public static <T, U> EdgeCases<U> flatMapArbitrary(
 		return EdgeCases.fromSuppliers(flatMappedSuppliers);
 	}
 
-	public static <T> EdgeCases<T> combine(
+	public static <T extends @Nullable Object> EdgeCases<T> combine(
 		final List<Arbitrary<Object>> arbitraries,
-		final Function<List<Object>, T> combineFunction,
+		final Function<? super List<?>, ? extends T> combineFunction,
 		int maxEdgeCases
 	) {
 		if (arbitraries.isEmpty() || maxEdgeCases <= 0) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyArbitrary.java
index eca9c70ea..4bdff7637 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyArbitrary.java
@@ -7,7 +7,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
-public class FrequencyArbitrary<T> extends UseGeneratorsArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class FrequencyArbitrary<T extends @Nullable Object> extends UseGeneratorsArbitrary<T> {
 	private final List<Tuple.Tuple2<Integer, T>> frequencies;
 
 	public FrequencyArbitrary(List<Tuple.Tuple2<Integer, T>> frequencies) {
@@ -19,7 +21,7 @@ public FrequencyArbitrary(List<Tuple.Tuple2<Integer, T>> frequencies) {
 		this.frequencies = frequencies;
 	}
 
-	private static <T> List<T> valuesOf(List<Tuple.Tuple2<Integer, T>> frequencies) {
+	private static <T extends @Nullable Object> List<T> valuesOf(List<? extends Tuple.Tuple2<Integer, T>> frequencies) {
 		return frequencies.stream().map(Tuple.Tuple2::get2).collect(Collectors.toList());
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
index 2fb69bcc5..31d19e281 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FrequencyOfArbitrary.java
@@ -11,7 +11,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
-public class FrequencyOfArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class FrequencyOfArbitrary<T extends @Nullable Object> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
 
 	private final List<Tuple2<Integer, ? extends Arbitrary<T>>> frequencies;
 	private final boolean isGeneratorMemoizable;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java
index 333d3eee8..8a9aa2acd 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java
@@ -8,7 +8,7 @@
 
 public class GenericEdgeCasesConfiguration<T> implements EdgeCases.Config<T> {
 	private boolean none;
-	private final List<Predicate<T>> filters = new ArrayList<>();
+	private final List<Predicate<? super T>> filters = new ArrayList<>();
 	private final List<T> additionalEdgeCases = new ArrayList<>();
 
 	@Override
@@ -18,7 +18,7 @@ public EdgeCases.Config<T> none() {
 	}
 
 	@Override
-	public EdgeCases.Config<T> filter(Predicate<T> filter) {
+	public EdgeCases.Config<T> filter(Predicate<? super T> filter) {
 		filters.add(filter);
 		return this;
 	}
@@ -44,7 +44,7 @@ public final EdgeCases.Config<T> includeOnly(T... includedValues) {
 		return filter(values::contains);
 	}
 
-	public EdgeCases<T> configure(Consumer<EdgeCases.Config<T>> configurator, Function<Integer, EdgeCases<T>> edgeCasesCreator, int maxEdgeCases) {
+	public EdgeCases<T> configure(Consumer<? super EdgeCases.Config<T>> configurator, Function<? super Integer, ? extends EdgeCases<T>> edgeCasesCreator, int maxEdgeCases) {
 		configurator.accept(this);
 
 		EdgeCases<T> configuredEdgeCases;
@@ -57,7 +57,7 @@ public EdgeCases<T> configure(Consumer<EdgeCases.Config<T>> configurator, Functi
 		}
 
 		List<Supplier<Shrinkable<T>>> suppliers = configuredEdgeCases.suppliers();
-		for (Predicate<T> filter : new ArrayList<>(filters)) {
+		for (Predicate<? super T> filter : new ArrayList<>(filters)) {
 			suppliers = suppliers.stream().filter(s -> filter.test(s.get().value())).collect(Collectors.toList());
 		}
 		for (T additionalEdgeCase : additionalEdgeCases) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/IntegralGeneratingArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/IntegralGeneratingArbitrary.java
index a1af33b79..549b5259f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/IntegralGeneratingArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/IntegralGeneratingArbitrary.java
@@ -21,7 +21,7 @@ class IntegralGeneratingArbitrary extends TypedCloneable implements Arbitrary<Bi
 	BigInteger shrinkingTarget;
 	RandomDistribution distribution = RandomDistribution.biased();
 
-	private Consumer<EdgeCases.Config<BigInteger>> edgeCasesConfigurator = EdgeCases.Config.noConfig();
+	private Consumer<? super EdgeCases.Config<BigInteger>> edgeCasesConfigurator = EdgeCases.Config.noConfig();
 
 	IntegralGeneratingArbitrary(BigInteger defaultMin, BigInteger defaultMax) {
 		this.min = defaultMin;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java
index 71e86ff3d..ce9c3b793 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java
@@ -6,6 +6,8 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 
+import org.jspecify.annotations.*;
+
 public class JustArbitrary<T> implements Arbitrary<T> {
 
 	private final T value;
@@ -15,13 +17,13 @@ public JustArbitrary(T value) {
 	}
 
 	@Override
-	public <U> Arbitrary<U> flatMap(Function<T, Arbitrary<U>> mapper) {
+	public <U extends @Nullable Object> Arbitrary<U> flatMap(Function<? super T, ? extends Arbitrary<U>> mapper) {
 		// Optimization: just(value).flatMap(mapper) -> mapper(value)
 		return mapper.apply(value);
 	}
 
 	@Override
-	public <U> Arbitrary<U> map(Function<T, U> mapper) {
+	public <U extends @Nullable Object> Arbitrary<U> map(Function<? super T, ? extends U> mapper) {
 		// Optimization: just(value).map(mapper) -> just(mapper(value))
 		return new JustArbitrary<>(mapper.apply(value));
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
index 9afd129f8..74878c7e9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
@@ -12,6 +12,8 @@
 import net.jqwik.engine.properties.shrinking.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 public class LazyOfArbitrary<T> implements Arbitrary<T> {
 
 	// Cached arbitraries only have to survive one property
@@ -24,7 +26,7 @@ private static Store<Map<Integer, LazyOfArbitrary<?>>> arbitrariesStore() {
 	}
 
 	@SuppressWarnings("unchecked")
-	public static <T> Arbitrary<T> of(int hashIdentifier, List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
+	public static <T extends @Nullable Object> Arbitrary<T> of(int hashIdentifier, List<? extends Supplier<? extends Arbitrary<T>>> suppliers) {
 		// It's important for good shrinking to work that the same arbitrary usage is handled by the same arbitrary instance
 		LazyOfArbitrary<?> arbitrary = arbitrariesStore().get().computeIfAbsent(hashIdentifier, ignore -> new LazyOfArbitrary<>(suppliers));
 		return (Arbitrary<T>) arbitrary;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MappedEdgeCasesConsumer.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MappedEdgeCasesConsumer.java
index e8899750b..7c7b3afd7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MappedEdgeCasesConsumer.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MappedEdgeCasesConsumer.java
@@ -31,7 +31,7 @@ public EdgeCases.Config<T> none() {
 			}
 
 			@Override
-			public EdgeCases.Config<T> filter(Predicate<T> filter) {
+			public EdgeCases.Config<T> filter(Predicate<? super T> filter) {
 				uConfig.filter(u -> filter.test(utMapper.apply(u)));
 				return this;
 			}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MultivalueArbitraryBase.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MultivalueArbitraryBase.java
index 0058b7752..46fa0c7e0 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MultivalueArbitraryBase.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/MultivalueArbitraryBase.java
@@ -10,6 +10,8 @@
 import net.jqwik.engine.properties.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
+import org.jspecify.annotations.*;
+
 import static java.util.Arrays.*;
 
 import static net.jqwik.engine.properties.UniquenessChecker.*;
@@ -68,7 +70,7 @@ public StreamableArbitrary<T, U> withSizeDistribution(RandomDistribution distrib
 	}
 
 	@Override
-	public <R> Arbitrary<R> reduce(R initial, BiFunction<R, T, R> accumulator) {
+	public <R extends @Nullable Object> Arbitrary<R> reduce(R initial, BiFunction<R, ? super T, R> accumulator) {
 		return this.map(streamable -> {
 			// Couldn't find a way to use Stream.reduce since it requires a combinator
 			@SuppressWarnings("unchecked")
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java
index bf1234fe3..ebc5c3795 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 public class ReportableStream<T> implements Stream<T> {
 
 	private final List<T> values;
@@ -31,7 +33,7 @@ public Stream<T> filter(final Predicate<? super T> predicate) {
 	}
 
 	@Override
-	public <R> Stream<R> map(final Function<? super T, ? extends R> mapper) {
+	public <R extends @Nullable Object> Stream<R> map(final Function<? super T, ? extends R> mapper) {
 		return stream.map(mapper);
 	}
 
@@ -51,7 +53,7 @@ public DoubleStream mapToDouble(final ToDoubleFunction<? super T> mapper) {
 	}
 
 	@Override
-	public <R> Stream<R> flatMap(final Function<? super T, ? extends Stream<? extends R>> mapper) {
+	public <R extends @Nullable Object> Stream<R> flatMap(final Function<? super T, ? extends Stream<? extends R>> mapper) {
 		return stream.flatMap(mapper);
 	}
 
@@ -136,7 +138,7 @@ public <U> U reduce(final U identity, final BiFunction<U, ? super T, U> accumula
 	}
 
 	@Override
-	public <R> R collect(final Supplier<R> supplier, final BiConsumer<R, ? super T> accumulator, final BiConsumer<R, R> combiner) {
+	public <R extends @Nullable Object> R collect(final Supplier<R> supplier, final BiConsumer<R, ? super T> accumulator, final BiConsumer<R, R> combiner) {
 		return stream.collect(supplier, accumulator, combiner);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/UseGeneratorsArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/UseGeneratorsArbitrary.java
index c5a2eb3fa..2b8090b2b 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/UseGeneratorsArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/UseGeneratorsArbitrary.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-abstract class UseGeneratorsArbitrary<T> implements Arbitrary<T> {
+import org.jspecify.annotations.*;
+
+abstract class UseGeneratorsArbitrary<T extends @Nullable Object> implements Arbitrary<T> {
 
 	private final RandomGenerator<T> randomGenerator;
 	private final Function<Long, Optional<ExhaustiveGenerator<T>>> exhaustiveGeneratorFunction;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java
index 7f97751f4..b6e052766 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java
@@ -12,11 +12,11 @@
 
 public class CombineArbitrary<R> implements Arbitrary<R> {
 
-	private final Function<List<Object>, R> combinator;
+	private final Function<? super List<?>, ? extends R> combinator;
 	private final List<Arbitrary<Object>> arbitraries;
 
 	@SuppressWarnings("unchecked")
-	public CombineArbitrary(Function<List<Object>, R> combinator, Arbitrary<?>... arbitraries) {
+	public CombineArbitrary(Function<? super List<?>, ? extends R> combinator, Arbitrary<?>... arbitraries) {
 		this.combinator = combinator;
 		this.arbitraries = Arrays.asList((Arbitrary<Object>[]) arbitraries);
 	}
@@ -75,7 +75,7 @@ private boolean isCombinedGeneratorMemoizable(List<Arbitrary<Object>> arbitrarie
 
 	private RandomGenerator<R> combineGenerator(
 		int genSize,
-		Function<List<Object>, R> combineFunction,
+		Function<? super List<?>, ? extends R> combineFunction,
 		List<Arbitrary<Object>> arbitraries
 	) {
 		List<RandomGenerator<Object>> generators = arbitraries.stream()
@@ -89,7 +89,7 @@ private RandomGenerator<R> combineGenerator(
 
 	private RandomGenerator<R> combineGeneratorWithEmbeddedEdgeCases(
 		int genSize,
-		Function<List<Object>, R> combineFunction,
+		Function<? super List<?>, ? extends R> combineFunction,
 		List<Arbitrary<Object>> arbitraries
 	) {
 		List<RandomGenerator<Object>> generators =
@@ -111,14 +111,14 @@ private List<Shrinkable<Object>> generateShrinkables(List<RandomGenerator<Object
 	}
 
 	private Shrinkable<R> combineShrinkables(
-		List<Shrinkable<Object>> shrinkables, Function<List<Object>, R> combineFunction
+		List<Shrinkable<Object>> shrinkables, Function<? super List<?>, ? extends R> combineFunction
 	) {
 		return new CombinedShrinkable<>(shrinkables, combineFunction);
 	}
 
 	private Optional<ExhaustiveGenerator<R>> combineExhaustive(
 		List<Arbitrary<Object>> arbitraries,
-		Function<List<Object>, R> combineFunction,
+		Function<? super List<?>, ? extends R> combineFunction,
 		long maxNumberOfSamples
 	) {
 		return ExhaustiveGenerators.combine(arbitraries, combineFunction, maxNumberOfSamples);
@@ -126,7 +126,7 @@ private Optional<ExhaustiveGenerator<R>> combineExhaustive(
 
 	private EdgeCases<R> combineEdgeCases(
 		final List<Arbitrary<Object>> arbitraries,
-		final Function<List<Object>, R> combineFunction,
+		final Function<? super List<?>, ? extends R> combineFunction,
 		int maxEdgeCases
 	) {
 		return EdgeCasesSupport.combine(arbitraries, combineFunction, maxEdgeCases);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator2.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator2.java
index 9dfd77994..1a399a750 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator2.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator2.java
@@ -7,7 +7,7 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator2<T1, T2> implements Combinators.Combinator2<T1, T2> {
+public class DefaultCombinator2<T1 extends @Nullable Object, T2 extends @Nullable Object> implements Combinators.Combinator2<T1, T2> {
 	protected final Arbitrary<T2> a2;
 	protected final Arbitrary<T1> a1;
 
@@ -17,43 +17,43 @@ public DefaultCombinator2(Arbitrary<T1> a1, Arbitrary<T2> a2) {
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F2<T1, T2, @NonNull R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F2<? super T1, ? super T2, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2);
 	}
 
 	@Override
-	public Combinators.Combinator2<T1, T2> filter(Combinators.F2<T1, T2, Boolean> filter) {
+	public Combinators.Combinator2<T1, T2> filter(Combinators.F2<? super T1, ? super T2, Boolean> filter) {
 		return new Filtered<>(a1, a2, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F2<T1, T2, R> combinator2) {
+	protected <R extends @Nullable Object> Function<? super List<?>, ? extends R> combineFunction(Combinators.F2<? super T1, ? super T2, ? extends R> combinator2) {
 		return params -> combinator2.apply((T1) params.get(0), (T2) params.get(1));
 	}
 
-	private static class Filtered<T1, T2> extends DefaultCombinator2<T1, T2> {
-		private final Combinators.F2<T1, T2, Boolean> filter;
+	private static class Filtered<T1 extends @Nullable Object, T2 extends @Nullable Object> extends DefaultCombinator2<T1, T2> {
+		private final Combinators.F2<? super T1, ? super T2, Boolean> filter;
 
-		private Filtered(Arbitrary<T1> a1, Arbitrary<T2> a2, Combinators.F2<T1, T2, Boolean> filter) {
+		private Filtered(Arbitrary<T1> a1, Arbitrary<T2> a2, Combinators.F2<? super T1, ? super T2, Boolean> filter) {
 			super(a1, a2);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F2<T1, T2, @NonNull R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F2<? super T1, ? super T2, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator2<T1, T2> filter(Combinators.F2<T1, T2, Boolean> filter) {
+		public Combinators.Combinator2<T1, T2> filter(Combinators.F2<? super T1, ? super T2, Boolean> filter) {
 			return new Filtered<>(a1, a2, combineFilters(this.filter, filter));
 		}
 
 		private Combinators.F2<T1, T2, Boolean> combineFilters(
-			Combinators.F2<T1, T2, Boolean> first,
-			Combinators.F2<T1, T2, Boolean> second
+			Combinators.F2<? super T1, ? super T2, Boolean> first,
+			Combinators.F2<? super T1, ? super T2, Boolean> second
 		) {
 			return (t1, t2) -> first.apply(t1, t2) && second.apply(t1, t2);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator3.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator3.java
index 53b148a96..bc858e38a 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator3.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator3.java
@@ -7,7 +7,7 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator3<T1, T2, T3>
+public class DefaultCombinator3<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object>
 	extends DefaultCombinator2<T1, T2>
 	implements Combinators.Combinator3<T1, T2, T3> {
 	protected final Arbitrary<T3> a3;
@@ -18,44 +18,44 @@ public DefaultCombinator3(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3)
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F3<T1, T2, T3, @NonNull R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F3<? super T1, ? super T2, ? super T3, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2, a3);
 	}
 
 	@Override
-	public Combinators.Combinator3<T1, T2, T3> filter(Combinators.F3<T1, T2, T3, Boolean> filter) {
+	public Combinators.Combinator3<T1, T2, T3> filter(Combinators.F3<? super T1, ? super T2, ? super T3, Boolean> filter) {
 		return new Filtered<>(a1, a2, a3, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F3<T1, T2, T3, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Combinators.F3<? super T1, ? super T2, ? super T3, ? extends R> combinator) {
 		return params -> combinator.apply((T1) params.get(0), (T2) params.get(1), (T3) params.get(2));
 	}
 
 	private static class Filtered<T1, T2, T3> extends DefaultCombinator3<T1, T2, T3> {
-		private final Combinators.F3<T1, T2, T3, Boolean> filter;
+		private final Combinators.F3<? super T1, ? super T2, ? super T3, Boolean> filter;
 
-		private Filtered(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3, Combinators.F3<T1, T2, T3, Boolean> filter) {
+		private Filtered(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3, Combinators.F3<? super T1, ? super T2, ? super T3, Boolean> filter) {
 			super(a1, a2, a3);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F3<T1, T2, T3, @NonNull R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F3<? super T1, ? super T2, ? super T3, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2, a3)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator3<T1, T2, T3> filter(Combinators.F3<T1, T2, T3, Boolean> filter) {
+		public Combinators.Combinator3<T1, T2, T3> filter(Combinators.F3<? super T1, ? super T2, ? super T3, Boolean> filter) {
 			return new Filtered<>(a1, a2, a3, combineFilters(this.filter, filter));
 		}
 
 		@SuppressWarnings("unchecked")
 		private Combinators.F3<T1, T2, T3, Boolean> combineFilters(
-			Combinators.F3<T1, T2, T3, Boolean> filter1,
-			Combinators.F3<T1, T2, T3, Boolean> filter2
+			Combinators.F3<? super T1, ? super T2, ? super T3, Boolean> filter1,
+			Combinators.F3<? super T1, ? super T2, ? super T3, Boolean> filter2
 		) {
 			return (t1, t2, t3) -> filter1.apply(t1, t2, t3) && filter2.apply(t1, t2, t3);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator4.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator4.java
index 0de2945b7..67c7697f9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator4.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator4.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator4<T1, T2, T3, T4>
+import org.jspecify.annotations.*;
+
+public class DefaultCombinator4<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object>
 	extends DefaultCombinator3<T1, T2, T3>
 	implements Combinators.Combinator4<T1, T2, T3, T4> {
 
@@ -17,49 +19,49 @@ public DefaultCombinator4(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F4<T1, T2, T3, T4, R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2, a3, a4);
 	}
 
 	@Override
-	public Combinators.Combinator4<T1, T2, T3, T4> filter(Combinators.F4<T1, T2, T3, T4, Boolean> filter) {
+	public Combinators.Combinator4<T1, T2, T3, T4> filter(Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> filter) {
 		return new Filtered<>(a1, a2, a3, a4, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F4<T1, T2, T3, T4, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, ? extends R> combinator) {
 		return params -> combinator.apply((T1) params.get(0), (T2) params.get(1), (T3) params.get(2), (T4) params.get(3));
 	}
 
 	private static class Filtered<T1, T2, T3, T4> extends DefaultCombinator4<T1, T2, T3, T4> {
-		private final Combinators.F4<T1, T2, T3, T4, Boolean> filter;
+		private final Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> filter;
 
 		private Filtered(
 			Arbitrary<T1> a1,
 			Arbitrary<T2> a2,
 			Arbitrary<T3> a3,
 			Arbitrary<T4> a4,
-			Combinators.F4<T1, T2, T3, T4, Boolean> filter
+			Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> filter
 		) {
 			super(a1, a2, a3, a4);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F4<T1, T2, T3, T4, R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2, a3, a4)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator4<T1, T2, T3, T4> filter(Combinators.F4<T1, T2, T3, T4, Boolean> filter) {
+		public Combinators.Combinator4<T1, T2, T3, T4> filter(Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> filter) {
 			return super.filter(combineFilters(this.filter, filter));
 		}
 
 		private Combinators.F4<T1, T2, T3, T4, Boolean> combineFilters(
-			Combinators.F4<T1, T2, T3, T4, Boolean> first,
-			Combinators.F4<T1, T2, T3, T4, Boolean> second
+			Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> first,
+			Combinators.F4<? super T1, ? super T2, ? super T3, ? super T4, Boolean> second
 		) {
 			return (p1, p2, p3, p4) -> first.apply(p1, p2, p3, p4) && second.apply(p1, p2, p3, p4);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator5.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator5.java
index 8eee660fc..2ba358525 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator5.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator5.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator5<T1, T2, T3, T4, T5>
+import org.jspecify.annotations.*;
+
+public class DefaultCombinator5<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object>
 	extends DefaultCombinator4<T1, T2, T3, T4>
 	implements Combinators.Combinator5<T1, T2, T3, T4, T5> {
 
@@ -17,17 +19,17 @@ public DefaultCombinator5(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F5<T1, T2, T3, T4, T5, R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2, a3, a4, a5);
 	}
 
 	@Override
-	public Combinators.Combinator5<T1, T2, T3, T4, T5> filter(Combinators.F5<T1, T2, T3, T4, T5, Boolean> filter) {
+	public Combinators.Combinator5<T1, T2, T3, T4, T5> filter(Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> filter) {
 		return new Filtered<>(a1, a2, a3, a4, a5, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F5<T1, T2, T3, T4, T5, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? extends R> combinator) {
 		return params -> combinator.apply(
 			(T1) params.get(0), (T2) params.get(1),
 			(T3) params.get(2), (T4) params.get(3),
@@ -36,7 +38,7 @@ protected <R> Function<List<Object>, R> combineFunction(Combinators.F5<T1, T2, T
 	}
 
 	private static class Filtered<T1, T2, T3, T4, T5> extends DefaultCombinator5<T1, T2, T3, T4, T5> {
-		private final Combinators.F5<T1, T2, T3, T4, T5, Boolean> filter;
+		private final Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> filter;
 
 		private Filtered(
 			Arbitrary<T1> a1,
@@ -44,27 +46,27 @@ private Filtered(
 			Arbitrary<T3> a3,
 			Arbitrary<T4> a4,
 			Arbitrary<T5> a5,
-			Combinators.F5<T1, T2, T3, T4, T5, Boolean> filter
+			Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> filter
 		) {
 			super(a1, a2, a3, a4, a5);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F5<T1, T2, T3, T4, T5, R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2, a3, a4, a5)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator5<T1, T2, T3, T4, T5> filter(Combinators.F5<T1, T2, T3, T4, T5, Boolean> filter) {
+		public Combinators.Combinator5<T1, T2, T3, T4, T5> filter(Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> filter) {
 			return super.filter(combineFilters(this.filter, filter));
 		}
 
 		private Combinators.F5<T1, T2, T3, T4, T5, Boolean> combineFilters(
-			Combinators.F5<T1, T2, T3, T4, T5, Boolean> first,
-			Combinators.F5<T1, T2, T3, T4, T5, Boolean> second
+			Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> first,
+			Combinators.F5<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, Boolean> second
 		) {
 			return (p1, p2, p3, p4, p5) -> first.apply(p1, p2, p3, p4, p5) && second.apply(p1, p2, p3, p4, p5);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator6.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator6.java
index 308877959..bf4184e95 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator6.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator6.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator6<T1, T2, T3, T4, T5, T6>
+import org.jspecify.annotations.*;
+
+public class DefaultCombinator6<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object>
 	extends DefaultCombinator5<T1, T2, T3, T4, T5>
 	implements Combinators.Combinator6<T1, T2, T3, T4, T5, T6> {
 
@@ -17,17 +19,17 @@ public DefaultCombinator6(Arbitrary<T1> a1, Arbitrary<T2> a2, Arbitrary<T3> a3,
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F6<T1, T2, T3, T4, T5, T6, R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2, a3, a4, a5, a6);
 	}
 
 	@Override
-	public Combinators.Combinator6<T1, T2, T3, T4, T5, T6> filter(Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> filter) {
+	public Combinators.Combinator6<T1, T2, T3, T4, T5, T6> filter(Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> filter) {
 		return new Filtered<>(a1, a2, a3, a4, a5, a6, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F6<T1, T2, T3, T4, T5, T6, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? extends R> combinator) {
 		return params -> combinator.apply(
 			(T1) params.get(0), (T2) params.get(1),
 			(T3) params.get(2), (T4) params.get(3),
@@ -36,7 +38,7 @@ protected <R> Function<List<Object>, R> combineFunction(Combinators.F6<T1, T2, T
 	}
 
 	private static class Filtered<T1, T2, T3, T4, T5, T6> extends DefaultCombinator6<T1, T2, T3, T4, T5, T6> {
-		private final Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> filter;
+		private final Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> filter;
 
 		private Filtered(
 			Arbitrary<T1> a1,
@@ -45,27 +47,27 @@ private Filtered(
 			Arbitrary<T4> a4,
 			Arbitrary<T5> a5,
 			Arbitrary<T6> a6,
-			Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> filter
+			Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> filter
 		) {
 			super(a1, a2, a3, a4, a5, a6);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F6<T1, T2, T3, T4, T5, T6, R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2, a3, a4, a5, a6)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator6<T1, T2, T3, T4, T5, T6> filter(Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> filter) {
+		public Combinators.Combinator6<T1, T2, T3, T4, T5, T6> filter(Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> filter) {
 			return super.filter(combineFilters(this.filter, filter));
 		}
 
 		private Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> combineFilters(
-			Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> first,
-			Combinators.F6<T1, T2, T3, T4, T5, T6, Boolean> second
+			Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> first,
+			Combinators.F6<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, Boolean> second
 		) {
 			return (p1, p2, p3, p4, p5, p6) -> first.apply(p1, p2, p3, p4, p5, p6)
 												   && second.apply(p1, p2, p3, p4, p5, p6);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator7.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator7.java
index bcb649513..c6c51e85c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator7.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator7.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator7<T1, T2, T3, T4, T5, T6, T7>
+import org.jspecify.annotations.*;
+
+public class DefaultCombinator7<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object>
 	extends DefaultCombinator6<T1, T2, T3, T4, T5, T6>
 	implements Combinators.Combinator7<T1, T2, T3, T4, T5, T6, T7> {
 
@@ -19,17 +21,17 @@ public DefaultCombinator7(
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F7<T1, T2, T3, T4, T5, T6, T7, R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2, a3, a4, a5, a6, a7);
 	}
 
 	@Override
-	public Combinators.Combinator7<T1, T2, T3, T4, T5, T6, T7> filter(Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> filter) {
+	public Combinators.Combinator7<T1, T2, T3, T4, T5, T6, T7> filter(Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> filter) {
 		return new Filtered<>(a1, a2, a3, a4, a5, a6, a7, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F7<T1, T2, T3, T4, T5, T6, T7, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? extends R> combinator) {
 		return params -> combinator.apply(
 			(T1) params.get(0), (T2) params.get(1),
 			(T3) params.get(2), (T4) params.get(3),
@@ -39,7 +41,7 @@ protected <R> Function<List<Object>, R> combineFunction(Combinators.F7<T1, T2, T
 	}
 
 	private static class Filtered<T1, T2, T3, T4, T5, T6, T7> extends DefaultCombinator7<T1, T2, T3, T4, T5, T6, T7> {
-		private final Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> filter;
+		private final Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> filter;
 
 		private Filtered(
 			Arbitrary<T1> a1,
@@ -49,27 +51,27 @@ private Filtered(
 			Arbitrary<T5> a5,
 			Arbitrary<T6> a6,
 			Arbitrary<T7> a7,
-			Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> filter
+			Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> filter
 		) {
 			super(a1, a2, a3, a4, a5, a6, a7);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F7<T1, T2, T3, T4, T5, T6, T7, R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2, a3, a4, a5, a6, a7)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator7<T1, T2, T3, T4, T5, T6, T7> filter(Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> filter) {
+		public Combinators.Combinator7<T1, T2, T3, T4, T5, T6, T7> filter(Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> filter) {
 			return super.filter(combineFilters(this.filter, filter));
 		}
 
 		private Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> combineFilters(
-			Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> first,
-			Combinators.F7<T1, T2, T3, T4, T5, T6, T7, Boolean> second
+			Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> first,
+			Combinators.F7<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, Boolean> second
 		) {
 			return (p1, p2, p3, p4, p5, p6, p7) -> first.apply(p1, p2, p3, p4, p5, p6, p7) && second.apply(p1, p2, p3, p4, p5, p6, p7);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator8.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator8.java
index 1cd2761f1..ec6ad98ac 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator8.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultCombinator8.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class DefaultCombinator8<T1, T2, T3, T4, T5, T6, T7, T8>
+import org.jspecify.annotations.*;
+
+public class DefaultCombinator8<T1 extends @Nullable Object, T2 extends @Nullable Object, T3 extends @Nullable Object, T4 extends @Nullable Object, T5 extends @Nullable Object, T6 extends @Nullable Object, T7 extends @Nullable Object, T8 extends @Nullable Object>
 	extends DefaultCombinator7<T1, T2, T3, T4, T5, T6, T7>
 	implements Combinators.Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> {
 
@@ -20,17 +22,17 @@ public DefaultCombinator8(
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), a1, a2, a3, a4, a5, a6, a7, a8);
 	}
 
 	@Override
-	public Combinators.Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> filter(Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> filter) {
+	public Combinators.Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> filter(Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> filter) {
 		return new Filtered<>(a1, a2, a3, a4, a5, a6, a7, a8, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, ? extends R> combinator) {
 		return params -> combinator.apply(
 			(T1) params.get(0), (T2) params.get(1),
 			(T3) params.get(2), (T4) params.get(3),
@@ -40,7 +42,7 @@ protected <R> Function<List<Object>, R> combineFunction(Combinators.F8<T1, T2, T
 	}
 
 	private static class Filtered<T1, T2, T3, T4, T5, T6, T7, T8> extends DefaultCombinator8<T1, T2, T3, T4, T5, T6, T7, T8> {
-		private final Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> filter;
+		private final Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> filter;
 
 		private Filtered(
 			Arbitrary<T1> a1,
@@ -51,27 +53,27 @@ private Filtered(
 			Arbitrary<T6> a6,
 			Arbitrary<T7> a7,
 			Arbitrary<T8> a8,
-			Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> filter
+			Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> filter
 		) {
 			super(a1, a2, a3, a4, a5, a6, a7, a8);
 			this.filter = filter;
 		}
 
 		@Override
-		public <R> Arbitrary<R> as(Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, R> combinator) {
+		public <R extends @Nullable Object> Arbitrary<R> as(Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, ? extends R> combinator) {
 			return new CombineArbitrary<>(Function.identity(), a1, a2, a3, a4, a5, a6, a7, a8)
 					   .filter(combineFunction(filter)::apply)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> filter(Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> filter) {
+		public Combinators.Combinator8<T1, T2, T3, T4, T5, T6, T7, T8> filter(Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> filter) {
 			return super.filter(combineFilters(this.filter, filter));
 		}
 
 		private Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> combineFilters(
-			Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> first,
-			Combinators.F8<T1, T2, T3, T4, T5, T6, T7, T8, Boolean> second
+			Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> first,
+			Combinators.F8<? super T1, ? super T2, ? super T3, ? super T4, ? super T5, ? super T6, ? super T7, ? super T8, Boolean> second
 		) {
 			return (p1, p2, p3, p4, p5, p6, p7, p8) -> first.apply(p1, p2, p3, p4, p5, p6, p7, p8) && second.apply(p1, p2, p3, p4, p5, p6, p7, p8);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
index 5c661c4f7..5594cf0d7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
@@ -24,44 +24,44 @@ private DefaultListCombinator(Arbitrary<T>[] arbitraries) {
 	}
 
 	@Override
-	public <R> Arbitrary<R> as(Function<List<T>, @NonNull R> combinator) {
+	public <R extends @Nullable Object> Arbitrary<R> as(Function<? super List<T>, ? extends R> combinator) {
 		return new CombineArbitrary<>(combineFunction(combinator), arbitraries);
 	}
 
 	@API(status = EXPERIMENTAL, since = "1.7.1")
-	public Combinators.ListCombinator<T> filter(Predicate<List<T>> filter) {
+	public Combinators.ListCombinator<T> filter(Predicate<? super List<? extends T>> filter) {
 		return new Filtered<>(arbitraries, filter);
 	}
 
 	@SuppressWarnings("unchecked")
-	protected <R> Function<List<Object>, R> combineFunction(Function<List<T>, R> combinator) {
+	protected <R extends @Nullable Object> Function<List<?>, R> combineFunction(Function<? super List<T>, ? extends R> combinator) {
 		return params -> combinator.apply((List<T>) params);
 	}
 
 	private static class Filtered<T> extends DefaultListCombinator<T> {
-		private final Predicate<List<T>> filter;
+		private final Predicate<? super List<? extends T>> filter;
 
-		private Filtered(Arbitrary<T>[] arbitraries, Predicate<List<T>> filter) {
+		private Filtered(Arbitrary<T>[] arbitraries, Predicate<? super List<? extends T>> filter) {
 			super(arbitraries);
 			this.filter = filter;
 		}
 
 		@SuppressWarnings("unchecked")
 		@Override
-		public <R> Arbitrary<R> as(Function<List<T>, @NonNull R> combinator) {
-			Predicate<List<Object>> filterPredicate = params -> filter.test((List<T>) params);
+		public <R extends @Nullable Object> Arbitrary<R> as(Function<? super List<T>, ? extends R> combinator) {
+			Predicate<? super List<?>> filterPredicate = params -> filter.test((List<T>) params);
 			return new CombineArbitrary<>(Function.identity(), arbitraries)
 					   .filter(filterPredicate)
 					   .map(combineFunction(combinator));
 		}
 
 		@Override
-		public Combinators.ListCombinator<T> filter(Predicate<List<T>> filter) {
+		public Combinators.ListCombinator<T> filter(Predicate<? super List<? extends T>> filter) {
 			return super.filter(combineFilters(this.filter, filter));
 		}
 
-		private Predicate<List<T>> combineFilters(Predicate<List<T>> first, Predicate<List<T>> second) {
-			return first.and(second);
+		private Predicate<? super List<? extends T>> combineFilters(Predicate<? super List<? extends T>> first, Predicate<? super List<? extends T>> second) {
+			return it -> first.test(it) && second.test(it);
 		}
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/package-info.java
new file mode 100644
index 000000000..ac173e93e
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.arbitraries.combinations;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java
index c60eacfc9..8701d9226 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java
@@ -10,7 +10,7 @@
 class CombinedExhaustiveGenerator<R> implements ExhaustiveGenerator<R> {
 	private final Long maxCount;
 	private final List<Arbitrary<Object>> arbitraries;
-	private final Function<List<Object>, R> combinator;
+	private final Function<? super List<?>, ? extends R> combinator;
 
 	static Optional<Long> calculateMaxCount(List<Arbitrary<Object>> arbitraries, long maxNumberOfSamples) {
 		long product = 1;
@@ -27,7 +27,7 @@ static Optional<Long> calculateMaxCount(List<Arbitrary<Object>> arbitraries, lon
 		return Optional.of(product);
 	}
 
-	CombinedExhaustiveGenerator(Long maxCount, List<Arbitrary<Object>> arbitraries, Function<List<Object>, R> combinator) {
+	CombinedExhaustiveGenerator(Long maxCount, List<Arbitrary<Object>> arbitraries, Function<? super List<?>, ? extends R> combinator) {
 		this.maxCount = maxCount;
 		this.arbitraries = arbitraries;
 		this.combinator = combinator;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
index 2c8b50ca7..da0e60611 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
@@ -7,15 +7,17 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.properties.UniquenessChecker.*;
 
 public class ExhaustiveGenerators {
 
-	public static <T> Optional<ExhaustiveGenerator<T>> create(Supplier<T> supplier, long maxNumberOfSamples) {
+	public static <T extends @Nullable Object> Optional<ExhaustiveGenerator<T>> create(Supplier<T> supplier, long maxNumberOfSamples) {
 		return fromIterable(() -> new SupplierIterator<>(supplier), 1, maxNumberOfSamples);
 	}
 
-	public static <T> Optional<ExhaustiveGenerator<T>> choose(List<T> values, long maxNumberOfSamples) {
+	public static <T extends @Nullable Object> Optional<ExhaustiveGenerator<T>> choose(List<T> values, long maxNumberOfSamples) {
 		return fromIterable(values, values.size(), maxNumberOfSamples);
 	}
 
@@ -27,17 +29,17 @@ public static Optional<ExhaustiveGenerator<Character>> choose(char[] characters,
 		return choose(validCharacters, maxNumberOfSamples);
 	}
 
-	public static <T> Optional<ExhaustiveGenerator<T>> fromIterable(Iterable<T> iterator, long maxCount, long maxNumberOfSamples) {
+	public static <T extends @Nullable Object> Optional<ExhaustiveGenerator<T>> fromIterable(Iterable<T> iterator, long maxCount, long maxNumberOfSamples) {
 		if (maxCount > maxNumberOfSamples) {
 			return Optional.empty();
 		}
 		return Optional.of(new IterableBasedExhaustiveGenerator<>(iterator, maxCount));
 	}
 
-	public static <T> Optional<ExhaustiveGenerator<List<T>>> list(
+	public static <T extends @Nullable Object> Optional<ExhaustiveGenerator<List<T>>> list(
 			Arbitrary<T> elementArbitrary,
 			int minSize, int maxSize,
-			Collection<FeatureExtractor<T>> uniquenessExtractors,
+			Collection<? extends FeatureExtractor<T>> uniquenessExtractors,
 			long maxNumberOfSamples
 	) {
 		Optional<Long> optionalMaxCount = ListExhaustiveGenerator.calculateMaxCount(elementArbitrary, minSize, maxSize, maxNumberOfSamples);
@@ -66,7 +68,7 @@ public static Optional<ExhaustiveGenerator<String>> strings(
 				));
 	}
 
-	public static <T> Optional<ExhaustiveGenerator<Set<T>>> set(
+	public static <T extends @Nullable Object> Optional<ExhaustiveGenerator<Set<T>>> set(
 			Arbitrary<T> elementArbitrary,
 			int minSize, int maxSize,
 			Collection<FeatureExtractor<T>> featureExtractors,
@@ -79,23 +81,23 @@ public static <T> Optional<ExhaustiveGenerator<Set<T>>> set(
 		);
 	}
 
-	public static <R> Optional<ExhaustiveGenerator<R>> combine(
+	public static <R extends @Nullable Object> Optional<ExhaustiveGenerator<R>> combine(
 			List<Arbitrary<Object>> arbitraries,
-			Function<List<Object>, R> combinator,
+			Function<? super List<?>, ? extends R> combinator,
 			long maxNumberOfSamples
 	) {
 		Optional<Long> optionalMaxCount = CombinedExhaustiveGenerator.calculateMaxCount(arbitraries, maxNumberOfSamples);
 		return optionalMaxCount.map(maxCount -> new CombinedExhaustiveGenerator<>(maxCount, arbitraries, combinator));
 	}
 
-	public static <T> Optional<ExhaustiveGenerator<List<T>>> shuffle(List<T> values, long maxNumberOfSamples) {
+	public static <T extends @Nullable Object> Optional<ExhaustiveGenerator<List<T>>> shuffle(List<T> values, long maxNumberOfSamples) {
 		Optional<Long> optionalMaxCount = PermutationExhaustiveGenerator.calculateMaxCount(values, maxNumberOfSamples);
 		return optionalMaxCount.map(
 			maxCount -> new PermutationExhaustiveGenerator<>(values, maxCount)
 		);
 	}
 
-	public static <U, T> Optional<ExhaustiveGenerator<U>> flatMap(
+	public static <U extends @Nullable Object, T extends @Nullable Object> Optional<ExhaustiveGenerator<U>> flatMap(
 		ExhaustiveGenerator<T> base,
 		Function<? super T, ? extends Arbitrary<U>> mapper,
 		long maxNumberOfSamples
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java
index d95caed54..11442b458 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java
@@ -7,10 +7,10 @@
 
 public class FilteredExhaustiveGenerator<T> implements ExhaustiveGenerator<T> {
 	private final ExhaustiveGenerator<T> toFilter;
-	private final Predicate<T> filter;
+	private final Predicate<? super T> filter;
 	private int maxMisses;
 
-	public FilteredExhaustiveGenerator(ExhaustiveGenerator<T> toFilter, Predicate<T> filter, int maxMisses) {
+	public FilteredExhaustiveGenerator(ExhaustiveGenerator<T> toFilter, Predicate<? super T> filter, int maxMisses) {
 		this.toFilter = toFilter;
 		this.filter = filter;
 		this.maxMisses = maxMisses;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
index 3db50eaa9..8f6f14ba4 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FlatMappedExhaustiveGenerator.java
@@ -7,7 +7,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-public class FlatMappedExhaustiveGenerator<U, T> implements ExhaustiveGenerator<U> {
+import org.jspecify.annotations.*;
+
+public class FlatMappedExhaustiveGenerator<U extends @Nullable Object, T extends @Nullable Object> implements ExhaustiveGenerator<U> {
 	private final ExhaustiveGenerator<T> baseGenerator;
 	private final long maxCount;
 	private final Function<? super T, ? extends Arbitrary<U>> mapper;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IgnoreExceptionExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IgnoreExceptionExhaustiveGenerator.java
index 2f201ff51..b63317ead 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IgnoreExceptionExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IgnoreExceptionExhaustiveGenerator.java
@@ -4,9 +4,11 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.support.JqwikExceptionSupport.*;
 
-public class IgnoreExceptionExhaustiveGenerator<T> implements ExhaustiveGenerator<T> {
+public class IgnoreExceptionExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<T> {
 	private final ExhaustiveGenerator<T> toFilter;
 	private final Class<? extends Throwable>[] exceptionTypes;
 	private final int maxThrows;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IterableBasedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IterableBasedExhaustiveGenerator.java
index b16cf872a..475561735 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IterableBasedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/IterableBasedExhaustiveGenerator.java
@@ -4,7 +4,9 @@
 
 import net.jqwik.api.*;
 
-class IterableBasedExhaustiveGenerator<T> implements ExhaustiveGenerator<T> {
+import org.jspecify.annotations.*;
+
+class IterableBasedExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<T> {
 
 	final private Iterable<T> iterable;
 	final private long maxCount;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ListExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ListExhaustiveGenerator.java
index af032d800..bcd3e2ad9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ListExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ListExhaustiveGenerator.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-class ListExhaustiveGenerator<T> implements ExhaustiveGenerator<List<T>> {
+import org.jspecify.annotations.*;
+
+class ListExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<List<T>> {
 	private final Arbitrary<T> elementArbitrary;
 	private final Long maxCount;
 	private final int minSize;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/MappedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/MappedExhaustiveGenerator.java
index b6d839fe2..600dfbc42 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/MappedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/MappedExhaustiveGenerator.java
@@ -5,11 +5,13 @@
 
 import net.jqwik.api.*;
 
-public class MappedExhaustiveGenerator<T, U> implements ExhaustiveGenerator<U> {
+import org.jspecify.annotations.*;
+
+public class MappedExhaustiveGenerator<T extends @Nullable Object, U extends @Nullable Object> implements ExhaustiveGenerator<U> {
 	private final ExhaustiveGenerator<T> toMap;
-	private final Function<T, U> mapper;
+	private final Function<? super T, ? extends U> mapper;
 
-	public MappedExhaustiveGenerator(ExhaustiveGenerator<T> toMap, Function<T, U> mapper) {this.toMap = toMap;
+	public MappedExhaustiveGenerator(ExhaustiveGenerator<T> toMap, Function<? super T, ? extends U> mapper) {this.toMap = toMap;
 		this.mapper = mapper;
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/PermutationExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/PermutationExhaustiveGenerator.java
index 08abaed66..49735895c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/PermutationExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/PermutationExhaustiveGenerator.java
@@ -5,9 +5,11 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.support.MathSupport.*;
 
-class PermutationExhaustiveGenerator<T> implements ExhaustiveGenerator<List<T>> {
+class PermutationExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<List<T>> {
 	private final List<T> values;
 	private final Long maxCount;
 
@@ -16,7 +18,7 @@ public PermutationExhaustiveGenerator(List<T> values, Long maxCount) {
 		this.maxCount = maxCount;
 	}
 
-	static <T> Optional<Long> calculateMaxCount(List<T> values, long maxNumberOfSamples) {
+	static Optional<Long> calculateMaxCount(List<?> values, long maxNumberOfSamples) {
 		try {
 			long choices = factorial(values.size());
 			if (choices > maxNumberOfSamples || choices < 0) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/SetExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/SetExhaustiveGenerator.java
index d8bd74c19..8475b4c8a 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/SetExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/SetExhaustiveGenerator.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-class SetExhaustiveGenerator<T> implements ExhaustiveGenerator<Set<T>> {
+import org.jspecify.annotations.*;
+
+class SetExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<Set<T>> {
 	private final Arbitrary<T> elementArbitrary;
 	private final long maxCount;
 	private final int minSize;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/WithNullExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/WithNullExhaustiveGenerator.java
index 40b15a12b..dc786d124 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/WithNullExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/WithNullExhaustiveGenerator.java
@@ -4,7 +4,9 @@
 
 import net.jqwik.api.*;
 
-public class WithNullExhaustiveGenerator<T> implements ExhaustiveGenerator<T> {
+import org.jspecify.annotations.*;
+
+public class WithNullExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<T> {
 	private final ExhaustiveGenerator<T> base;
 
 	public WithNullExhaustiveGenerator(ExhaustiveGenerator<T> base) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/package-info.java
new file mode 100644
index 000000000..a3a6194df
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.arbitraries.exhaustive;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/package-info.java
new file mode 100644
index 000000000..f556b14f3
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.arbitraries;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java
index c0364015b..7122e33c5 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java
@@ -8,9 +8,9 @@
 
 public class CollectGenerator<T> implements RandomGenerator<List<T>> {
 	private final RandomGenerator<T> elementGenerator;
-	private final Predicate<List<T>> until;
+	private final Predicate<? super List<? extends T>> until;
 
-	public CollectGenerator(RandomGenerator<T> elementGenerator, Predicate<List<T>> until) {
+	public CollectGenerator(RandomGenerator<T> elementGenerator, Predicate<? super List<? extends T>> until) {
 		this.elementGenerator = elementGenerator;
 		this.until = until;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ContainerGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ContainerGenerator.java
index 1b142dffd..1ab410f8d 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ContainerGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/ContainerGenerator.java
@@ -10,10 +10,10 @@
 
 class ContainerGenerator<T, C> implements RandomGenerator<C> {
 	private final RandomGenerator<T> elementGenerator;
-	private final Function<List<Shrinkable<T>>, Shrinkable<C>> createShrinkable;
+	private final Function<? super List<? extends Shrinkable<T>>, ? extends Shrinkable<C>> createShrinkable;
 	private final int minSize;
 	private final long maxUniqueElements;
-	private final Collection<FeatureExtractor<T>> uniquenessExtractors;
+	private final Collection<? extends FeatureExtractor<T>> uniquenessExtractors;
 	private final Function<Random, Integer> sizeGenerator;
 	private final long maxAttempts;
 
@@ -28,13 +28,13 @@ private static Function<Random, Integer> sizeGenerator(
 
 	ContainerGenerator(
 		RandomGenerator<T> elementGenerator,
-		Function<List<Shrinkable<T>>, Shrinkable<C>> createShrinkable,
+		Function<? super List<? extends Shrinkable<T>>, ? extends Shrinkable<C>> createShrinkable,
 		int minSize,
 		int maxSize,
 		long maxUniqueElements,
 		int genSize,
 		RandomDistribution sizeDistribution,
-		Collection<FeatureExtractor<T>> uniquenessExtractors
+		Collection<? extends FeatureExtractor<T>> uniquenessExtractors
 	) {
 		this.elementGenerator = elementGenerator;
 		this.createShrinkable = createShrinkable;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java
index 07f42e86a..a5f3c8c14 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java
@@ -8,10 +8,10 @@
 
 public class FilteredGenerator<T> implements RandomGenerator<T> {
 	private final RandomGenerator<T> toFilter;
-	private final Predicate<T> filterPredicate;
+	private final Predicate<? super T> filterPredicate;
 	private int maxMisses;
 
-	public FilteredGenerator(RandomGenerator<T> toFilter, Predicate<T> filterPredicate, int maxMisses) {
+	public FilteredGenerator(RandomGenerator<T> toFilter, Predicate<? super T> filterPredicate, int maxMisses) {
 		this.toFilter = toFilter;
 		this.filterPredicate = filterPredicate;
 		this.maxMisses = maxMisses;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
index 5e6fb3a1e..02bbfeeca 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/RandomGenerators.java
@@ -10,6 +10,8 @@
 import net.jqwik.engine.properties.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 import static java.lang.Math.*;
 
 public class RandomGenerators {
@@ -24,8 +26,8 @@ public static int collectionMaxSize(int minSize, Integer maxSize) {
 		return max(RandomGenerators.DEFAULT_COLLECTION_SIZE, doubleMinSize);
 	}
 
-	public static <U> RandomGenerator<U> choose(List<U> values) {
-		if (values.size() == 0) {
+	public static <U extends @Nullable Object> RandomGenerator<U> choose(List<? extends U> values) {
+		if (values.isEmpty()) {
 			return fail("empty set of values");
 		}
 		return random -> {
@@ -34,12 +36,12 @@ public static <U> RandomGenerator<U> choose(List<U> values) {
 		};
 	}
 
-	public static <U> U chooseValue(List<U> values, Random random) {
+	public static <U extends @Nullable Object> U chooseValue(List<? extends U> values, Random random) {
 		int index = random.nextInt(values.size());
 		return values.get(index);
 	}
 
-	public static <U> RandomGenerator<U> choose(U[] values) {
+	public static <U extends @Nullable Object> RandomGenerator<U> choose(U[] values) {
 		return choose(Arrays.asList(values));
 	}
 
@@ -84,11 +86,11 @@ public static RandomGenerator<BigDecimal> bigDecimals(
 		return RandomDecimalGenerators.bigDecimals(1000, range, scale, distribution, shrinkingTarget);
 	}
 
-	public static <T> RandomGenerator<T> oneOf(List<RandomGenerator<T>> all) {
+	public static <T extends @Nullable Object> RandomGenerator<T> oneOf(List<? extends RandomGenerator<T>> all) {
 		return choose(all).flatMap(Function.identity());
 	}
 
-	public static <T> RandomGenerator<List<T>> shuffle(List<T> values) {
+	public static <T extends @Nullable Object> RandomGenerator<List<T>> shuffle(List<T> values) {
 		return random -> {
 			List<T> clone = new ArrayList<>(values);
 			Collections.shuffle(clone, random);
@@ -103,17 +105,17 @@ public static RandomGenerator<String> strings(
 		Arbitrary<Character> characterArbitrary,
 		boolean uniqueChars
 	) {
-		Function<List<Shrinkable<Character>>, Shrinkable<String>> createShrinkable = elements -> new ShrinkableString(elements, minLength, maxLength, characterArbitrary, uniqueChars);
+		Function<List<? extends Shrinkable<Character>>, Shrinkable<String>> createShrinkable = elements -> new ShrinkableString(elements, minLength, maxLength, characterArbitrary, uniqueChars);
 		Set<FeatureExtractor<Character>> featureExtractors = uniqueChars ? ShrinkableString.UNIQUE_CHARS_EXTRACTOR : Collections.emptySet();
 		return container(elementGenerator, createShrinkable, minLength, maxLength, maxUniqueChars, genSize, lengthDistribution, featureExtractors);
 	}
 
-	private static <T, C> RandomGenerator<C> container(
+	private static <T extends @Nullable Object, C extends @Nullable Object> RandomGenerator<C> container(
 		RandomGenerator<T> elementGenerator,
-		Function<List<Shrinkable<T>>, Shrinkable<C>> createShrinkable,
+		Function<? super List<? extends Shrinkable<T>>, ? extends Shrinkable<C>> createShrinkable,
 		int minSize, int maxSize, long maxUniqueElements,
 		int genSize, RandomDistribution sizeDistribution,
-		Set<FeatureExtractor<T>> uniquenessExtractors
+		Set<? extends FeatureExtractor<T>> uniquenessExtractors
 	) {
 		if (minSize > maxSize) {
 			String message = String.format("minSize <%s> must not be larger than maxSize <%s>.", minSize, maxSize);
@@ -127,19 +129,19 @@ private static <T, C> RandomGenerator<C> container(
 		);
 	}
 
-	public static <T> RandomGenerator<List<T>> list(
+	public static <T extends @Nullable Object> RandomGenerator<List<T>> list(
 		RandomGenerator<T> elementGenerator,
 		int minSize, int maxSize, long maxUniqueElements,
 		int genSize, RandomDistribution sizeDistribution,
-		Set<FeatureExtractor<T>> uniquenessExtractors,
+		Set<? extends FeatureExtractor<T>> uniquenessExtractors,
 		Arbitrary<T> elementArbitrary
 	) {
-		Function<List<Shrinkable<T>>, Shrinkable<List<T>>> createShrinkable =
+		Function<List<? extends Shrinkable<T>>, Shrinkable<List<T>>> createShrinkable =
 			elements -> new ShrinkableList<>(elements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
 		return container(elementGenerator, createShrinkable, minSize, maxSize, maxUniqueElements, genSize, sizeDistribution, uniquenessExtractors);
 	}
 
-	public static <T> RandomGenerator<Set<T>> set(
+	public static <T extends @Nullable Object> RandomGenerator<Set<T>> set(
 		RandomGenerator<T> elementGenerator,
 		int minSize, int maxSize, int genSize,
 		Arbitrary<T> elementArbitrary
@@ -147,20 +149,20 @@ public static <T> RandomGenerator<Set<T>> set(
 		return set(elementGenerator, minSize, maxSize, genSize, null, Collections.emptySet(), elementArbitrary);
 	}
 
-	public static <T> RandomGenerator<Set<T>> set(
+	public static <T extends @Nullable Object> RandomGenerator<Set<T>> set(
 			RandomGenerator<T> elementGenerator,
 			int minSize, int maxSize, int genSize, RandomDistribution sizeDistribution,
-			Set<FeatureExtractor<T>> uniquenessExtractors,
+			Set<? extends FeatureExtractor<T>> uniquenessExtractors,
 			Arbitrary<T> elementArbitrary
 	) {
 		Set<FeatureExtractor<T>> extractors = new LinkedHashSet<>(uniquenessExtractors);
 		extractors.add(FeatureExtractor.identity());
-		Function<List<Shrinkable<T>>, Shrinkable<Set<T>>> createShrinkable =
+		Function<List<? extends Shrinkable<T>>, ? extends Shrinkable<Set<T>>> createShrinkable =
 			elements -> new ShrinkableSet<T>(elements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
 		return container(elementGenerator, createShrinkable, minSize, maxSize, maxSize, genSize, sizeDistribution, extractors);
 	}
 
-	public static <T> RandomGenerator<T> samplesFromShrinkables(List<Shrinkable<T>> samples) {
+	public static <T extends @Nullable Object> RandomGenerator<T> samplesFromShrinkables(List<? extends Shrinkable<T>> samples) {
 		AtomicInteger tryCount = new AtomicInteger(0);
 		return ignored -> {
 			if (tryCount.get() >= samples.size())
@@ -169,16 +171,16 @@ public static <T> RandomGenerator<T> samplesFromShrinkables(List<Shrinkable<T>>
 		};
 	}
 
-	public static <T> RandomGenerator<T> samples(T[] samples) {
+	public static <T extends @Nullable Object> RandomGenerator<T> samples(T[] samples) {
 		List<Shrinkable<T>> shrinkables = SampleShrinkable.listOf(samples);
 		return samplesFromShrinkables(shrinkables);
 	}
 
-	public static <T> RandomGenerator<T> frequency(List<? extends Tuple2<Integer, ? extends T>> frequencies) {
+	public static <T extends @Nullable Object> RandomGenerator<T> frequency(List<? extends Tuple2<Integer, ? extends T>> frequencies) {
 		return new FrequencyGenerator<>(frequencies);
 	}
 
-	public static <T> RandomGenerator<T> frequencyOf(
+	public static <T extends @Nullable Object> RandomGenerator<T> frequencyOf(
 			List<? extends Tuple2<Integer, ? extends Arbitrary<T>>> frequencies,
 			int genSize,
 			boolean withEmbeddedEdgeCases
@@ -186,14 +188,14 @@ public static <T> RandomGenerator<T> frequencyOf(
 		return frequency(frequencies).flatMap(Function.identity(), genSize, withEmbeddedEdgeCases);
 	}
 
-	public static <T> RandomGenerator<T> withEdgeCases(RandomGenerator<T> self, int genSize, EdgeCases<T> edgeCases) {
+	public static <T extends @Nullable Object> RandomGenerator<T> withEdgeCases(RandomGenerator<T> self, int genSize, EdgeCases<T> edgeCases) {
 		if (edgeCases.isEmpty()) {
 			return self;
 		}
 		return new WithEdgeCasesGenerator<>(self, edgeCases, genSize);
 	}
 
-	public static <T> RandomGenerator<T> fail(String message) {
+	public static <T extends @Nullable Object> RandomGenerator<T> fail(String message) {
 		return ignored -> {
 			throw new JqwikException(message);
 		};
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java
index b961dd5ed..6f20ad0a6 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java
@@ -6,6 +6,8 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.*;
 
+import org.jspecify.annotations.*;
+
 class WithEdgeCasesGenerator<T> implements RandomGenerator<T> {
 
 	private final RandomGenerator<T> base;
@@ -27,7 +29,7 @@ public Shrinkable<T> next(final Random random) {
 		}
 	}
 
-	private static <T> RandomGenerator<T> chooseEdgeCase(EdgeCases<T> edgeCases) {
+	private static <T extends @Nullable Object> RandomGenerator<T> chooseEdgeCase(EdgeCases<T> edgeCases) {
 		final List<Supplier<Shrinkable<T>>> suppliers = edgeCases.suppliers();
 		return random -> RandomGenerators.chooseValue(suppliers, random).get();
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/package-info.java
new file mode 100644
index 000000000..ac018d97f
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.arbitraries.randomized;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/configurators/UniqueElementsConfigurator.java b/engine/src/main/java/net/jqwik/engine/properties/configurators/UniqueElementsConfigurator.java
index 72b3e32b0..fedc890ad 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/configurators/UniqueElementsConfigurator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/configurators/UniqueElementsConfigurator.java
@@ -12,12 +12,14 @@
 import net.jqwik.api.providers.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 @SuppressWarnings("unchecked")
 public class UniqueElementsConfigurator implements ArbitraryConfigurator {
 
 	@SuppressWarnings("OverlyComplexMethod")
 	@Override
-	public <T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
+	public <T extends @Nullable Object> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
 		return targetType.findAnnotation(UniqueElements.class).map(uniqueness -> {
 			if (arbitrary instanceof SetArbitrary) {
 				// Handle SetArbitrary explicitly for optimization
@@ -36,7 +38,7 @@ public <T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType)
 			}
 			if (targetType.isArray()) {
 				Arbitrary<Object[]> arrayArbitrary = (Arbitrary<Object[]>) arbitrary;
-				Predicate<List<? extends Object>> predicate = isUnique(uniqueness);
+				Predicate<List<?>> predicate = isUnique(uniqueness);
 				return (Arbitrary<T>) arrayArbitrary.filter(array -> predicate.test(Arrays.asList(array)));
 			}
 			if (targetType.isAssignableFrom(Stream.class)) {
@@ -56,7 +58,7 @@ public <T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType)
 		}).orElse(arbitrary);
 	}
 
-	private <T> List<T> toList(Iterator<T> i) {
+	private <T extends @Nullable Object> List<T> toList(Iterator<T> i) {
 		List<T> list = new ArrayList<>();
 		while (i.hasNext()) {
 			list.add(i.next());
@@ -64,7 +66,7 @@ private <T> List<T> toList(Iterator<T> i) {
 		return list;
 	}
 
-	private static <C extends Collection<? extends Object>> Predicate<C> isUnique(UniqueElements uniqueness) {
+	private static <C extends Collection<?>> Predicate<C> isUnique(UniqueElements uniqueness) {
 		Class<? extends Function<?, Object>> extractorClass = uniqueness.by();
 		if (extractorClass.equals(UniqueElements.NOT_SET.class)) {
 			return items -> {
@@ -98,7 +100,7 @@ private static <C extends Collection<? extends Object>> Predicate<C> isUnique(Un
 		};
 	}
 
-	private <T> Arbitrary<?> configureStreamableArbitrary(StreamableArbitrary<T, ?> arbitrary, UniqueElements uniqueness) {
+	private <T extends @Nullable Object> Arbitrary<?> configureStreamableArbitrary(StreamableArbitrary<T, ?> arbitrary, UniqueElements uniqueness) {
 		Class<? extends Function<?, Object>> extractorClass = uniqueness.by();
 		if (extractorClass.equals(UniqueElements.NOT_SET.class)) {
 			return arbitrary.uniqueElements();
@@ -107,7 +109,7 @@ private <T> Arbitrary<?> configureStreamableArbitrary(StreamableArbitrary<T, ?>
 		return arbitrary.uniqueElements(extractor);
 	}
 
-	private <T> Arbitrary<?> configureSetArbitrary(SetArbitrary<T> arbitrary, UniqueElements uniqueness) {
+	private <T extends @Nullable Object> Arbitrary<?> configureSetArbitrary(SetArbitrary<T> arbitrary, UniqueElements uniqueness) {
 		Class<? extends Function<?, Object>> extractorClass = uniqueness.by();
 		if (extractorClass.equals(UniqueElements.NOT_SET.class)) {
 			return arbitrary;
@@ -116,7 +118,7 @@ private <T> Arbitrary<?> configureSetArbitrary(SetArbitrary<T> arbitrary, Unique
 		return arbitrary.uniqueElements(extractor);
 	}
 
-	private static <T> Function<T, Object> extractor(Class<? extends Function<?, Object>> extractorClass) {
+	private static <T extends @Nullable Object> Function<T, Object> extractor(Class<? extends Function<?, Object>> extractorClass) {
 		return (Function<T, Object>) (
 			extractorClass.equals(UniqueElements.NOT_SET.class)
 				? Function.identity()
diff --git a/engine/src/main/java/net/jqwik/engine/properties/configurators/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/configurators/package-info.java
new file mode 100644
index 000000000..be011f43f
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/configurators/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.configurators;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/package-info.java
new file mode 100644
index 000000000..821b5a4dd
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/AggressiveSizeOfListShrinker.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/AggressiveSizeOfListShrinker.java
index d3d5b38f8..e3a296a9e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/AggressiveSizeOfListShrinker.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/AggressiveSizeOfListShrinker.java
@@ -13,7 +13,7 @@ public AggressiveSizeOfListShrinker(int minSize) {
 		this.minSize = minSize;
 	}
 
-	public Stream<List<T>> shrink(List<T> toShrink) {
+	public Stream<List<T>> shrink(List<? extends T> toShrink) {
 		if (toShrink.size() <= minSize) {
 			return Stream.empty();
 		}
@@ -24,14 +24,14 @@ public Stream<List<T>> shrink(List<T> toShrink) {
 		).filter(l -> l.size() >= minSize);
 	}
 
-	public Stream<List<T>> cutsToMinsize(List<T> toShrink) {
+	public Stream<List<T>> cutsToMinsize(List<? extends T> toShrink) {
 		Set<List<T>> lists = new LinkedHashSet<>();
 		appendLeftCut(toShrink, lists, minSize);
 		appendRightCut(toShrink, lists, minSize);
 		return lists.stream();
 	}
 
-	public Stream<List<T>> cutsToMinsizePlus1(List<T> toShrink) {
+	public Stream<List<T>> cutsToMinsizePlus1(List<? extends T> toShrink) {
 		if (toShrink.size() <= minSize + 1) {
 			return Stream.empty();
 		}
@@ -41,7 +41,7 @@ public Stream<List<T>> cutsToMinsizePlus1(List<T> toShrink) {
 		return lists.stream();
 	}
 
-	public Stream<List<T>> cutInHalves(List<T> toShrink) {
+	public Stream<List<T>> cutInHalves(List<? extends T> toShrink) {
 		int halfSize = toShrink.size() / 2;
 		if (halfSize < minSize) {
 			return Stream.empty();
@@ -52,12 +52,12 @@ public Stream<List<T>> cutInHalves(List<T> toShrink) {
 		return lists.stream();
 	}
 
-	private void appendLeftCut(List<T> toShrink, Set<List<T>> lists, int elementsToKeep) {
+	private void appendLeftCut(List<? extends T> toShrink, Set<List<T>> lists, int elementsToKeep) {
 		List<T> cut = new ArrayList<>(toShrink);
 		lists.add(cut.subList(0, elementsToKeep));
 	}
 
-	private void appendRightCut(List<T> toShrink, Set<List<T>> lists, int elementsToKeep) {
+	private void appendRightCut(List<? extends T> toShrink, Set<List<T>> lists, int elementsToKeep) {
 		List<T> cut = new ArrayList<>(toShrink);
 		int elementsToCut = toShrink.size() - elementsToKeep;
 		lists.add(cut.subList(elementsToCut, toShrink.size()));
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java
index c5d275a26..5268a4263 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java
@@ -8,10 +8,10 @@
 import net.jqwik.engine.support.*;
 
 public class CollectShrinkable<T> implements Shrinkable<List<T>> {
-	private final List<Shrinkable<T>> elements;
-	private final Predicate<List<T>> until;
+	private final List<? extends Shrinkable<T>> elements;
+	private final Predicate<? super List<? extends T>> until;
 
-	public CollectShrinkable(List<Shrinkable<T>> elements, Predicate<List<T>> until) {
+	public CollectShrinkable(List<? extends Shrinkable<T>> elements, Predicate<? super List<? extends T>> until) {
 		this.elements = elements;
 		this.until = until;
 	}
@@ -21,7 +21,7 @@ public List<T> value() {
 		return createValue(elements);
 	}
 
-	private List<T> createValue(List<Shrinkable<T>> elements) {
+	private List<T> createValue(List<? extends Shrinkable<T>> elements) {
 		return elements
 				   .stream()
 				   .map(Shrinkable::value)
@@ -53,10 +53,10 @@ private Stream<Shrinkable<List<T>>> shrinkElementsOneAfterTheOther() {
 	}
 
 	private Stream<Shrinkable<List<T>>> sortElements() {
-		return ShrinkingCommons.sortElements(elements, this::createShrinkable);
+		return ShrinkingCommons.sortElements(elements, (ShrinkingCommons.ContainerCreator<List<T>, T>) this::createShrinkable);
 	}
 
-	private CollectShrinkable<T> createShrinkable(List<Shrinkable<T>> pairSwap) {
+	private CollectShrinkable<T> createShrinkable(List<? extends Shrinkable<T>> pairSwap) {
 		return new CollectShrinkable<>(pairSwap, until);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java
index d2c35a489..ab539f030 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java
@@ -9,9 +9,9 @@
 
 public class CombinedShrinkable<T> implements Shrinkable<T> {
 	private final List<Shrinkable<Object>> parts;
-	private final Function<List<Object>, T> combinator;
+	private final Function<? super List<?>, ? extends T> combinator;
 
-	public CombinedShrinkable(List<Shrinkable<Object>> parts, Function<List<Object>, T> combinator) {
+	public CombinedShrinkable(List<Shrinkable<Object>> parts, Function<? super List<?>, ? extends T> combinator) {
 		this.parts = parts;
 		this.combinator = combinator;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java
index ced0aa877..0815b42a7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java
@@ -14,9 +14,9 @@ public class FilteredShrinkable<T> implements Shrinkable<T> {
 	private final AtomicInteger countBaseShrinks = new AtomicInteger(0);
 
 	private final Shrinkable<T> toFilter;
-	private final Predicate<T> filter;
+	private final Predicate<? super T> filter;
 
-	public FilteredShrinkable(Shrinkable<T> toFilter, Predicate<T> filter) {
+	public FilteredShrinkable(Shrinkable<T> toFilter, Predicate<? super T> filter) {
 		this.toFilter = toFilter;
 		this.filter = filter;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FixedValueFlatMappedShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FixedValueFlatMappedShrinkable.java
index 9d6eb5545..f67c9fc2e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FixedValueFlatMappedShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FixedValueFlatMappedShrinkable.java
@@ -4,14 +4,16 @@
 
 import net.jqwik.api.*;
 
-public class FixedValueFlatMappedShrinkable<T, U> extends FlatMappedShrinkable<T, U> {
+import org.jspecify.annotations.*;
 
-	private final Supplier<Shrinkable<U>> shrinkableSupplier;
+public class FixedValueFlatMappedShrinkable<T extends @Nullable Object, U extends @Nullable Object> extends FlatMappedShrinkable<T, U> {
+
+	private final Supplier<? extends Shrinkable<U>> shrinkableSupplier;
 
 	public FixedValueFlatMappedShrinkable(
 		Shrinkable<T> toMap,
-		Function<T, Shrinkable<U>> mapper,
-		Supplier<Shrinkable<U>> shrinkableSupplier
+		Function<? super T, ? extends Shrinkable<U>> mapper,
+		Supplier<? extends Shrinkable<U>> shrinkableSupplier
 	) {
 		super(toMap, mapper);
 		this.shrinkableSupplier = shrinkableSupplier;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FlatMappedShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FlatMappedShrinkable.java
index 4cdb6f3b0..546376f7b 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FlatMappedShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FlatMappedShrinkable.java
@@ -9,14 +9,16 @@
 import net.jqwik.engine.*;
 import net.jqwik.engine.support.*;
 
-public class FlatMappedShrinkable<T, U> implements Shrinkable<U> {
+import org.jspecify.annotations.*;
+
+public class FlatMappedShrinkable<T extends @Nullable Object, U extends @Nullable Object> implements Shrinkable<U> {
 
 	private final Shrinkable<T> toMap;
-	private final Function<T, Shrinkable<U>> mapper;
+	private final Function<? super T, ? extends Shrinkable<U>> mapper;
 
 	public FlatMappedShrinkable(
 		Shrinkable<T> toMap,
-		Function<T, Arbitrary<U>> toArbitraryMapper,
+		Function<? super T, ? extends Arbitrary<U>> toArbitraryMapper,
 		int genSize,
 		long randomSeed,
 		boolean withEmbeddedEdgeCases
@@ -27,11 +29,11 @@ public FlatMappedShrinkable(
 		}, randomSeed);
 	}
 
-	public FlatMappedShrinkable(Shrinkable<T> toMap, Function<T, RandomGenerator<U>> toGeneratorMapper, long randomSeed) {
+	public FlatMappedShrinkable(Shrinkable<T> toMap, Function<? super T, ? extends RandomGenerator<U>> toGeneratorMapper, long randomSeed) {
 		this(toMap, t -> toGeneratorMapper.apply(t).next(SourceOfRandomness.newRandom(randomSeed)));
 	}
 
-	protected FlatMappedShrinkable(Shrinkable<T> toMap, Function<T, Shrinkable<U>> mapper) {
+	protected FlatMappedShrinkable(Shrinkable<T> toMap, Function<? super T, ? extends Shrinkable<U>> mapper) {
 		this.toMap = toMap;
 		this.mapper = mapper;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/MappedShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/MappedShrinkable.java
index c0799dbfd..c29814454 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/MappedShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/MappedShrinkable.java
@@ -9,9 +9,9 @@
 public class MappedShrinkable<T, U> implements Shrinkable<U> {
 
 	private final Shrinkable<T> toMap;
-	private final Function<T, U> mapper;
+	private final Function<? super T, ? extends U> mapper;
 
-	public MappedShrinkable(Shrinkable<T> toMap, Function<T, U> mapper) {
+	public MappedShrinkable(Shrinkable<T> toMap, Function<? super T, ? extends U> mapper) {
 		this.toMap = toMap;
 		this.mapper = mapper;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/OneAfterTheOtherParameterShrinker.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/OneAfterTheOtherParameterShrinker.java
index ce5a560a6..23095bc36 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/OneAfterTheOtherParameterShrinker.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/OneAfterTheOtherParameterShrinker.java
@@ -7,6 +7,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.lifecycle.*;
 
+import org.jspecify.annotations.*;
+
 class OneAfterTheOtherParameterShrinker extends AbstractSampleShrinker {
 
 	public OneAfterTheOtherParameterShrinker(Map<List<Object>, TryExecutionResult> falsificationCache) {
@@ -49,7 +51,7 @@ private FalsifiedSample shrinkSingleParameter(
 		);
 	}
 
-	private <T> List<T> replaceIn(T object, int index, List<T> old) {
+	private <T extends @Nullable Object> List<T> replaceIn(T object, int index, List<T> old) {
 		List<T> newList = new ArrayList<>(old);
 		newList.set(index, object);
 		return newList;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/PropertyShrinker.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/PropertyShrinker.java
index 2e8d7bcd5..9647b3516 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/PropertyShrinker.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/PropertyShrinker.java
@@ -46,7 +46,7 @@ public PropertyShrinker(
 		this.targetMethod = targetMethod;
 	}
 
-	public ShrunkFalsifiedSample shrink(Falsifier<List<Object>> forAllFalsifier) {
+	public ShrunkFalsifiedSample shrink(Falsifier<? super List<Object>> forAllFalsifier) {
 		if (shrinkingMode == ShrinkingMode.OFF) {
 			return unshrunkOriginalSample();
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableContainer.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableContainer.java
index 725f46e63..d22280917 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableContainer.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableContainer.java
@@ -13,18 +13,17 @@
 import static net.jqwik.engine.properties.UniquenessChecker.*;
 
 abstract class ShrinkableContainer<C, E> implements Shrinkable<C> {
-	protected final List<Shrinkable<E>> elements;
+	protected final List<? extends Shrinkable<E>> elements;
 	protected final int minSize;
 	protected final int maxSize;
-	protected final Collection<FeatureExtractor<E>> uniquenessExtractors;
+	protected final Collection<? extends FeatureExtractor<E>> uniquenessExtractors;
 
-	@Nullable
-	protected final Arbitrary<E> elementArbitrary;
+	protected final @Nullable Arbitrary<E> elementArbitrary;
 
 	ShrinkableContainer(
-		List<Shrinkable<E>> elements,
+		List<? extends Shrinkable<E>> elements,
 		int minSize, int maxSize,
-		Collection<FeatureExtractor<E>> uniquenessExtractors,
+		Collection<? extends FeatureExtractor<E>> uniquenessExtractors,
 		@Nullable Arbitrary<E> elementArbitrary
 	) {
 		this.elements = elements;
@@ -34,7 +33,7 @@ abstract class ShrinkableContainer<C, E> implements Shrinkable<C> {
 		this.elementArbitrary = elementArbitrary;
 	}
 
-	abstract C createValue(List<Shrinkable<E>> shrinkables);
+	abstract C createValue(List<? extends Shrinkable<E>> shrinkables);
 
 	@Override
 	public C value() {
@@ -159,7 +158,7 @@ protected Stream<Shrinkable<C>> shrinkPairsOfElements() {
 	}
 
 	protected Stream<Shrinkable<C>> sortElements() {
-		return ShrinkingCommons.sortElements(elements, this::createShrinkable);
+		return ShrinkingCommons.sortElements(elements, (ShrinkingCommons.ContainerCreator<C, E>) this::createShrinkable);
 	}
 
 	@Override
@@ -190,6 +189,6 @@ public String toString() {
 		);
 	}
 
-	abstract Shrinkable<C> createShrinkable(List<Shrinkable<E>> shrunkElements);
+	abstract Shrinkable<C> createShrinkable(List<? extends Shrinkable<E>> shrunkElements);
 
 }
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java
index 26a51f7cf..b2bbacb74 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java
@@ -17,16 +17,16 @@ public class ShrinkableList<E> extends ShrinkableContainer<List<E>, E> {
 	}
 
 	public ShrinkableList(
-		List<Shrinkable<E>> elements,
+		List<? extends Shrinkable<E>> elements,
 		int minSize, int maxSize,
-		Collection<FeatureExtractor<E>> uniquenessExtractors,
+		Collection<? extends FeatureExtractor<E>> uniquenessExtractors,
 		Arbitrary<E> elementArbitrary
 	) {
 		super(elements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
 	}
 
 	@Override
-	List<E> createValue(List<Shrinkable<E>> shrinkables) {
+	List<E> createValue(List<? extends Shrinkable<E>> shrinkables) {
 		// Using loop instead of stream to make stack traces more readable
 		List<E> values = new ArrayList<>(shrinkables.size());
 		for (Shrinkable<E> shrinkable : shrinkables) {
@@ -36,7 +36,7 @@ List<E> createValue(List<Shrinkable<E>> shrinkables) {
 	}
 
 	@Override
-	Shrinkable<List<E>> createShrinkable(List<Shrinkable<E>> shrunkElements) {
+	Shrinkable<List<E>> createShrinkable(List<? extends Shrinkable<E>> shrunkElements) {
 		return new ShrinkableList<>(shrunkElements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java
index c45adbe10..aaacd69b8 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java
@@ -11,18 +11,18 @@
 public class ShrinkableSet<E> extends ShrinkableContainer<Set<E>, E> {
 
 	public ShrinkableSet(
-		Collection<Shrinkable<E>> elements,
+		Collection<? extends Shrinkable<E>> elements,
 		int minSize, int maxSize,
-		Collection<FeatureExtractor<E>> uniquenessExtractors,
+		Collection<? extends FeatureExtractor<E>> uniquenessExtractors,
 		@Nullable Arbitrary<E> elementArbitrary
 	) {
 		this(new ArrayList<>(elements), minSize, maxSize, uniquenessExtractors, elementArbitrary);
 	}
 
 	private ShrinkableSet(
-		List<Shrinkable<E>> elements,
+		List<? extends Shrinkable<E>> elements,
 		int minSize, int maxSize,
-		Collection<FeatureExtractor<E>> uniquenessExtractors,
+		Collection<? extends FeatureExtractor<E>> uniquenessExtractors,
 		@Nullable Arbitrary<E> elementArbitrary
 	) {
 		super(elements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
@@ -39,7 +39,7 @@ protected boolean hasReallyGrown(Shrinkable<Set<E>> grownShrinkable) {
 	}
 
 	@Override
-	Set<E> createValue(List<Shrinkable<E>> shrinkables) {
+	Set<E> createValue(List<? extends Shrinkable<E>> shrinkables) {
 		// See https://richardstartin.github.io/posts/5-java-mundane-performance-tricks#size-hashmaps-whenever-possible
 		//     for how to compute initial capacity of hash maps
 		int capacityWithLoadFactor = shrinkables.size() * 4 / 3;
@@ -53,7 +53,7 @@ Set<E> createValue(List<Shrinkable<E>> shrinkables) {
 	}
 
 	@Override
-	Shrinkable<Set<E>> createShrinkable(List<Shrinkable<E>> shrunkElements) {
+	Shrinkable<Set<E>> createShrinkable(List<? extends Shrinkable<E>> shrunkElements) {
 		return new ShrinkableSet<>(shrunkElements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableString.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableString.java
index ea9a67a54..0b31d58c4 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableString.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableString.java
@@ -1,7 +1,6 @@
 package net.jqwik.engine.properties.shrinking;
 
 import java.util.*;
-import java.util.function.*;
 import java.util.stream.*;
 
 import net.jqwik.api.*;
@@ -13,7 +12,7 @@ public class ShrinkableString extends ShrinkableContainer<String, Character> {
 	public static final Set<FeatureExtractor<Character>> UNIQUE_CHARS_EXTRACTOR = Collections.singleton(FeatureExtractor.identity());
 
 	public ShrinkableString(
-		List<Shrinkable<Character>> elements, int minSize, int maxSize,
+		List<? extends Shrinkable<Character>> elements, int minSize, int maxSize,
 		Arbitrary<Character> characterArbitrary,
 		boolean uniqueChars
 	) {
@@ -21,8 +20,8 @@ public ShrinkableString(
 	}
 
 	private ShrinkableString(
-		List<Shrinkable<Character>> elements, int minSize, int maxSize,
-		Collection<FeatureExtractor<Character>> uniquenessExtractors,
+		List<? extends Shrinkable<Character>> elements, int minSize, int maxSize,
+		Collection<? extends FeatureExtractor<Character>> uniquenessExtractors,
 		Arbitrary<Character> characterArbitrary
 	) {
 		super(elements, minSize, maxSize, uniquenessExtractors, characterArbitrary);
@@ -33,7 +32,7 @@ private static Collection<FeatureExtractor<Character>> uniquenessExtractors(bool
 	}
 
 	@Override
-	String createValue(List<Shrinkable<Character>> shrinkables) {
+	String createValue(List<? extends Shrinkable<Character>> shrinkables) {
 		// Using loop instead of stream to make stack traces more readable
 		StringBuilder builder = new StringBuilder(shrinkables.size());
 		for (Shrinkable<Character> shrinkable : shrinkables) {
@@ -43,7 +42,7 @@ String createValue(List<Shrinkable<Character>> shrinkables) {
 	}
 
 	@Override
-	Shrinkable<String> createShrinkable(List<Shrinkable<Character>> shrunkElements) {
+	Shrinkable<String> createShrinkable(List<? extends Shrinkable<Character>> shrunkElements) {
 		return new ShrinkableString(shrunkElements, minSize, maxSize, uniquenessExtractors, elementArbitrary);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkingCommons.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkingCommons.java
index b1791d423..e2137009b 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkingCommons.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkingCommons.java
@@ -7,12 +7,14 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.properties.UniquenessChecker.*;
 
 class ShrinkingCommons {
 
 	@FunctionalInterface
-	interface ContainerCreator<C, E> extends Function<List<Shrinkable<E>>, Shrinkable<C>> {}
+	interface ContainerCreator<C extends @Nullable Object, E extends @Nullable Object> extends Function<List<? extends Shrinkable<E>>, Shrinkable<C>> {}
 
 	/**
 	 * Shrink elements of a container pairwise
@@ -23,7 +25,7 @@ interface ContainerCreator<C, E> extends Function<List<Shrinkable<E>>, Shrinkabl
 	 * @param <E>             type of elements
 	 * @return stream of shrunk containers
 	 */
-	static  <C, E> Stream<Shrinkable<C>> shrinkPairsOfElements(List<Shrinkable<E>> elements, ContainerCreator<C, E> createContainer) {
+	static  <C extends @Nullable Object, E extends @Nullable Object> Stream<Shrinkable<C>> shrinkPairsOfElements(List<? extends Shrinkable<E>> elements, ContainerCreator<C, E> createContainer) {
 		return Combinatorics
 			.distinctPairs(elements.size())
 			.flatMap(pair -> JqwikStreamSupport.zip(
@@ -47,7 +49,7 @@ static  <C, E> Stream<Shrinkable<C>> shrinkPairsOfElements(List<Shrinkable<E>> e
 	 * @param <E>             type of elements
 	 * @return stream of shrunk containers
 	 */
-	static <C, E> Stream<Shrinkable<C>> sortElements(List<Shrinkable<E>> elements, ContainerCreator<C, E> createContainer) {
+	static <C extends @Nullable Object, E extends @Nullable Object> Stream<Shrinkable<C>> sortElements(List<? extends Shrinkable<E>> elements, ContainerCreator<C, E> createContainer) {
 		List<Shrinkable<E>> sortedElements = new ArrayList<>(elements);
 		sortedElements.sort(Comparator.comparing(Shrinkable::distance));
 		if (elements.equals(sortedElements)) {
@@ -59,11 +61,11 @@ static <C, E> Stream<Shrinkable<C>> sortElements(List<Shrinkable<E>> elements, C
 		);
 	}
 
-	private static <C, E> Stream<Shrinkable<C>> fullSort(List<Shrinkable<E>> sortedElements, ContainerCreator<C, E> createContainer) {
+	private static <C extends @Nullable Object, E extends @Nullable Object> Stream<Shrinkable<C>> fullSort(List<? extends Shrinkable<E>> sortedElements, ContainerCreator<C, E> createContainer) {
 		return Stream.of(createContainer.apply(sortedElements));
 	}
 
-	private static <C, E> Stream<Shrinkable<C>> pairwiseSort(List<Shrinkable<E>> elements, ContainerCreator<C, E> createContainer) {
+	private static <C extends @Nullable Object, E extends @Nullable Object> Stream<Shrinkable<C>> pairwiseSort(List<? extends Shrinkable<E>> elements, ContainerCreator<C, E> createContainer) {
 		return Combinatorics.distinctPairs(elements.size())
 							.map(pair -> {
 								int firstIndex = Math.min(pair.get1(), pair.get2());
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/SizeOfListShrinker.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/SizeOfListShrinker.java
index 7628fa8c2..d819a2850 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/SizeOfListShrinker.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/SizeOfListShrinker.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.engine.support.*;
 
-public class SizeOfListShrinker<T> {
+import org.jspecify.annotations.*;
+
+public class SizeOfListShrinker<T extends @Nullable Object> {
 
 	private final int minSize;
 
@@ -13,7 +15,7 @@ public SizeOfListShrinker(int minSize) {
 		this.minSize = minSize;
 	}
 
-	public Stream<List<T>> shrink(List<T> toShrink) {
+	public Stream<List<T>> shrink(List<? extends T> toShrink) {
 		if (toShrink.size() <= minSize)
 			return Stream.empty();
 		return JqwikStreamSupport.concat(
@@ -30,14 +32,14 @@ private Stream<List<T>> emptyList() {
 		}
 	}
 
-	public Stream<List<T>> cuts(List<T> toShrink) {
+	public Stream<List<T>> cuts(List<? extends T> toShrink) {
 		Set<List<T>> lists = new LinkedHashSet<>();
 		appendRightCuts(toShrink, lists);
 		appendLeftCuts(toShrink, lists);
 		return lists.stream();
 	}
 
-	private void appendLeftCuts(List<T> toShrink, Set<List<T>> lists) {
+	private void appendLeftCuts(List<? extends T> toShrink, Set<List<T>> lists) {
 		int elementsToCut = calculateElementsToCut(toShrink.size());
 		appendLeftCut(toShrink, lists, elementsToCut);
 		if (elementsToCut != 1) {
@@ -45,11 +47,11 @@ private void appendLeftCuts(List<T> toShrink, Set<List<T>> lists) {
 		}
 	}
 
-	private void appendLeftCut(List<T> toShrink, Set<List<T>> lists, int elementsToCut) {
+	private void appendLeftCut(List<? extends T> toShrink, Set<List<T>> lists, int elementsToCut) {
 		lists.add(new ArrayList<>(cutFromLeft(toShrink, elementsToCut)));
 	}
 
-	private List<T> cutFromLeft(List<T> toShrink, int elementsToCut) {
+	private List<? extends T> cutFromLeft(List<? extends T> toShrink, int elementsToCut) {
 		return toShrink.subList(elementsToCut, toShrink.size());
 	}
 
@@ -68,7 +70,7 @@ private int rawElementsToCut(int listSize) {
 		return listSize / 2;
 	}
 
-	private void appendRightCuts(List<T> toShrink, Set<List<T>> lists) {
+	private void appendRightCuts(List<? extends T> toShrink, Set<List<T>> lists) {
 		int elementsToCut = calculateElementsToCut(toShrink.size());
 		appendRightCut(toShrink, lists, elementsToCut);
 		if (elementsToCut != 1) {
@@ -76,11 +78,11 @@ private void appendRightCuts(List<T> toShrink, Set<List<T>> lists) {
 		}
 	}
 
-	private void appendRightCut(List<T> toShrink, Set<List<T>> lists, int elementsToCut) {
+	private void appendRightCut(List<? extends T> toShrink, Set<List<T>> lists, int elementsToCut) {
 		lists.add(new ArrayList<>(cutFromRight(toShrink, elementsToCut)));
 	}
 
-	private List<T> cutFromRight(List<T> toShrink, int elementsToCut) {
+	private List<? extends T> cutFromRight(List<? extends T> toShrink, int elementsToCut) {
 		return toShrink.subList(0, toShrink.size() - elementsToCut);
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java
index 71de04812..fb8644003 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java
@@ -9,9 +9,9 @@
 
 public class Unshrinkable<T> implements Shrinkable<T> {
 	private final ShrinkingDistance distance;
-	private final Supplier<T> valueSupplier;
+	private final Supplier<? extends T> valueSupplier;
 
-	public Unshrinkable(Supplier<T> valueSupplier, ShrinkingDistance distance) {
+	public Unshrinkable(Supplier<? extends T> valueSupplier, ShrinkingDistance distance) {
 		this.valueSupplier = valueSupplier;
 		this.distance = distance;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/package-info.java
new file mode 100644
index 000000000..697e10f54
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.shrinking;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java
index 18d6d8cca..5c15ae319 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java
@@ -11,7 +11,7 @@
 public class DefaultChainArbitrary<T> extends TypedCloneable implements ChainArbitrary<T> {
 
 	private int maxTransformations = Integer.MIN_VALUE;
-	private Supplier<ChangeDetector<T>> changeDetectorSupplier = ChangeDetector::alwaysTrue;
+	private Supplier<? extends ChangeDetector<? super T>> changeDetectorSupplier = ChangeDetector::alwaysTrue;
 	private List<Tuple.Tuple2<Integer, Transformation<T>>> weightedTransformations = new ArrayList<>();
 	private final Supplier<? extends T> initialSupplier;
 
@@ -24,7 +24,7 @@ public RandomGenerator<Chain<T>> generator(int genSize) {
 		final int effectiveMaxTransformations =
 			this.maxTransformations != Integer.MIN_VALUE ? this.maxTransformations : (int) Math.max(Math.round(Math.sqrt(genSize)), 10);
 		Function<Random, Transformation<T>> transformationGenerator = new ChooseRandomlyByFrequency<>(weightedTransformations);
-		return random -> new ShrinkableChain<>(
+		return random -> new ShrinkableChain<T>(
 			random.nextLong(),
 			initialSupplier,
 			transformationGenerator,
@@ -54,7 +54,7 @@ public ChainArbitrary<T> withMaxTransformations(int maxTransformations) {
 	}
 
 	@Override
-	public ChainArbitrary<T> improveShrinkingWith(Supplier<ChangeDetector<T>> changeDetectorSupplier) {
+	public ChainArbitrary<T> improveShrinkingWith(Supplier<? extends ChangeDetector<? super T>> changeDetectorSupplier) {
 		DefaultChainArbitrary<T> clone = typedClone();
 		clone.changeDetectorSupplier = changeDetectorSupplier;
 		return clone;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/SequentialActionChain.java b/engine/src/main/java/net/jqwik/engine/properties/state/SequentialActionChain.java
index 9934456a8..340f6be3c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/SequentialActionChain.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/SequentialActionChain.java
@@ -12,12 +12,12 @@
 import net.jqwik.api.support.*;
 import net.jqwik.engine.support.*;
 
-public class SequentialActionChain<T> implements ActionChain<T> {
+public class SequentialActionChain<T extends @Nullable Object> implements ActionChain<T> {
 	private final Chain<T> chain;
 
 	private volatile T currentValue = null;
 	private volatile RunningState currentRunning = RunningState.NOT_RUN;
-	private final List<Consumer<T>> peekers = new ArrayList<>();
+	private final List<Consumer<? super T>> peekers = new ArrayList<>();
 	private final List<Tuple.Tuple2<String, Consumer<T>>> invariants = new ArrayList<>();
 
 	public SequentialActionChain(Chain<T> chain) {
@@ -25,7 +25,6 @@ public SequentialActionChain(Chain<T> chain) {
 	}
 
 	@Override
-	@NonNull
 	public List<String> transformations() {
 		return chain.transformations();
 	}
@@ -36,7 +35,6 @@ public List<Transformer<T>> transformers() {
 	}
 
 	@Override
-	@NonNull
 	public synchronized T run() {
 		currentRunning = RunningState.RUNNING;
 		for (Iterator<T> iterator = chain.iterator(); iterator.hasNext(); ) {
@@ -66,7 +64,7 @@ private void nextAction(Iterator<T> iterator) {
 	}
 
 	private void callPeekers() {
-		for (Consumer<T> peeker : peekers) {
+		for (Consumer<? super T> peeker : peekers) {
 			peeker.accept(currentValue);
 		}
 	}
@@ -98,7 +96,6 @@ private String createErrorMessage(String name, String causeMessage) {
 	}
 
 	@Override
-	@NonNull
 	public ActionChain<T> withInvariant(@Nullable String label, Consumer<T> invariant) {
 		String invariantLabel = label == null ? "Invariant" : String.format("Invariant '%s'", label);
 		invariants.add(Tuple.of(invariantLabel, invariant));
@@ -106,20 +103,17 @@ public ActionChain<T> withInvariant(@Nullable String label, Consumer<T> invarian
 	}
 
 	@Override
-	@NonNull
 	public synchronized Optional<T> finalState() {
 		return Optional.ofNullable(currentValue);
 	}
 
 	@Override
-	// @NonNull // TODO: Why does this not work?
 	public ActionChain.RunningState running() {
 		return currentRunning;
 	}
 
 	@Override
-	@NonNull
-	public synchronized ActionChain<T> peek(@NonNull Consumer<T> peeker) {
+	public synchronized ActionChain<T> peek(Consumer<? super T> peeker) {
 		peekers.add(peeker);
 		return this;
 	}
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java b/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java
index a1efc4c53..3103e257a 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java
@@ -20,17 +20,17 @@ public class ShrinkableChain<T> implements Shrinkable<Chain<T>> {
 	public static final int MAX_TRANSFORMER_TRIES = 1000;
 	private final long randomSeed;
 	private final Supplier<? extends T> initialSupplier;
-	private final Function<Random, Transformation<T>> transformationGenerator;
+	private final Function<? super Random, ? extends Transformation<T>> transformationGenerator;
 	private final int maxTransformations;
 	private final int genSize;
 	private final List<ShrinkableChainIteration<T>> iterations;
-	private final Supplier<ChangeDetector<T>> changeDetectorSupplier;
+	private final Supplier<? extends ChangeDetector<? super T>> changeDetectorSupplier;
 
 	public ShrinkableChain(
 		long randomSeed,
 		Supplier<? extends T> initialSupplier,
-		Function<Random, Transformation<T>> transformationGenerator,
-		Supplier<ChangeDetector<T>> changeDetectorSupplier,
+		Function<? super Random, ? extends Transformation<T>> transformationGenerator,
+		Supplier<? extends ChangeDetector<? super T>> changeDetectorSupplier,
 		int maxTransformations,
 		int genSize
 	) {
@@ -39,8 +39,8 @@ public ShrinkableChain(
 
 	private ShrinkableChain(
 		long randomSeed, Supplier<? extends T> initialSupplier,
-		Function<Random, Transformation<T>> transformationGenerator,
-		Supplier<ChangeDetector<T>> changeDetectorSupplier,
+		Function<? super Random, ? extends Transformation<T>> transformationGenerator,
+		Supplier<? extends ChangeDetector<? super T>> changeDetectorSupplier,
 		int maxTransformations,
 		int genSize,
 		List<ShrinkableChainIteration<T>> iterations
@@ -55,13 +55,11 @@ private ShrinkableChain(
 	}
 
 	@Override
-	@NonNull
 	public Chain<T> value() {
 		return new ChainInstance();
 	}
 
 	@Override
-	@NonNull
 	public Stream<Shrinkable<Chain<T>>> shrink() {
 		return new ShrinkableChainShrinker<>(this, iterations, maxTransformations).shrink();
 	}
@@ -79,7 +77,6 @@ ShrinkableChain<T> cloneWith(List<ShrinkableChainIteration<T>> shrunkIterations,
 	}
 
 	@Override
-	@NonNull
 	public ShrinkingDistance distance() {
 		List<Shrinkable<Transformer<T>>> shrinkablesForDistance = new ArrayList<>();
 		for (int i = 0; i < maxTransformations; i++) {
@@ -100,7 +97,6 @@ public String toString() {
 	private class ChainInstance implements Chain<T> {
 
 		@Override
-		@NonNull
 		public Iterator<T> start() {
 			return new ChainIterator(initialSupplier.get());
 		}
@@ -111,13 +107,11 @@ public int maxTransformations() {
 		}
 
 		@Override
-		@NonNull
 		public List<String> transformations() {
 			return iterations.stream().map(i -> i.transformation()).collect(Collectors.toList());
 		}
 
 		@Override
-		@NonNull
 		public List<Transformer<T>> transformers() {
 			return iterations.stream().map(i -> i.transformer()).collect(Collectors.toList());
 		}
@@ -187,7 +181,7 @@ private Transformer<T> nextTransformer() {
 		}
 
 		private T transformState(Transformer<T> transformer, T before) {
-			ChangeDetector<T> changeDetector = changeDetectorSupplier.get();
+			ChangeDetector<? super T> changeDetector = changeDetectorSupplier.get();
 			changeDetector.before(before);
 			try {
 				T after = transformer.apply(before);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChainIteration.java b/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChainIteration.java
index be11ffdb0..c20d5f055 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChainIteration.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChainIteration.java
@@ -6,16 +6,18 @@
 import net.jqwik.api.*;
 import net.jqwik.api.state.*;
 
+import org.jspecify.annotations.*;
+
 class ShrinkableChainIteration<T> {
 	final Shrinkable<Transformer<T>> shrinkable;
 	final boolean accessState;
 	final boolean changeState;
 
-	private final Predicate<T> precondition;
-	private final Transformer<T> cachedTransformer;
+	private final @Nullable Predicate<T> precondition;
+	private final @Nullable Transformer<T> cachedTransformer;
 
 	ShrinkableChainIteration(
-		Predicate<T> precondition,
+		@Nullable Predicate<T> precondition,
 		boolean accessState,
 		Shrinkable<Transformer<T>> shrinkable
 	) {
@@ -24,7 +26,7 @@ class ShrinkableChainIteration<T> {
 	}
 
 	private ShrinkableChainIteration(
-		Predicate<T> precondition,
+		@Nullable Predicate<T> precondition,
 		boolean accessState,
 		boolean changeState,
 		Shrinkable<Transformer<T>> shrinkable
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/state/package-info.java
new file mode 100644
index 000000000..1f8a77b63
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.state;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionGenerator.java
index 9b0b89830..2e3d2f330 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionGenerator.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.stateful.*;
 
-interface ActionGenerator<M> {
+import org.jspecify.annotations.*;
+
+interface ActionGenerator<M extends @Nullable Object> {
 
 	Action<M> next(M model);
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/RandomActionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/RandomActionGenerator.java
index dba103baa..1ef178a43 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/RandomActionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/RandomActionGenerator.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.stateful.*;
 
-class RandomActionGenerator<T> implements ActionGenerator<T> {
+import org.jspecify.annotations.*;
+
+class RandomActionGenerator<T extends @Nullable Object> implements ActionGenerator<T> {
 
 	private static final int MAX_TRIES = 1000;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkablesActionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkablesActionGenerator.java
index fed75bbc2..5e6baf059 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkablesActionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkablesActionGenerator.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.stateful.*;
 
-class ShrinkablesActionGenerator<T> implements ActionGenerator<T> {
+import org.jspecify.annotations.*;
+
+class ShrinkablesActionGenerator<T extends @Nullable Object> implements ActionGenerator<T> {
 
 	private Iterator<Shrinkable<Action<T>>> iterator;
 	private List<Shrinkable<Action<T>>> shrinkables = new ArrayList<>();
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/package-info.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/package-info.java
new file mode 100644
index 000000000..181c00e4d
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.properties.stateful;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/providers/package-info.java b/engine/src/main/java/net/jqwik/engine/providers/package-info.java
new file mode 100644
index 000000000..13b56dff6
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/providers/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.providers;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/recording/package-info.java b/engine/src/main/java/net/jqwik/engine/recording/package-info.java
new file mode 100644
index 000000000..2f557efc7
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/recording/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.recording;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java b/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java
index 19a55c413..348b5f7ac 100644
--- a/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java
+++ b/engine/src/main/java/net/jqwik/engine/support/Combinatorics.java
@@ -9,11 +9,13 @@
 import net.jqwik.api.Tuple.*;
 import net.jqwik.engine.support.combinatorics.*;
 
+import org.jspecify.annotations.*;
+
 import static java.util.Arrays.*;
 
 public class Combinatorics {
 
-	public static <T> Iterator<List<T>> combine(List<Iterable<T>> listOfIterables) {
+	public static <T extends @Nullable Object> Iterator<List<T>> combine(List<? extends Iterable<? extends T>> listOfIterables) {
 		if (listOfIterables.isEmpty()) {
 			return emptyListSingleton();
 		}
@@ -21,16 +23,16 @@ public static <T> Iterator<List<T>> combine(List<Iterable<T>> listOfIterables) {
 	}
 
 	@SuppressWarnings("unchecked")
-	private static <T> Iterator<List<T>> emptyListSingleton() {
+	private static <T extends @Nullable Object> Iterator<List<T>> emptyListSingleton() {
 		return asList((List<T>) new ArrayList<>()).iterator();
 	}
 
 	@SuppressWarnings("unchecked")
-	private static <T> Iterator<Set<T>> emptySetSingleton() {
+	private static <T extends @Nullable Object> Iterator<Set<T>> emptySetSingleton() {
 		return asList((Set<T>) new LinkedHashSet<>()).iterator();
 	}
 
-	public static <T> Iterator<List<T>> listCombinations(Iterable<T> elementIterable, int minSize, int maxSize) {
+	public static <T extends @Nullable Object> Iterator<List<T>> listCombinations(Iterable<? extends T> elementIterable, int minSize, int maxSize) {
 		List<Iterator<List<T>>> iterators = new ArrayList<>();
 		for(int listSize = minSize; listSize <= maxSize; listSize++) {
 			iterators.add(listIterator(elementIterable, listSize));
@@ -38,15 +40,15 @@ public static <T> Iterator<List<T>> listCombinations(Iterable<T> elementIterable
 		return concatIterators(iterators);
 	}
 
-	private static <T> Iterator<List<T>> listIterator(Iterable<T> elementIterable, int listSize) {
-		List<Iterable<T>> listOfIterables = new ArrayList<>();
+	private static <T extends @Nullable Object> Iterator<List<T>> listIterator(Iterable<? extends T> elementIterable, int listSize) {
+		List<Iterable<? extends T>> listOfIterables = new ArrayList<>();
 		for (int i = 0; i < listSize; i++) {
 			listOfIterables.add(elementIterable);
 		}
 		return combine(listOfIterables);
 	}
 
-	public static <T> Iterator<Set<T>> setCombinations(Iterable<T> elementIterable, int minSize, int maxSize) {
+	public static <T extends @Nullable Object> Iterator<Set<T>> setCombinations(Iterable<? extends T> elementIterable, int minSize, int maxSize) {
 		List<Iterator<Set<T>>> iterators = new ArrayList<>();
 		for(int setSize = minSize; setSize <= maxSize; setSize++) {
 			Iterator<Set<T>> setIterator = setIterator(elementIterable, setSize);
@@ -55,26 +57,26 @@ public static <T> Iterator<Set<T>> setCombinations(Iterable<T> elementIterable,
 		return concatIterators(iterators);
 	}
 
-	private static <T> Iterator<Set<T>> setIterator(Iterable<T> elementIterable, int setSize) {
+	private static <T extends @Nullable Object> Iterator<Set<T>> setIterator(Iterable<? extends T> elementIterable, int setSize) {
 		if (setSize == 0) {
 			return emptySetSingleton();
 		}
 		return new SetIterator<>(elementIterable, setSize);
 	}
 
-	public static <T> Iterator<List<T>> listPermutations(List<T> values) {
+	public static <T extends @Nullable Object> Iterator<List<T>> listPermutations(List<? extends T> values) {
 		if (values.isEmpty()) {
 			return emptyListSingleton();
 		}
 		return new PermutationIterator<>(values);
 	}
 
-	public static <T> Iterator<T> concat(List<Iterable<T>> iterables) {
-		List<Iterator<T>> iterators = iterables.stream().map(Iterable::iterator).collect(Collectors.toList());
+	public static <T extends @Nullable Object> Iterator<T> concat(List<? extends Iterable<? extends T>> iterables) {
+		List<? extends Iterator<? extends T>> iterators = iterables.stream().map(Iterable::iterator).collect(Collectors.toList());
 		return new ConcatIterator<>(iterators);
 	}
 
-	private static <T> Iterator<T> concatIterators(List<Iterator<T>> iterators) {
+	private static <T extends @Nullable Object> Iterator<T> concatIterators(List<Iterator<T>> iterators) {
 		return new ConcatIterator<>(iterators);
 	}
 
diff --git a/engine/src/main/java/net/jqwik/engine/support/JqwikStreamSupport.java b/engine/src/main/java/net/jqwik/engine/support/JqwikStreamSupport.java
index f6a7f0a14..7bd50350d 100644
--- a/engine/src/main/java/net/jqwik/engine/support/JqwikStreamSupport.java
+++ b/engine/src/main/java/net/jqwik/engine/support/JqwikStreamSupport.java
@@ -1,5 +1,7 @@
 package net.jqwik.engine.support;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 import java.util.concurrent.atomic.*;
 import java.util.function.*;
@@ -40,13 +42,13 @@ public boolean tryAdvance(Consumer<? super T> action) {
 	}
 
 	@SafeVarargs
-	public static <T> Stream<T> concat(Stream<T>... streams) {
+	public static <T extends @Nullable Object> Stream<T> concat(Stream<T>... streams) {
 		// See discussion in https://github.com/jqwik-team/jqwik/issues/526 why StreamConcatenation is used
 		return StreamConcatenation.concat(streams);
 	}
 
 	@SuppressWarnings("unchecked")
-	public static <T> Stream<T> concat(List<Stream<T>> streams) {
+	public static <T extends @Nullable Object> Stream<T> concat(List<Stream<T>> streams) {
 		return concat(streams.toArray(new Stream[0]));
 	}
 
@@ -61,7 +63,7 @@ public static <T> Stream<T> concat(List<Stream<T>> streams) {
 	 * @param <T>    a type
 	 * @return a stream
 	 */
-	public static <T> Stream<T> takeWhile(Stream<T> stream, Predicate<? super T> p) {
+	public static <T extends @Nullable Object> Stream<T> takeWhile(Stream<T> stream, Predicate<? super T> p) {
 		class Taking extends Spliterators.AbstractSpliterator<T> implements Consumer<T> {
 			private static final int CANCEL_CHECK_COUNT = 63;
 			private final Spliterator<T> s;
@@ -114,7 +116,7 @@ public Spliterator<T> trySplit() {
 		return StreamSupport.stream(new Taking(stream.spliterator()), stream.isParallel()).onClose(stream::close);
 	}
 
-	public static <T> Stream<T> toStream(Optional<T> optional) {
+	public static <T extends @Nullable Object> Stream<T> toStream(Optional<T> optional) {
 		return optional.map(Stream::of).orElse(Stream.empty());
 	}
 }
diff --git a/engine/src/main/java/net/jqwik/engine/support/StreamConcatenation.java b/engine/src/main/java/net/jqwik/engine/support/StreamConcatenation.java
index 1e7350f57..64e861b51 100644
--- a/engine/src/main/java/net/jqwik/engine/support/StreamConcatenation.java
+++ b/engine/src/main/java/net/jqwik/engine/support/StreamConcatenation.java
@@ -1,5 +1,7 @@
 package net.jqwik.engine.support;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 import java.util.function.*;
 import java.util.stream.*;
@@ -48,7 +50,7 @@ private StreamConcatenation() {
 	 */
 	@SafeVarargs
 	@SuppressWarnings({"unchecked", "varargs"}) // TODO: Explain why this is ok.
-	public static <T> Stream<T> concat(Stream<? extends T>... streams) {
+	public static <T extends @Nullable Object> Stream<T> concat(Stream<? extends T>... streams) {
 		return concatInternal(
 			(Stream<T>[]) streams,
 			size -> (Spliterator<T>[]) new Spliterator<?>[size],
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java
index b2cc4c8d9..a621463ef 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java
@@ -4,13 +4,13 @@
 import java.util.stream.*;
 
 public class CombinedIterator<T> implements Iterator<List<T>> {
-	private final List<Iterable<T>> iterables;
-	private final List<Iterator<T>> iterators;
+	private final List<? extends Iterable<? extends T>> iterables;
+	private final List<Iterator<? extends T>> iterators;
 	private final List<T> elements;
 	private final boolean isEmpty;
 	private int position = -1;
 
-	public CombinedIterator(List<Iterable<T>> iterables) {
+	public CombinedIterator(List<? extends Iterable<? extends T>> iterables) {
 		this.iterables = iterables;
 		elements = new ArrayList<>(Collections.nCopies(iterables.size(), null));
 		iterators = iterables.stream().map(Iterable::iterator).collect(Collectors.toCollection(ArrayList::new));
@@ -34,7 +34,7 @@ public List<T> next() {
 			// The first initialization of the values
 			resetValuesFrom(0);
 		} else {
-			Iterator<T> it = iterators.get(position);
+			Iterator<? extends T> it = iterators.get(position);
 			if (it.hasNext()) {
 				// Just advance the current iterator
 				elements.set(position, it.next());
@@ -53,14 +53,14 @@ public List<T> next() {
 	}
 
 	private void resetValuesFrom(int startPosition) {
-		List<Iterator<T>> iterators = this.iterators;
-		List<Iterable<T>> iterables = this.iterables;
+		List<Iterator<? extends T>> iterators = this.iterators;
+		List<? extends Iterable<? extends T>> iterables = this.iterables;
 		List<T> elements = this.elements;
 		// In the initial reset, we can reuse the existing iterators
 		// It might slightly optimize the behavior, and it supports Stream#iterator which can't be executed twice
 		boolean initialReset = position == -1;
 		for (int i = startPosition; i < iterables.size(); i++) {
-			Iterator<T> newIt = initialReset ? iterators.get(i) : iterables.get(i).iterator();
+			Iterator<? extends T> newIt = initialReset ? iterators.get(i) : iterables.get(i).iterator();
 			iterators.set(i, newIt);
 			elements.set(i, newIt.next());
 		}
@@ -68,7 +68,7 @@ private void resetValuesFrom(int startPosition) {
 	}
 
 	private int nextAvailablePosition() {
-		List<Iterator<T>> iterators = this.iterators;
+		List<Iterator<? extends T>> iterators = this.iterators;
 		for (int i = position; i >= 0; i--) {
 			if (iterators.get(i).hasNext()) {
 				return i;
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java
index 506c2f540..b7d742fa1 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java
@@ -5,11 +5,11 @@
 
 public class ConcatIterator<T> implements Iterator<T> {
 
-	private final List<Iterator<T>> iterators;
+	private final List<? extends Iterator<? extends T>> iterators;
 	private final AtomicInteger position;
-	private Iterator<T> next;
+	private Iterator<? extends T> next;
 
-	public ConcatIterator(List<Iterator<T>> iterators) {
+	public ConcatIterator(List<? extends Iterator<? extends T>> iterators) {
 		this.iterators = iterators;
 		position = new AtomicInteger(0);
 		if (!iterators.isEmpty()) {
@@ -17,7 +17,7 @@ public ConcatIterator(List<Iterator<T>> iterators) {
 		}
 	}
 
-	private Iterator<T> findNext() {
+	private Iterator<? extends T> findNext() {
 		while (!iterators.get(position.get()).hasNext()) {
 			if (position.get() >= iterators.size() -1)
 				return null;
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java
index e11739aa9..560979c3b 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java
@@ -8,7 +8,7 @@ public class PermutationIterator<T> implements Iterator<List<T>> {
 	private List<T> next;
 	private int[] indices;
 
-	public PermutationIterator(List<T> values) {
+	public PermutationIterator(List<? extends T> values) {
 		this.values.addAll(values);
 		initializeIndices(values);
 
@@ -19,7 +19,7 @@ public PermutationIterator(List<T> values) {
 		}
 	}
 
-	private void initializeIndices(List<T> values) {
+	private void initializeIndices(List<? extends T> values) {
 		this.indices = new int[values.size()];
 		for (int i = 0; i < indices.length; ++i) {
 			indices[i] = i;
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java
index 7e6c9112d..3779206e4 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java
@@ -9,9 +9,9 @@ public class SetIterator<T> implements Iterator<Set<T>> {
 	private final int setSize;
 	private Set<T> next;
 
-	public SetIterator(Iterable<T> elementIterable, int setSize) {
+	public SetIterator(Iterable<? extends T> elementIterable, int setSize) {
 		this.setSize = setSize;
-		List<Iterable<T>> iterables = new ArrayList<>();
+		List<Iterable<? extends T>> iterables = new ArrayList<>();
 		for (int i = 0; i < setSize; i++) {
 			iterables.add(elementIterable);
 		}
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/package-info.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/package-info.java
new file mode 100644
index 000000000..2d14d3084
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.support.combinatorics;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/support/package-info.java b/engine/src/main/java/net/jqwik/engine/support/package-info.java
new file mode 100644
index 000000000..cebd7e05e
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/support/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.support;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/main/java/net/jqwik/engine/support/types/package-info.java b/engine/src/main/java/net/jqwik/engine/support/types/package-info.java
new file mode 100644
index 000000000..9d8a0060a
--- /dev/null
+++ b/engine/src/main/java/net/jqwik/engine/support/types/package-info.java
@@ -0,0 +1,4 @@
+@NullMarked
+package net.jqwik.engine.support.types;
+
+import org.jspecify.annotations.*;
\ No newline at end of file
diff --git a/engine/src/test/java/net/jqwik/api/ArbitraryTestHelper.java b/engine/src/test/java/net/jqwik/api/ArbitraryTestHelper.java
index a081639a2..471f19140 100644
--- a/engine/src/test/java/net/jqwik/api/ArbitraryTestHelper.java
+++ b/engine/src/test/java/net/jqwik/api/ArbitraryTestHelper.java
@@ -5,9 +5,11 @@
 import net.jqwik.engine.*;
 import net.jqwik.testing.*;
 
+import org.jspecify.annotations.*;
+
 public class ArbitraryTestHelper {
 
-	public static <T> void assertAllGenerated(RandomGenerator<? extends T> generator, Consumer<T> assertions) {
+	public static <T extends @Nullable Object> void assertAllGenerated(RandomGenerator<? extends T> generator, Consumer<? super T> assertions) {
 		Predicate<T> checker = value -> {
 			try {
 				assertions.accept(value);
diff --git a/engine/src/test/java/net/jqwik/api/ArbitraryTests.java b/engine/src/test/java/net/jqwik/api/ArbitraryTests.java
index 369a07d35..631b07752 100644
--- a/engine/src/test/java/net/jqwik/api/ArbitraryTests.java
+++ b/engine/src/test/java/net/jqwik/api/ArbitraryTests.java
@@ -233,7 +233,7 @@ public MyString(String string) {
 		}
 	}
 
-	private static int sum(List<Integer> list) {
+	private static int sum(List<? extends Integer> list) {
 		return list.stream().mapToInt(i -> i).sum();
 	}
 
diff --git a/engine/src/test/java/net/jqwik/api/ListArbitraryTests.java b/engine/src/test/java/net/jqwik/api/ListArbitraryTests.java
index 539d25ccf..38c28ea3b 100644
--- a/engine/src/test/java/net/jqwik/api/ListArbitraryTests.java
+++ b/engine/src/test/java/net/jqwik/api/ListArbitraryTests.java
@@ -279,10 +279,11 @@ private boolean isUniqueModulo(List<Integer> list, int modulo) {
 	@Example
 	void mapEach(@ForAll Random random) {
 		Arbitrary<Integer> integerArbitrary = Arbitraries.integers().between(1, 10);
+		@SuppressWarnings("unchecked")
 		Arbitrary<List<Tuple.Tuple2<Integer, List<Integer>>>> setArbitrary =
 			integerArbitrary
 				.list().ofSize(5)
-				.mapEach((all, each) -> Tuple.of(each, all));
+				.mapEach((all, each) -> Tuple.of(each, (List<Integer>) all));
 
 		RandomGenerator<List<Tuple.Tuple2<Integer, List<Integer>>>> generator = setArbitrary.generator(1, true);
 
diff --git a/engine/src/test/java/net/jqwik/api/SetArbitraryTests.java b/engine/src/test/java/net/jqwik/api/SetArbitraryTests.java
index f617d7553..73f601524 100644
--- a/engine/src/test/java/net/jqwik/api/SetArbitraryTests.java
+++ b/engine/src/test/java/net/jqwik/api/SetArbitraryTests.java
@@ -54,10 +54,11 @@ void setWithLessElementsThanMaxSize(@ForAll Random random) {
 	@Example
 	void mapEach(@ForAll Random random) {
 		Arbitrary<Integer> integerArbitrary = Arbitraries.integers().between(1, 10);
+		@SuppressWarnings("unchecked")
 		Arbitrary<Set<Tuple.Tuple2<Integer, Set<Integer>>>> setArbitrary =
 			integerArbitrary
 				.set().ofSize(5)
-				.mapEach((all, each) -> Tuple.of(each, all));
+				.mapEach((all, each) -> Tuple.of(each, (Set<Integer>) all));
 
 		RandomGenerator<Set<Tuple.Tuple2<Integer, Set<Integer>>>> generator = setArbitrary.generator(1, true);
 
diff --git a/engine/src/test/java/net/jqwik/api/constraints/UniqueElementsProperties.java b/engine/src/test/java/net/jqwik/api/constraints/UniqueElementsProperties.java
index b8e23ee22..7361363de 100644
--- a/engine/src/test/java/net/jqwik/api/constraints/UniqueElementsProperties.java
+++ b/engine/src/test/java/net/jqwik/api/constraints/UniqueElementsProperties.java
@@ -6,6 +6,8 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static java.util.Arrays.*;
 
 @PropertyDefaults(tries = 100)
@@ -207,7 +209,7 @@ public Object apply(String string) {
 		}
 	}
 
-	private <T> boolean hasNoDuplicates(Collection<T> collection, Function<T, Object> by) {
+	private <T extends @Nullable Object> boolean hasNoDuplicates(Collection<T> collection, Function<? super T, ?> by) {
 		Set<Object> set = collection.stream().map(by).collect(Collectors.toSet());
 		return set.size() == collection.size();
 	}
diff --git a/engine/src/test/java/net/jqwik/api/domains/ContextIsArbitraryConfigurator.java b/engine/src/test/java/net/jqwik/api/domains/ContextIsArbitraryConfigurator.java
index 185b7d581..b4947119e 100644
--- a/engine/src/test/java/net/jqwik/api/domains/ContextIsArbitraryConfigurator.java
+++ b/engine/src/test/java/net/jqwik/api/domains/ContextIsArbitraryConfigurator.java
@@ -4,11 +4,13 @@
 import net.jqwik.api.configurators.*;
 import net.jqwik.api.providers.*;
 
+import org.jspecify.annotations.*;
+
 class ContextIsArbitraryConfigurator extends DomainContextBase implements ArbitraryConfigurator {
 
 	@SuppressWarnings("unchecked")
 	@Override
-	public <T> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
+	public <T extends @Nullable Object> Arbitrary<T> configure(Arbitrary<T> arbitrary, TypeUsage targetType) {
 		if (targetType.isOfType(String.class) && targetType.isAnnotated(Doubled.class)) {
 			Arbitrary<String> stringArbitrary = (Arbitrary<String>) arbitrary;
 			return (Arbitrary<T>) stringArbitrary.map(s -> s + s);
diff --git a/engine/src/test/java/net/jqwik/engine/properties/shrinking/ArbitraryShrinkingTests.java b/engine/src/test/java/net/jqwik/engine/properties/shrinking/ArbitraryShrinkingTests.java
index 2d24ef7c8..d76714572 100644
--- a/engine/src/test/java/net/jqwik/engine/properties/shrinking/ArbitraryShrinkingTests.java
+++ b/engine/src/test/java/net/jqwik/engine/properties/shrinking/ArbitraryShrinkingTests.java
@@ -78,7 +78,7 @@ void collectedListShrinksElementsAndSize(@ForAll Random random) {
 		assertThat(shrunkValue).containsExactly(3, 3, 3, 3);
 	}
 
-	private int sum(List<Integer> list) {
+	private int sum(List<? extends Integer> list) {
 		return list.stream().mapToInt(i -> i).sum();
 	}
 
diff --git a/engine/src/test/java/net/jqwik/engine/properties/shrinking/CollectShrinkableTests.java b/engine/src/test/java/net/jqwik/engine/properties/shrinking/CollectShrinkableTests.java
index bb27cef5d..2c073bd8d 100644
--- a/engine/src/test/java/net/jqwik/engine/properties/shrinking/CollectShrinkableTests.java
+++ b/engine/src/test/java/net/jqwik/engine/properties/shrinking/CollectShrinkableTests.java
@@ -26,7 +26,7 @@ void creation() {
 
 		List<Shrinkable<Integer>> shrinkables = asList(shrinkable3, shrinkable2, shrinkable1);
 
-		Predicate<List<Integer>> untilIsIgnoredHere = l -> true;
+		Predicate<List<?>> untilIsIgnoredHere = l -> true;
 		CollectShrinkable<Integer> shrinkable = new CollectShrinkable<>(shrinkables, untilIsIgnoredHere);
 
 		assertThat(shrinkable.distance()).isEqualTo(ShrinkingDistance.of(3, 6));
@@ -43,7 +43,7 @@ void shrinkingWithoutSizeReduction() {
 
 			List<Shrinkable<Integer>> shrinkables = asList(shrinkable4, shrinkable1);
 
-			Predicate<List<Integer>> untilSizeAtLeast2 = l -> l.size() >= 2;
+			Predicate<List<?>> untilSizeAtLeast2 = l -> l.size() >= 2;
 			Shrinkable<List<Integer>> shrinkable = new CollectShrinkable<>(shrinkables, untilSizeAtLeast2);
 
 			List<Integer> shrunkValue = shrink(shrinkable, alwaysFalsify(), null);
@@ -57,7 +57,7 @@ void shrinkingWithFilter() {
 
 			List<Shrinkable<Integer>> shrinkables = asList(shrinkable4, shrinkable1);
 
-			Predicate<List<Integer>> untilSizeAtLeast2 = l -> l.size() >= 2;
+			Predicate<List<?>> untilSizeAtLeast2 = l -> l.size() >= 2;
 			Shrinkable<List<Integer>> shrinkable = new CollectShrinkable<>(shrinkables, untilSizeAtLeast2);
 
 			Falsifier<List<Integer>> falsifier = collection -> {
@@ -78,7 +78,7 @@ void shrinkingWithoutSizeReductionButNotAllShrinkingStepsWork() {
 
 			List<Shrinkable<Integer>> shrinkables = asList(shrinkable4, shrinkable1);
 
-			Predicate<List<Integer>> untilSizeAtLeast2 = l -> l.size() >= 2;
+			Predicate<List<?>> untilSizeAtLeast2 = l -> l.size() >= 2;
 			Shrinkable<List<Integer>> shrinkable = new CollectShrinkable<>(shrinkables, untilSizeAtLeast2);
 
 			TestingFalsifier<List<Integer>> sumMustNotBeEven = listOfInts -> {
@@ -96,7 +96,7 @@ void shrinkingWithSizeReduction() {
 			List<Shrinkable<Integer>> shrinkables =
 				asList(shrinkable1, shrinkable1, shrinkable1, shrinkable1, shrinkable1, shrinkable1);
 
-			Predicate<List<Integer>> sumAtLeast6 = l -> {
+			Predicate<List<? extends Integer>> sumAtLeast6 = l -> {
 				int sum = l.stream().mapToInt(i -> i).sum();
 				return sum >= 6;
 			};
@@ -114,7 +114,7 @@ void shrinkToFullySorted() {
 
 			List<Shrinkable<Integer>> shrinkables = asList(shrinkable3, shrinkable4, shrinkable2, shrinkable1);
 
-			Predicate<List<Integer>> sumAtLeast10 = l -> {
+			Predicate<List<? extends Integer>> sumAtLeast10 = l -> {
 				int sum = l.stream().mapToInt(i -> i).sum();
 				return sum >= 10;
 			};
@@ -132,7 +132,7 @@ void shrinkToPartiallySorted() {
 
 			List<Shrinkable<Integer>> shrinkables = asList(shrinkable3, shrinkable4, shrinkable2, shrinkable1);
 
-			Predicate<List<Integer>> sumAtLeast10 = l -> {
+			Predicate<List<? extends Integer>> sumAtLeast10 = l -> {
 				int sum = l.stream().mapToInt(i -> i).sum();
 				return sum >= 10 && l.get(0) != 1;
 			};
diff --git a/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java b/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java
index 150cbd926..a9575e9cc 100644
--- a/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java
+++ b/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java
@@ -22,7 +22,7 @@ class CombinedShrinkableTests {
 	void creation() {
 		Shrinkable three = new OneStepShrinkable(3);
 		Shrinkable hello = Shrinkable.unshrinkable("hello");
-		Function<List<Object>, String> combinator = shrinkables -> {
+		Function<List<?>, String> combinator = shrinkables -> {
 			int anInt = (int) shrinkables.get(0);
 			String aString = (String) shrinkables.get(1);
 			return aString + anInt;
@@ -37,7 +37,7 @@ void creation() {
 
 	@Example
 	void creationWithEmptyShrinkablesList() {
-		Function<List<Object>, String> combinator = shrinkables -> {
+		Function<List<?>, String> combinator = shrinkables -> {
 			return "constant";
 		};
 
@@ -56,7 +56,7 @@ class Shrinking {
 		void shrinkingToBottom() {
 			Shrinkable three = new OneStepShrinkable(3);
 			Shrinkable five = new OneStepShrinkable(5);
-			Function<List<Object>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
+			Function<List<?>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
 				int first = (int) shrinkables.get(0);
 				int second = (int) shrinkables.get(1);
 				return Tuple.of(first, second);
@@ -74,7 +74,7 @@ void shrinkingToBottom() {
 		void shrinkToCondition() {
 			Shrinkable three = new OneStepShrinkable(3);
 			Shrinkable five = new OneStepShrinkable(5);
-			Function<List<Object>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
+			Function<List<?>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
 				int first = (int) shrinkables.get(0);
 				int second = (int) shrinkables.get(1);
 				return Tuple.of(first, second);
@@ -92,7 +92,7 @@ void shrinkToCondition() {
 		void shrinkingWithFilter() {
 			Shrinkable three = new OneStepShrinkable(3);
 			Shrinkable five = new OneStepShrinkable(5);
-			Function<List<Object>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
+			Function<List<?>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
 				int first = (int) shrinkables.get(0);
 				int second = (int) shrinkables.get(1);
 				return Tuple.of(first, second);
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/AnyForSubtypeOfDsl.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/AnyForSubtypeOfDsl.kt
index c754a58e9..34ca145bf 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/AnyForSubtypeOfDsl.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/AnyForSubtypeOfDsl.kt
@@ -17,14 +17,14 @@ val <T : Any> KClass<T>.allSealedSubclasses: List<KClass<out T>>
     }
 
 @API(status = API.Status.EXPERIMENTAL, since = "1.8.4")
-class SubtypeScope<T> {
+class SubtypeScope<T: Any> {
     val customProviders = mutableListOf<CustomProvider<T>>()
 
     /**
      * Registers a custom provider for subtype [U], instead of default one created by [anyForSubtypeOf].
      */
     inline fun <reified U> provide(noinline customProvider: () -> Arbitrary<U>) where U : T {
-        customProviders.add(CustomProvider(U::class as KClass<Any>, customProvider) as CustomProvider<T>)
+        customProviders.add(CustomProvider(U::class, customProvider))
     }
 
     /**
@@ -33,8 +33,8 @@ class SubtypeScope<T> {
     fun getProviderFor(targetType: KClass<*>) =
         customProviders.firstOrNull { it.targetType == targetType }?.arbitraryFactory?.invoke()
 
-    class CustomProvider<T>(
-        val targetType: KClass<Any>,
-        val arbitraryFactory: () -> Arbitrary<T>
+    class CustomProvider<out T: Any>(
+        val targetType: KClass<out T>,
+        val arbitraryFactory: () -> Arbitrary<out T>
     )
 }
\ No newline at end of file
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/ArbitraryExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/ArbitraryExtensions.kt
index 39d268385..260fe7031 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/ArbitraryExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/ArbitraryExtensions.kt
@@ -58,5 +58,5 @@ fun <T> Arbitrary<T>.triple(): Arbitrary<Triple<T, T, T>> {
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
 inline fun <T, reified A: Any> Arbitrary<T>.array(): ArrayArbitrary<T, A> {
-    return array(A::class.java) as ArrayArbitrary<T, A>
+    return array(A::class.java)
 }
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
index b8a2771a8..97cdfc055 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
@@ -9,7 +9,7 @@ import java.util.function.BiFunction
  * Convenience function for Kotlin to not use backticked `in` function.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <B, T: Any?> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
+fun <B, T> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
     where B : Any {
     val toFunction = BiFunction<B, T, B> { b, t -> combinator(b, t) }
     return this.use(arbitrary).`in`(toFunction)
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/CollectionExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/CollectionExtensions.kt
index eebb91e20..6e15b8d50 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/CollectionExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/CollectionExtensions.kt
@@ -9,10 +9,10 @@ import org.apiguardian.api.API
  * Convenience function to replace Arbitraries.subsetOf(..)
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.5")
-fun <T> Collection<T>.anySubset(): SetArbitrary<T> = Arbitraries.subsetOf(this) as SetArbitrary<T>
+fun <T> Collection<T>.anySubset(): SetArbitrary<T> = Arbitraries.subsetOf(this)
 
 /**
  * Convenience function to replace Arbitraries.of(..)
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.7.1")
-fun <T> Collection<T>.anyValue(): Arbitrary<T> = Arbitraries.of(this) as Arbitrary<T>
+fun <T> Collection<T>.anyValue(): Arbitrary<T> = Arbitraries.of(this)
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/JqwikGlobals.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/JqwikGlobals.kt
index a375f2ad0..d0b88ae99 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/JqwikGlobals.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/JqwikGlobals.kt
@@ -41,8 +41,7 @@ fun anyFunction(kClass: KClass<*>): Functions.FunctionWrapper {
  * Create a [FunctionArbitrary] for Kotlin function without parameters.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <R> anyFunction0(returning: Arbitrary<R>): FunctionArbitrary<Function0<R>, R>
-    where R : Any {
+fun <R> anyFunction0(returning: Arbitrary<R>): FunctionArbitrary<Function0<R>, R> {
     return anyFunction(Function0::class).returning(returning)
 }
 
@@ -50,8 +49,7 @@ fun <R> anyFunction0(returning: Arbitrary<R>): FunctionArbitrary<Function0<R>, R
  * Create a [FunctionArbitrary] for Kotlin function with 1 parameter.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <A, R> anyFunction1(returning: Arbitrary<R>): FunctionArbitrary<Function1<A, R>, R>
-    where R : Any {
+fun <A, R> anyFunction1(returning: Arbitrary<R>): FunctionArbitrary<Function1<A, R>, R> {
     return anyFunction(Function1::class).returning(returning)
 }
 
@@ -59,8 +57,7 @@ fun <A, R> anyFunction1(returning: Arbitrary<R>): FunctionArbitrary<Function1<A,
  * Create a [FunctionArbitrary] for Kotlin function with 2 parameters.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <A, B, R> anyFunction2(returning: Arbitrary<R>): FunctionArbitrary<Function2<A, B, R>, R>
-    where R : Any {
+fun <A, B, R> anyFunction2(returning: Arbitrary<R>): FunctionArbitrary<Function2<A, B, R>, R> {
     return anyFunction(Function2::class).returning(returning)
 }
 
@@ -68,8 +65,7 @@ fun <A, B, R> anyFunction2(returning: Arbitrary<R>): FunctionArbitrary<Function2
  * Create a [FunctionArbitrary] for Kotlin function with 3 parameters.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <A, B, C, R> anyFunction3(returning: Arbitrary<R>): FunctionArbitrary<Function3<A, B, C, R>, R>
-    where R : Any {
+fun <A, B, C, R> anyFunction3(returning: Arbitrary<R>): FunctionArbitrary<Function3<A, B, C, R>, R> {
     return anyFunction(Function3::class).returning(returning)
 }
 
@@ -77,8 +73,7 @@ fun <A, B, C, R> anyFunction3(returning: Arbitrary<R>): FunctionArbitrary<Functi
  * Create a [FunctionArbitrary] for Kotlin function with 4 parameters.
  */
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
-fun <A, B, C, D, R> anyFunction4(returning: Arbitrary<R>): FunctionArbitrary<Function4<A, B, C, D, R>, R>
-    where R : Any {
+fun <A, B, C, D, R> anyFunction4(returning: Arbitrary<R>): FunctionArbitrary<Function4<A, B, C, D, R>, R> {
     return anyFunction(Function4::class).returning(returning)
 }
 
@@ -127,14 +122,14 @@ inline fun <reified T> anyForSubtypeOf(
     val scope = SubtypeScope<T>().apply(subtypeScope)
     return Arbitraries.of(T::class.allSealedSubclasses).flatMap {
         scope.getProviderFor(it)
-            ?: Arbitraries.forType(it.java as Class<T>).run {
+            ?: Arbitraries.forType(it.java).run {
                 if (enableArbitraryRecursion) {
                     enableRecursion()
                 } else {
                     this
                 }
             }
-    }.map { obj -> obj as T }
+    }.map { obj -> obj }
 }
 
 /**
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
index 1b75d2170..31d1abe87 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/SequenceArbitrary.kt
@@ -15,7 +15,7 @@ import java.util.function.Function
  * of type [Sequence]
  */
 @API(status = EXPERIMENTAL, since = "1.6.0")
-class SequenceArbitrary<T: Any?>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<Sequence<T>>(),
+class SequenceArbitrary<T>(elementArbitrary: Arbitrary<T>) : ArbitraryDecorator<Sequence<T>>(),
     SizableArbitrary<Sequence<T>> {
 
     private var listArbitrary: ListArbitrary<T>
diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/internal/KotlinUniqueElementsConfigurator.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/internal/KotlinUniqueElementsConfigurator.kt
index 7dff246ec..91295f9c9 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/internal/KotlinUniqueElementsConfigurator.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/internal/KotlinUniqueElementsConfigurator.kt
@@ -12,7 +12,7 @@ import java.util.function.Function
 
 class KotlinUniqueElementsConfigurator : ArbitraryConfigurator {
     @Suppress("UNCHECKED_CAST")
-    override fun <T:Any> configure(arbitrary: Arbitrary<T>, targetType: TypeUsage): Arbitrary<T> {
+    override fun <T> configure(arbitrary: Arbitrary<T>, targetType: TypeUsage): Arbitrary<T> {
         return targetType.findAnnotation(UniqueElements::class.java).map { uniqueness ->
             return@map when {
                 arbitrary is SequenceArbitrary<*> -> configureSequenceArbitrary(arbitrary, uniqueness)
diff --git a/testing/src/main/java/net/jqwik/testing/CheckReporting.java b/testing/src/main/java/net/jqwik/testing/CheckReporting.java
index 9aae434b8..66a8908f4 100644
--- a/testing/src/main/java/net/jqwik/testing/CheckReporting.java
+++ b/testing/src/main/java/net/jqwik/testing/CheckReporting.java
@@ -25,5 +25,5 @@ public PropertyExecutionResult aroundProperty(PropertyLifecycleContext context,
 		return result;
 	}
 
-	public abstract void check(@NonNull Reporter mockReporter);
+	public abstract void check(Reporter mockReporter);
 }
diff --git a/testing/src/main/java/net/jqwik/testing/ShrinkingSupport.java b/testing/src/main/java/net/jqwik/testing/ShrinkingSupport.java
index 3ae3b2424..6a4d2d3bc 100644
--- a/testing/src/main/java/net/jqwik/testing/ShrinkingSupport.java
+++ b/testing/src/main/java/net/jqwik/testing/ShrinkingSupport.java
@@ -11,6 +11,8 @@
 import net.jqwik.api.facades.*;
 import net.jqwik.api.lifecycle.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 import static org.assertj.core.api.Assertions.*;
 
@@ -20,35 +22,35 @@ public class ShrinkingSupport {
 	private ShrinkingSupport() {
 	}
 
-	public static <T> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random) {
+	public static <T extends @Nullable Object> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random) {
 		return falsifyThenShrink(arbitrary, random, ignore -> TryExecutionResult.falsified(null));
 	}
 
-	public static <T> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random, Falsifier<T> falsifier) {
+	public static <T extends @Nullable Object> T falsifyThenShrink(Arbitrary<? extends T> arbitrary, Random random, Falsifier<? super T> falsifier) {
 		return ShrinkingSupportFacade.implementation.falsifyThenShrink(arbitrary, random, falsifier);
 	}
 
-	public static <T> T falsifyThenShrink(RandomGenerator<? extends T> generator, Random random, Falsifier<T> falsifier) {
+	public static <T extends @Nullable Object> T falsifyThenShrink(RandomGenerator<? extends T> generator, Random random, Falsifier<? super T> falsifier) {
 		return ShrinkingSupportFacade.implementation.falsifyThenShrink(generator, random, falsifier);
 	}
 
-	public static <T> T shrink(
+	public static <T extends @Nullable Object> T shrink(
 			Shrinkable<T> falsifiedShrinkable,
-			Falsifier<T> falsifier,
+			Falsifier<? super T> falsifier,
 			Throwable originalError
 	) {
 		return ShrinkingSupportFacade.implementation.shrink(falsifiedShrinkable, falsifier, originalError);
 	}
 
-	public static <T> ShrunkFalsifiedSample shrinkToSample(
+	public static <T extends @Nullable Object> ShrunkFalsifiedSample shrinkToSample(
 			Shrinkable<T> falsifiedShrinkable,
-			Falsifier<T> falsifier,
+			Falsifier<? super T> falsifier,
 			Throwable originalError
 	) {
 		return ShrinkingSupportFacade.implementation.shrinkToSample(falsifiedShrinkable, falsifier, originalError);
 	}
 
-	public static <T> void assertWhileShrinking(Shrinkable<T> shrinkable, Predicate<T> condition) {
+	public static <T extends @Nullable Object> void assertWhileShrinking(Shrinkable<T> shrinkable, Predicate<? super T> condition) {
 		while(true) {
 			List<Shrinkable<T>> collect = shrinkable.shrink().collect(Collectors.toList());
 			assertThat(collect).allMatch(s -> condition.test(s.value()));
@@ -59,7 +61,7 @@ public static <T> void assertWhileShrinking(Shrinkable<T> shrinkable, Predicate<
 		}
 	}
 
-	public static <T> void assertAllValuesAreShrunkTo(Arbitrary<? extends T> arbitrary, Random random, T expectedShrunkValue) {
+	public static <T extends @Nullable Object> void assertAllValuesAreShrunkTo(Arbitrary<T> arbitrary, Random random, T expectedShrunkValue) {
 		T value = falsifyThenShrink(arbitrary, random);
 		Assertions.assertThat(value).isEqualTo(expectedShrunkValue);
 	}
diff --git a/testing/src/main/java/net/jqwik/testing/TestingFalsifier.java b/testing/src/main/java/net/jqwik/testing/TestingFalsifier.java
index c9151eae9..3bebdda1a 100644
--- a/testing/src/main/java/net/jqwik/testing/TestingFalsifier.java
+++ b/testing/src/main/java/net/jqwik/testing/TestingFalsifier.java
@@ -3,6 +3,7 @@
 import java.util.function.*;
 
 import org.apiguardian.api.*;
+import org.jspecify.annotations.*;
 import org.opentest4j.*;
 
 import net.jqwik.api.*;
@@ -11,13 +12,13 @@
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = EXPERIMENTAL, since = "1.4.0")
-public interface TestingFalsifier<T> extends Falsifier<T>, Predicate<T> {
+public interface TestingFalsifier<T extends @Nullable Object> extends Falsifier<T>, Predicate<T> {
 
-	static <T> TestingFalsifier<T> alwaysFalsify() {
+	static <T extends @Nullable Object> TestingFalsifier<T> alwaysFalsify() {
 		return ignore -> false;
 	}
 
-	static <T> TestingFalsifier<T> falsifier(Predicate<T> predicate) {
+	static <T extends @Nullable Object> TestingFalsifier<T> falsifier(Predicate<? super T> predicate) {
 		return predicate::test;
 	}
 
diff --git a/testing/src/main/java/net/jqwik/testing/TestingSupport.java b/testing/src/main/java/net/jqwik/testing/TestingSupport.java
index 8cee2e708..467ce90a4 100644
--- a/testing/src/main/java/net/jqwik/testing/TestingSupport.java
+++ b/testing/src/main/java/net/jqwik/testing/TestingSupport.java
@@ -10,6 +10,8 @@
 import net.jqwik.api.*;
 import net.jqwik.api.facades.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 import static org.assertj.core.api.Assertions.*;
 
@@ -19,12 +21,12 @@ public class TestingSupport {
 	private TestingSupport() {
 	}
 
-	public static <T> void checkAllGenerated(Arbitrary<? extends T> arbitrary, Random random, Predicate<T> checker) {
+	public static <T extends @Nullable Object> void checkAllGenerated(Arbitrary<T> arbitrary, Random random, Predicate<? super T> checker) {
 		checkAllGenerated(arbitrary.generator(1000), random, checker);
 	}
 
-	public static <T> void checkAllGenerated(RandomGenerator<? extends T> generator, Random random, Predicate<T> checker) {
-		Optional<? extends Shrinkable<? extends T>> failure =
+	public static <T extends @Nullable Object> void checkAllGenerated(RandomGenerator<T> generator, Random random, Predicate<? super T> checker) {
+		Optional<Shrinkable<T>> failure =
 			generator
 				.stream(random)
 				.limit(100)
@@ -36,11 +38,11 @@ public static <T> void checkAllGenerated(RandomGenerator<? extends T> generator,
 		});
 	}
 
-	public static <T> void assertAllGenerated(Arbitrary<? extends T> arbitrary, Random random, Consumer<T> assertions) {
+	public static <T extends @Nullable Object> void assertAllGenerated(Arbitrary<T> arbitrary, Random random, Consumer<? super T> assertions) {
 		assertAllGenerated(arbitrary.generator(1000), random, assertions);
 	}
 
-	public static <T> void assertAllGenerated(RandomGenerator<? extends T> generator, Random random, Consumer<T> assertions) {
+	public static <T extends @Nullable Object> void assertAllGenerated(RandomGenerator<T> generator, Random random, Consumer<? super T> assertions) {
 		Predicate<T> checker = value -> {
 			assertions.accept(value);
 			return true;
@@ -48,7 +50,7 @@ public static <T> void assertAllGenerated(RandomGenerator<? extends T> generator
 		checkAllGenerated(generator, random, checker);
 	}
 
-	public static <T> void assertAllGeneratedEqualTo(RandomGenerator<? extends T> generator, Random random, T expected) {
+	public static <T extends @Nullable Object> void assertAllGeneratedEqualTo(RandomGenerator<T> generator, Random random, T expected) {
 		assertAllGenerated(
 			generator,
 			random,
@@ -56,7 +58,7 @@ public static <T> void assertAllGeneratedEqualTo(RandomGenerator<? extends T> ge
 		);
 	}
 
-	public static <T> void assertAllGeneratedEqualTo(Arbitrary<? extends T> arbitrary, Random random, T expected) {
+	public static <T extends @Nullable Object> void assertAllGeneratedEqualTo(Arbitrary<T> arbitrary, Random random, T expected) {
 		assertAllGeneratedEqualTo(
 			arbitrary.generator(1000),
 			random,
@@ -64,13 +66,13 @@ public static <T> void assertAllGeneratedEqualTo(Arbitrary<? extends T> arbitrar
 		);
 	}
 
-	public static <T> void checkAtLeastOneGenerated(
-		RandomGenerator<? extends T> generator,
+	public static <T extends @Nullable Object> void checkAtLeastOneGenerated(
+		RandomGenerator<T> generator,
 		Random random,
-		Predicate<T> checker,
+		Predicate<? super T> checker,
 		String failureMessage
 	) {
-		Optional<? extends Shrinkable<? extends T>> success =
+		Optional<Shrinkable<T>> success =
 			generator
 				.stream(random)
 				.limit(5000)
@@ -81,15 +83,15 @@ public static <T> void checkAtLeastOneGenerated(
 		}
 	}
 
-	public static <T> void checkAtLeastOneGenerated(
-		RandomGenerator<? extends T> generator,
+	public static <T extends @Nullable Object> void checkAtLeastOneGenerated(
+		RandomGenerator<T> generator,
 		Random random,
-		Predicate<T> checker
+		Predicate<? super T> checker
 	) {
 		checkAtLeastOneGenerated(generator, random, checker, "Failed to generate at least one");
 	}
 
-	public static <T> void checkAtLeastOneGenerated(
+	public static <T extends @Nullable Object> void checkAtLeastOneGenerated(
 		Arbitrary<? extends T> arbitrary,
 		Random random,
 		Predicate<T> checker
@@ -98,18 +100,18 @@ public static <T> void checkAtLeastOneGenerated(
 	}
 
 	@SafeVarargs
-	public static <T> void assertAtLeastOneGeneratedOf(
+	public static <T extends @Nullable Object> void assertAtLeastOneGeneratedOf(
 		RandomGenerator<? extends T> generator,
 		Random random,
 		T... values
 	) {
 		for (T value : values) {
-			checkAtLeastOneGenerated(generator, random, value::equals, "Failed to generate " + value);
+			checkAtLeastOneGenerated(generator, random, x -> Objects.equals(x, value), "Failed to generate " + value);
 		}
 	}
 
 	@SafeVarargs
-	public static <T> void assertGeneratedExactly(RandomGenerator<? extends T> generator, Random random, T... expectedValues) {
+	public static <T extends @Nullable Object> void assertGeneratedExactly(RandomGenerator<? extends T> generator, Random random, T... expectedValues) {
 		List<T> generated = generator
 			.stream(random)
 			.limit(expectedValues.length)
@@ -119,7 +121,7 @@ public static <T> void assertGeneratedExactly(RandomGenerator<? extends T> gener
 		assertThat(generated).containsExactly(expectedValues);
 	}
 
-	public static <T> Set<Shrinkable<T>> collectEdgeCaseShrinkables(EdgeCases<T> edgeCases) {
+	public static <T extends @Nullable Object> Set<Shrinkable<T>> collectEdgeCaseShrinkables(EdgeCases<T> edgeCases) {
 		Set<Shrinkable<T>> shrinkables = new HashSet<>();
 		for (Shrinkable<T> edgeCase : edgeCases) {
 			shrinkables.add(edgeCase);
@@ -127,7 +129,7 @@ public static <T> Set<Shrinkable<T>> collectEdgeCaseShrinkables(EdgeCases<T> edg
 		return shrinkables;
 	}
 
-	public static <T> Set<T> collectEdgeCaseValues(EdgeCases<T> edgeCases) {
+	public static <T extends @Nullable Object> Set<T> collectEdgeCaseValues(EdgeCases<T> edgeCases) {
 		Set<T> values = new HashSet<>();
 		for (Shrinkable<T> edgeCase : edgeCases) {
 			values.add(edgeCase.value());
@@ -135,12 +137,12 @@ public static <T> Set<T> collectEdgeCaseValues(EdgeCases<T> edgeCases) {
 		return values;
 	}
 
-	public static <T> T generateFirst(Arbitrary<T> arbitrary, Random random) {
+	public static <T extends @Nullable Object> T generateFirst(Arbitrary<T> arbitrary, Random random) {
 		RandomGenerator<T> generator = arbitrary.generator(1, true);
 		return generator.next(random).value();
 	}
 
-	public static <T> Map<T, Long> count(RandomGenerator<T> generator, int tries, Random random) {
+	public static <T extends @Nullable Object> Map<T, Long> count(RandomGenerator<T> generator, int tries, Random random) {
 		return generator
 			.stream(random)
 			.limit(tries)
@@ -148,7 +150,7 @@ public static <T> Map<T, Long> count(RandomGenerator<T> generator, int tries, Ra
 			.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
 	}
 
-	public static <T> Shrinkable<T> generateUntil(RandomGenerator<T> generator, Random random, Function<T, Boolean> condition) {
+	public static <T extends @Nullable Object> Shrinkable<T> generateUntil(RandomGenerator<T> generator, Random random, Function<? super T, Boolean> condition) {
 		return TestingSupportFacade.implementation.generateUntil(generator, random, condition);
 	}
 

From f9e59ebf69202e5c4c7d3c2884fb36846002feea Mon Sep 17 00:00:00 2001
From: Vladimir Sitnikov <sitnikov@netcracker.com>
Date: Fri, 7 Jun 2024 15:23:49 +0300
Subject: [PATCH 3/5] style: remove excessive variable in
 BuilderCombinator<B>.use

---
 .../src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
index 97cdfc055..a06674d57 100644
--- a/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
+++ b/kotlin/src/main/kotlin/net/jqwik/kotlin/api/BuildersExtensions.kt
@@ -11,6 +11,5 @@ import java.util.function.BiFunction
 @API(status = API.Status.EXPERIMENTAL, since = "1.6.0")
 fun <B, T> BuilderCombinator<B>.use(arbitrary: Arbitrary<T>, combinator: (B, T) -> B): BuilderCombinator<B>
     where B : Any {
-    val toFunction = BiFunction<B, T, B> { b, t -> combinator(b, t) }
-    return this.use(arbitrary).`in`(toFunction)
+    return this.use(arbitrary).`in`(combinator)
 }

From abe7b1e99f6656eb26cdeea864f90f03867e4313 Mon Sep 17 00:00:00 2001
From: Vladimir Sitnikov <sitnikov@netcracker.com>
Date: Fri, 7 Jun 2024 15:30:09 +0300
Subject: [PATCH 4/5] WIP: fix for javac8

---
 .../shrinking/CombinedShrinkableTests.java         | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java b/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java
index a9575e9cc..acde0eab7 100644
--- a/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java
+++ b/engine/src/test/java/net/jqwik/engine/properties/shrinking/CombinedShrinkableTests.java
@@ -23,7 +23,7 @@ void creation() {
 		Shrinkable three = new OneStepShrinkable(3);
 		Shrinkable hello = Shrinkable.unshrinkable("hello");
 		Function<List<?>, String> combinator = shrinkables -> {
-			int anInt = (int) shrinkables.get(0);
+			int anInt = (Integer) shrinkables.get(0);
 			String aString = (String) shrinkables.get(1);
 			return aString + anInt;
 		};
@@ -57,8 +57,8 @@ void shrinkingToBottom() {
 			Shrinkable three = new OneStepShrinkable(3);
 			Shrinkable five = new OneStepShrinkable(5);
 			Function<List<?>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
-				int first = (int) shrinkables.get(0);
-				int second = (int) shrinkables.get(1);
+				int first = (Integer) shrinkables.get(0);
+				int second = (Integer) shrinkables.get(1);
 				return Tuple.of(first, second);
 			};
 
@@ -75,8 +75,8 @@ void shrinkToCondition() {
 			Shrinkable three = new OneStepShrinkable(3);
 			Shrinkable five = new OneStepShrinkable(5);
 			Function<List<?>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
-				int first = (int) shrinkables.get(0);
-				int second = (int) shrinkables.get(1);
+				int first = (Integer) shrinkables.get(0);
+				int second = (Integer) shrinkables.get(1);
 				return Tuple.of(first, second);
 			};
 
@@ -93,8 +93,8 @@ void shrinkingWithFilter() {
 			Shrinkable three = new OneStepShrinkable(3);
 			Shrinkable five = new OneStepShrinkable(5);
 			Function<List<?>, Tuple2<Integer, Integer>> combinator = shrinkables -> {
-				int first = (int) shrinkables.get(0);
-				int second = (int) shrinkables.get(1);
+				int first = (Integer) shrinkables.get(0);
+				int second = (Integer) shrinkables.get(1);
 				return Tuple.of(first, second);
 			};
 

From 389a32858aa6aaec60ef29fa6a87bfda736acb2a Mon Sep 17 00:00:00 2001
From: Vladimir Sitnikov <sitnikov@netcracker.com>
Date: Fri, 7 Jun 2024 16:30:25 +0300
Subject: [PATCH 5/5] fix: add extends @Nullable Object to class/interface
 declarations

---
 api/src/main/java/net/jqwik/api/ArbitrarySupplier.java |  2 +-
 api/src/main/java/net/jqwik/api/EdgeCases.java         |  2 +-
 .../java/net/jqwik/api/arbitraries/TypeArbitrary.java  |  2 +-
 .../java/net/jqwik/api/state/ActionChainArbitrary.java |  4 +++-
 .../main/java/net/jqwik/api/state/ChainArbitrary.java  |  4 +++-
 .../main/java/net/jqwik/api/state/Transformation.java  |  2 +-
 api/src/main/java/net/jqwik/api/state/Transformer.java |  2 +-
 .../main/java/net/jqwik/engine/ArbitraryDelegator.java |  4 +++-
 .../jqwik/engine/execution/lifecycle/ScopedStore.java  |  3 ++-
 .../properties/arbitraries/ArbitraryCollect.java       |  4 +++-
 .../engine/properties/arbitraries/ArbitraryFilter.java |  4 +++-
 .../properties/arbitraries/ChooseValueArbitrary.java   |  4 +++-
 .../arbitraries/DefaultIteratorArbitrary.java          |  4 +++-
 .../properties/arbitraries/DefaultSetArbitrary.java    |  4 +++-
 .../properties/arbitraries/DefaultStreamArbitrary.java |  4 +++-
 .../arbitraries/DefaultTraverseArbitrary.java          |  5 +++--
 .../properties/arbitraries/DefaultTypeArbitrary.java   |  3 ++-
 .../arbitraries/FromGeneratorWithSizeArbitrary.java    |  4 +++-
 .../arbitraries/GenericEdgeCasesConfiguration.java     |  4 +++-
 .../engine/properties/arbitraries/JustArbitrary.java   |  2 +-
 .../engine/properties/arbitraries/LazyArbitrary.java   |  6 ++++--
 .../engine/properties/arbitraries/LazyOfArbitrary.java |  2 +-
 .../engine/properties/arbitraries/OneOfArbitrary.java  |  4 +++-
 .../properties/arbitraries/RecursiveArbitrary.java     |  4 +++-
 .../properties/arbitraries/ReportableStream.java       |  2 +-
 .../properties/arbitraries/ShuffleArbitrary.java       |  4 +++-
 .../arbitraries/combinations/CombineArbitrary.java     |  4 +++-
 .../combinations/DefaultListCombinator.java            |  4 ++--
 .../exhaustive/CombinedExhaustiveGenerator.java        |  4 +++-
 .../arbitraries/exhaustive/ExhaustiveGenerators.java   |  2 +-
 .../exhaustive/FilteredExhaustiveGenerator.java        |  4 +++-
 .../arbitraries/randomized/CollectGenerator.java       |  4 +++-
 .../arbitraries/randomized/FilteredGenerator.java      |  4 +++-
 .../arbitraries/randomized/FrequencyGenerator.java     |  4 +++-
 .../randomized/IgnoreExceptionGenerator.java           |  4 +++-
 .../randomized/InjectDuplicatesGenerator.java          |  4 +++-
 .../arbitraries/randomized/WithEdgeCasesGenerator.java |  2 +-
 .../properties/shrinking/AbstractValueShrinkable.java  | 10 +++++++---
 .../properties/shrinking/ChooseValueShrinkable.java    |  4 +++-
 .../engine/properties/shrinking/CollectShrinkable.java |  4 +++-
 .../properties/shrinking/CombinedShrinkable.java       |  4 +++-
 .../properties/shrinking/FilteredShrinkable.java       |  4 +++-
 .../shrinking/IgnoreExceptionShrinkable.java           |  4 +++-
 .../engine/properties/shrinking/LazyOfShrinkable.java  |  4 +++-
 .../engine/properties/shrinking/SampleShrinkable.java  |  4 +++-
 .../engine/properties/shrinking/ShrinkableList.java    |  4 +++-
 .../engine/properties/shrinking/ShrinkableSet.java     |  2 +-
 .../engine/properties/shrinking/Unshrinkable.java      |  4 +++-
 .../properties/state/DefaultActionChainArbitrary.java  |  3 ++-
 .../engine/properties/state/DefaultChainArbitrary.java |  4 +++-
 .../jqwik/engine/properties/state/ShrinkableChain.java |  6 +++---
 .../properties/stateful/ActionSequenceGenerator.java   |  4 +++-
 .../stateful/DefaultActionSequenceArbitrary.java       |  4 +++-
 .../properties/stateful/SequentialActionSequence.java  |  5 +++--
 .../properties/stateful/ShrinkableActionSequence.java  |  6 ++++--
 .../engine/support/ChooseRandomlyByFrequency.java      |  4 +++-
 .../engine/support/combinatorics/CombinedIterator.java |  4 +++-
 .../engine/support/combinatorics/ConcatIterator.java   |  8 +++++---
 .../support/combinatorics/PermutationIterator.java     |  8 +++++---
 .../engine/support/combinatorics/SetIterator.java      |  4 +++-
 60 files changed, 164 insertions(+), 73 deletions(-)

diff --git a/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java b/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java
index 7a604414c..bb48ac91c 100644
--- a/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java
+++ b/api/src/main/java/net/jqwik/api/ArbitrarySupplier.java
@@ -24,7 +24,7 @@
  * @see From
  */
 @API(status = MAINTAINED, since = "1.7.0")
-public interface ArbitrarySupplier<T> extends Supplier<Arbitrary<T>> {
+public interface ArbitrarySupplier<T extends @Nullable Object> extends Supplier<Arbitrary<T>> {
 
 	/**
 	 * Override this method if you need more information about the target type,
diff --git a/api/src/main/java/net/jqwik/api/EdgeCases.java b/api/src/main/java/net/jqwik/api/EdgeCases.java
index fab4c9a3f..592ecefd4 100644
--- a/api/src/main/java/net/jqwik/api/EdgeCases.java
+++ b/api/src/main/java/net/jqwik/api/EdgeCases.java
@@ -9,7 +9,7 @@
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = EXPERIMENTAL, since = "1.3.0")
-public interface EdgeCases<T> extends Iterable<Shrinkable<T>> {
+public interface EdgeCases<T extends @Nullable Object> extends Iterable<Shrinkable<T>> {
 
 	@API(status = INTERNAL)
 	abstract class EdgeCasesFacade {
diff --git a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
index 57dcf14fa..0f74373ba 100644
--- a/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/arbitraries/TypeArbitrary.java
@@ -26,7 +26,7 @@
  * @see net.jqwik.api.domains.DomainContext
  */
 @API(status = MAINTAINED, since = "1.2")
-public interface TypeArbitrary<T> extends Arbitrary<T> {
+public interface TypeArbitrary<T extends @Nullable Object> extends Arbitrary<T> {
 
 	/**
 	 * Add public constructors of class {@code T} to be used
diff --git a/api/src/main/java/net/jqwik/api/state/ActionChainArbitrary.java b/api/src/main/java/net/jqwik/api/state/ActionChainArbitrary.java
index 336288452..e0b3e8e92 100644
--- a/api/src/main/java/net/jqwik/api/state/ActionChainArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/state/ActionChainArbitrary.java
@@ -6,10 +6,12 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface ActionChainArbitrary<S> extends Arbitrary<ActionChain<S>> {
+public interface ActionChainArbitrary<S extends @Nullable Object> extends Arbitrary<ActionChain<S>> {
 
 	/**
 	 * Allow an additional action with default weight of 1.
diff --git a/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java b/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java
index 12d24ca2f..7fc39b28a 100644
--- a/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java
+++ b/api/src/main/java/net/jqwik/api/state/ChainArbitrary.java
@@ -6,10 +6,12 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static org.apiguardian.api.API.Status.*;
 
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface ChainArbitrary<T> extends Arbitrary<Chain<T>> {
+public interface ChainArbitrary<T extends @Nullable Object> extends Arbitrary<Chain<T>> {
 
 	/**
 	 * Allow an additional {@linkplain Transformation} on the generated chain.
diff --git a/api/src/main/java/net/jqwik/api/state/Transformation.java b/api/src/main/java/net/jqwik/api/state/Transformation.java
index d0efbd2db..b242d055b 100644
--- a/api/src/main/java/net/jqwik/api/state/Transformation.java
+++ b/api/src/main/java/net/jqwik/api/state/Transformation.java
@@ -24,7 +24,7 @@
  */
 @FunctionalInterface
 @API(status = EXPERIMENTAL, since = "1.7.0")
-public interface Transformation<T> extends Function<Supplier<T>, Arbitrary<Transformer<T>>> {
+public interface Transformation<T extends @Nullable Object> extends Function<Supplier<T>, Arbitrary<Transformer<T>>> {
 
 	Predicate<?> NO_PRECONDITION = ignore -> false;
 
diff --git a/api/src/main/java/net/jqwik/api/state/Transformer.java b/api/src/main/java/net/jqwik/api/state/Transformer.java
index 05f6de95e..28090b49a 100644
--- a/api/src/main/java/net/jqwik/api/state/Transformer.java
+++ b/api/src/main/java/net/jqwik/api/state/Transformer.java
@@ -117,7 +117,7 @@ static <T> Transformer<T> noop() {
 	 * @param <S>         The type of the state to transform
 	 * @return a new instance of a transformer
 	 */
-	static <S extends @Nullable Object> Transformer<S> transform(String description, Function<? super S, S> transform) {
+	static <S extends @Nullable Object> Transformer<S> transform(String description, Function<? super S, ? extends S> transform) {
 		return new Transformer<S>() {
 			@Override
 			public S apply(S s) {
diff --git a/engine/src/main/java/net/jqwik/engine/ArbitraryDelegator.java b/engine/src/main/java/net/jqwik/engine/ArbitraryDelegator.java
index 8f8e0c23b..4cbded6bf 100644
--- a/engine/src/main/java/net/jqwik/engine/ArbitraryDelegator.java
+++ b/engine/src/main/java/net/jqwik/engine/ArbitraryDelegator.java
@@ -3,7 +3,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.arbitraries.*;
 
-public class ArbitraryDelegator<T> extends ArbitraryDecorator<T> {
+import org.jspecify.annotations.*;
+
+public class ArbitraryDelegator<T extends @Nullable Object> extends ArbitraryDecorator<T> {
 
 	private final Arbitrary<T> delegate;
 
diff --git a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java
index aeeb5e9a3..1d2c6e286 100644
--- a/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java
+++ b/engine/src/main/java/net/jqwik/engine/execution/lifecycle/ScopedStore.java
@@ -3,6 +3,7 @@
 import java.util.function.*;
 import java.util.logging.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.engine.*;
 
 import net.jqwik.api.lifecycle.*;
@@ -10,7 +11,7 @@
 
 import static net.jqwik.engine.support.JqwikStringSupport.*;
 
-public class ScopedStore<T> implements Store<T> {
+public class ScopedStore<T extends @Nullable Object> implements Store<T> {
 
 	private static final Logger LOG = Logger.getLogger(ScopedStore.class.getName());
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java
index 517c47a74..503aeb753 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryCollect.java
@@ -6,7 +6,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.support.*;
 
-public class ArbitraryCollect<T> implements Arbitrary<List<T>> {
+import org.jspecify.annotations.*;
+
+public class ArbitraryCollect<T extends @Nullable Object> implements Arbitrary<List<T>> {
 
 	private final Arbitrary<T> elementArbitrary;
 	private final Predicate<? super List<? extends T>> until;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java
index 4128cbf96..ad9566364 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ArbitraryFilter.java
@@ -7,7 +7,9 @@
 import net.jqwik.api.support.*;
 import net.jqwik.engine.*;
 
-public class ArbitraryFilter<T> extends ArbitraryDelegator<T> {
+import org.jspecify.annotations.*;
+
+public class ArbitraryFilter<T extends @Nullable Object> extends ArbitraryDelegator<T> {
 	private final Predicate<? super T> filterPredicate;
 	private final int maxMisses;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ChooseValueArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ChooseValueArbitrary.java
index 93cf0cf18..5a739a51d 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ChooseValueArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ChooseValueArbitrary.java
@@ -5,7 +5,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
-public class ChooseValueArbitrary<T> extends UseGeneratorsArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class ChooseValueArbitrary<T extends @Nullable Object> extends UseGeneratorsArbitrary<T> {
 
 	private final int hashCode;
 	private final List<T> values;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
index cd14bc789..2a845a9c8 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultIteratorArbitrary.java
@@ -9,7 +9,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.shrinking.*;
 
-public class DefaultIteratorArbitrary<T> extends MultivalueArbitraryBase<T, Iterator<T>> implements IteratorArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class DefaultIteratorArbitrary<T extends @Nullable Object> extends MultivalueArbitraryBase<T, Iterator<T>> implements IteratorArbitrary<T> {
 
 	public DefaultIteratorArbitrary(Arbitrary<T> elementArbitrary) {
 		super(elementArbitrary);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
index b9138641f..f234b942f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultSetArbitrary.java
@@ -12,7 +12,9 @@
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 import net.jqwik.engine.properties.shrinking.*;
 
-public class DefaultSetArbitrary<T> extends MultivalueArbitraryBase<T, Set<T>> implements SetArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class DefaultSetArbitrary<T extends @Nullable Object> extends MultivalueArbitraryBase<T, Set<T>> implements SetArbitrary<T> {
 
 	public DefaultSetArbitrary(Arbitrary<T> elementArbitrary) {
 		super(elementArbitrary);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
index c706cdb5a..01aafc56c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultStreamArbitrary.java
@@ -10,7 +10,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.shrinking.*;
 
-public class DefaultStreamArbitrary<T> extends MultivalueArbitraryBase<T, Stream<T>> implements StreamArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class DefaultStreamArbitrary<T extends @Nullable Object> extends MultivalueArbitraryBase<T, Stream<T>> implements StreamArbitrary<T> {
 
 	public DefaultStreamArbitrary(Arbitrary<T> elementArbitrary) {
 		super(elementArbitrary);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTraverseArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTraverseArbitrary.java
index 28b27ff8a..2ff0bd21f 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTraverseArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTraverseArbitrary.java
@@ -5,6 +5,7 @@
 import java.util.function.*;
 import java.util.stream.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.commons.support.*;
 
 import net.jqwik.api.*;
@@ -17,7 +18,7 @@
 
 import static net.jqwik.engine.support.JqwikReflectionSupport.*;
 
-public class DefaultTraverseArbitrary<T> extends ArbitraryDecorator<T> implements TraverseArbitrary<T> {
+public class DefaultTraverseArbitrary<T extends @Nullable Object> extends ArbitraryDecorator<T> implements TraverseArbitrary<T> {
 
 	private final Class<T> targetType;
 	private final Traverser traverser;
@@ -189,7 +190,7 @@ private T generateNext(Object[] params, Combinator combinator) {
 
 	@FunctionalInterface
 	private interface Combinator {
-		Object combine(Object[] params) throws Throwable;
+		@Nullable Object combine(Object[] params) throws Throwable;
 	}
 
 	private static class GenerationError extends RuntimeException {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java
index 490966a8d..31c93e529 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/DefaultTypeArbitrary.java
@@ -4,6 +4,7 @@
 import java.util.*;
 import java.util.function.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.commons.support.*;
 
 import net.jqwik.api.*;
@@ -12,7 +13,7 @@
 
 import static org.junit.platform.commons.support.ModifierSupport.*;
 
-public class DefaultTypeArbitrary<T> extends ArbitraryDecorator<T> implements TypeArbitrary<T> {
+public class DefaultTypeArbitrary<T extends @Nullable Object> extends ArbitraryDecorator<T> implements TypeArbitrary<T> {
 
 	private final Class<T> targetType;
 	private final Set<Executable> explicitCreators = new LinkedHashSet<>();
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
index 38971e03b..300fa980e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/FromGeneratorWithSizeArbitrary.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.support.*;
 
-public class FromGeneratorWithSizeArbitrary<T> implements Arbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class FromGeneratorWithSizeArbitrary<T extends @Nullable Object> implements Arbitrary<T> {
 
 	private final IntFunction<? extends RandomGenerator<T>> supplier;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java
index 8a9aa2acd..155e17a0b 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/GenericEdgeCasesConfiguration.java
@@ -6,7 +6,9 @@
 
 import net.jqwik.api.*;
 
-public class GenericEdgeCasesConfiguration<T> implements EdgeCases.Config<T> {
+import org.jspecify.annotations.*;
+
+public class GenericEdgeCasesConfiguration<T extends @Nullable Object> implements EdgeCases.Config<T> {
 	private boolean none;
 	private final List<Predicate<? super T>> filters = new ArrayList<>();
 	private final List<T> additionalEdgeCases = new ArrayList<>();
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java
index ce9c3b793..77c7722de 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/JustArbitrary.java
@@ -8,7 +8,7 @@
 
 import org.jspecify.annotations.*;
 
-public class JustArbitrary<T> implements Arbitrary<T> {
+public class JustArbitrary<T extends @Nullable Object> implements Arbitrary<T> {
 
 	private final T value;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
index 9da910f76..d950117a6 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyArbitrary.java
@@ -7,10 +7,12 @@
 import net.jqwik.api.configurators.*;
 import net.jqwik.api.providers.*;
 
-public class LazyArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class LazyArbitrary<T extends @Nullable Object> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
 	private final Supplier<? extends Arbitrary<T>> arbitrarySupplier;
 	private final List<Tuple.Tuple2<ArbitraryConfigurator, TypeUsage>> configurations = new ArrayList<>();
-	private Arbitrary<T> arbitrary;
+	private @Nullable Arbitrary<T> arbitrary;
 
 	public LazyArbitrary(Supplier<? extends Arbitrary<T>> arbitrarySupplier) {
 		this.arbitrarySupplier = arbitrarySupplier;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
index 74878c7e9..e7447990e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/LazyOfArbitrary.java
@@ -14,7 +14,7 @@
 
 import org.jspecify.annotations.*;
 
-public class LazyOfArbitrary<T> implements Arbitrary<T> {
+public class LazyOfArbitrary<T extends @Nullable Object> implements Arbitrary<T> {
 
 	// Cached arbitraries only have to survive one property
 	private static Store<Map<Integer, LazyOfArbitrary<?>>> arbitrariesStore() {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
index a928246f6..f8981024c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/OneOfArbitrary.java
@@ -11,7 +11,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
-public class OneOfArbitrary<T> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class OneOfArbitrary<T extends @Nullable Object> implements Arbitrary<T>, SelfConfiguringArbitrary<T> {
 	private final List<Arbitrary<T>> all = new ArrayList<>();
 	private final boolean isGeneratorMemoizable;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
index 1494c3b00..bb976eab5 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/RecursiveArbitrary.java
@@ -6,7 +6,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.support.*;
 
-public class RecursiveArbitrary<T> implements Arbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class RecursiveArbitrary<T extends @Nullable Object> implements Arbitrary<T> {
 	private final Supplier<? extends Arbitrary<T>> base;
 	private final Function<? super Arbitrary<T>, ? extends Arbitrary<T>> recur;
 	private final int depth;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java
index ebc5c3795..f39d22213 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ReportableStream.java
@@ -8,7 +8,7 @@
 
 import org.jspecify.annotations.*;
 
-public class ReportableStream<T> implements Stream<T> {
+public class ReportableStream<T extends @Nullable Object> implements Stream<T> {
 
 	private final List<T> values;
 	private final Stream<T> stream;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ShuffleArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ShuffleArbitrary.java
index e9bf60a28..8d2833952 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ShuffleArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/ShuffleArbitrary.java
@@ -6,7 +6,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.arbitraries.randomized.*;
 
-public class ShuffleArbitrary<T> extends UseGeneratorsArbitrary<List<T>> {
+import org.jspecify.annotations.*;
+
+public class ShuffleArbitrary<T extends @Nullable Object> extends UseGeneratorsArbitrary<List<T>> {
 	private final List<T> values;
 
 	public ShuffleArbitrary(List<T> values) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java
index b6e052766..f1e9f1a02 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/CombineArbitrary.java
@@ -10,7 +10,9 @@
 import net.jqwik.engine.properties.arbitraries.exhaustive.*;
 import net.jqwik.engine.properties.shrinking.*;
 
-public class CombineArbitrary<R> implements Arbitrary<R> {
+import org.jspecify.annotations.*;
+
+public class CombineArbitrary<R extends @Nullable Object> implements Arbitrary<R> {
 
 	private final Function<? super List<?>, ? extends R> combinator;
 	private final List<Arbitrary<Object>> arbitraries;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
index 5594cf0d7..fd81531a1 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/combinations/DefaultListCombinator.java
@@ -10,7 +10,7 @@
 
 import static org.apiguardian.api.API.Status.*;
 
-public class DefaultListCombinator<T> implements Combinators.ListCombinator<T> {
+public class DefaultListCombinator<T extends @Nullable Object> implements Combinators.ListCombinator<T> {
 
 	protected final Arbitrary<T>[] arbitraries;
 
@@ -38,7 +38,7 @@ public Combinators.ListCombinator<T> filter(Predicate<? super List<? extends T>>
 		return params -> combinator.apply((List<T>) params);
 	}
 
-	private static class Filtered<T> extends DefaultListCombinator<T> {
+	private static class Filtered<T extends @Nullable Object> extends DefaultListCombinator<T> {
 		private final Predicate<? super List<? extends T>> filter;
 
 		private Filtered(Arbitrary<T>[] arbitraries, Predicate<? super List<? extends T>> filter) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java
index 8701d9226..69c44dac3 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/CombinedExhaustiveGenerator.java
@@ -7,7 +7,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-class CombinedExhaustiveGenerator<R> implements ExhaustiveGenerator<R> {
+import org.jspecify.annotations.*;
+
+class CombinedExhaustiveGenerator<R extends @Nullable Object> implements ExhaustiveGenerator<R> {
 	private final Long maxCount;
 	private final List<Arbitrary<Object>> arbitraries;
 	private final Function<? super List<?>, ? extends R> combinator;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
index da0e60611..9724a3218 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/ExhaustiveGenerators.java
@@ -108,7 +108,7 @@ public static Optional<ExhaustiveGenerator<String>> strings(
 		);
 	}
 
-	private static class SupplierIterator<T> implements Iterator<T> {
+	private static class SupplierIterator<T extends @Nullable Object> implements Iterator<T> {
 
 		private final Supplier<T> supplier;
 		private volatile boolean generated = false;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java
index 11442b458..aedb2f3f9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/exhaustive/FilteredExhaustiveGenerator.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class FilteredExhaustiveGenerator<T> implements ExhaustiveGenerator<T> {
+import org.jspecify.annotations.*;
+
+public class FilteredExhaustiveGenerator<T extends @Nullable Object> implements ExhaustiveGenerator<T> {
 	private final ExhaustiveGenerator<T> toFilter;
 	private final Predicate<? super T> filter;
 	private int maxMisses;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java
index 7122e33c5..fb47f8a65 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/CollectGenerator.java
@@ -6,7 +6,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.shrinking.*;
 
-public class CollectGenerator<T> implements RandomGenerator<List<T>> {
+import org.jspecify.annotations.*;
+
+public class CollectGenerator<T extends @Nullable Object> implements RandomGenerator<List<T>> {
 	private final RandomGenerator<T> elementGenerator;
 	private final Predicate<? super List<? extends T>> until;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java
index a5f3c8c14..eea20c216 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FilteredGenerator.java
@@ -6,7 +6,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.shrinking.*;
 
-public class FilteredGenerator<T> implements RandomGenerator<T> {
+import org.jspecify.annotations.*;
+
+public class FilteredGenerator<T extends @Nullable Object> implements RandomGenerator<T> {
 	private final RandomGenerator<T> toFilter;
 	private final Predicate<? super T> filterPredicate;
 	private int maxMisses;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
index cedb08da0..a2806616c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/FrequencyGenerator.java
@@ -6,7 +6,9 @@
 import net.jqwik.engine.properties.shrinking.*;
 import net.jqwik.engine.support.*;
 
-class FrequencyGenerator<T> extends ChooseRandomlyByFrequency<T> implements RandomGenerator<T> {
+import org.jspecify.annotations.*;
+
+class FrequencyGenerator<T extends @Nullable Object> extends ChooseRandomlyByFrequency<T> implements RandomGenerator<T> {
 
 	FrequencyGenerator(List<? extends Tuple.Tuple2<Integer, ? extends T>> frequencies) {
 		super(frequencies);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/IgnoreExceptionGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/IgnoreExceptionGenerator.java
index ad4b917ec..60df08e7c 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/IgnoreExceptionGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/IgnoreExceptionGenerator.java
@@ -6,9 +6,11 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.shrinking.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.support.JqwikExceptionSupport.*;
 
-public class IgnoreExceptionGenerator<T> implements RandomGenerator<T> {
+public class IgnoreExceptionGenerator<T extends @Nullable Object> implements RandomGenerator<T> {
 
 	private final RandomGenerator<T> base;
 	private final Class<? extends Throwable>[] exceptionTypes;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/InjectDuplicatesGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/InjectDuplicatesGenerator.java
index 764c144bb..7321590eb 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/InjectDuplicatesGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/InjectDuplicatesGenerator.java
@@ -6,7 +6,9 @@
 import net.jqwik.api.lifecycle.*;
 import net.jqwik.engine.*;
 
-public class InjectDuplicatesGenerator<T> implements RandomGenerator<T> {
+import org.jspecify.annotations.*;
+
+public class InjectDuplicatesGenerator<T extends @Nullable Object> implements RandomGenerator<T> {
 
 	private final RandomGenerator<T> base;
 	private final double duplicateProbability;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java
index 6f20ad0a6..535c057ae 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/arbitraries/randomized/WithEdgeCasesGenerator.java
@@ -8,7 +8,7 @@
 
 import org.jspecify.annotations.*;
 
-class WithEdgeCasesGenerator<T> implements RandomGenerator<T> {
+class WithEdgeCasesGenerator<T extends @Nullable Object> implements RandomGenerator<T> {
 
 	private final RandomGenerator<T> base;
 	private final int baseToEdgeCaseRatio;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/AbstractValueShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/AbstractValueShrinkable.java
index 2b4ee0ecc..521ae125d 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/AbstractValueShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/AbstractValueShrinkable.java
@@ -2,7 +2,11 @@
 
 import net.jqwik.api.*;
 
-public abstract class AbstractValueShrinkable<T> implements Shrinkable<T> {
+import org.jspecify.annotations.*;
+
+import java.util.*;
+
+public abstract class AbstractValueShrinkable<T extends @Nullable Object> implements Shrinkable<T> {
 
 	private final T value;
 
@@ -22,12 +26,12 @@ public boolean equals(Object o) {
 
 		AbstractValueShrinkable<?> that = (AbstractValueShrinkable<?>) o;
 
-		return value.equals(that.value);
+		return Objects.equals(value, that.value);
 	}
 
 	@Override
 	public int hashCode() {
-		return value.hashCode();
+		return Objects.hash(value);
 	}
 
 	@Override
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
index c3bf061a7..217a47f22 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ChooseValueShrinkable.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class ChooseValueShrinkable<T> extends AbstractValueShrinkable<T> {
+import org.jspecify.annotations.*;
+
+public class ChooseValueShrinkable<T extends @Nullable Object> extends AbstractValueShrinkable<T> {
 
 	private final List<? extends T> values;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java
index 5268a4263..8d348efa7 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CollectShrinkable.java
@@ -7,7 +7,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-public class CollectShrinkable<T> implements Shrinkable<List<T>> {
+import org.jspecify.annotations.*;
+
+public class CollectShrinkable<T extends @Nullable Object> implements Shrinkable<List<T>> {
 	private final List<? extends Shrinkable<T>> elements;
 	private final Predicate<? super List<? extends T>> until;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java
index ab539f030..9a291bad6 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/CombinedShrinkable.java
@@ -7,7 +7,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-public class CombinedShrinkable<T> implements Shrinkable<T> {
+import org.jspecify.annotations.*;
+
+public class CombinedShrinkable<T extends @Nullable Object> implements Shrinkable<T> {
 	private final List<Shrinkable<Object>> parts;
 	private final Function<? super List<?>, ? extends T> combinator;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java
index 0815b42a7..51bf3bed9 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/FilteredShrinkable.java
@@ -8,7 +8,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-public class FilteredShrinkable<T> implements Shrinkable<T> {
+import org.jspecify.annotations.*;
+
+public class FilteredShrinkable<T extends @Nullable Object> implements Shrinkable<T> {
 
 	private static final int MAX_BASE_SHRINKS = 100;
 	private final AtomicInteger countBaseShrinks = new AtomicInteger(0);
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/IgnoreExceptionShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/IgnoreExceptionShrinkable.java
index 866fa3d06..6fa4554fc 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/IgnoreExceptionShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/IgnoreExceptionShrinkable.java
@@ -4,9 +4,11 @@
 
 import net.jqwik.api.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.support.JqwikExceptionSupport.*;
 
-public class IgnoreExceptionShrinkable<T> implements Shrinkable<T> {
+public class IgnoreExceptionShrinkable<T extends @Nullable Object> implements Shrinkable<T> {
 
 	private final Shrinkable<T> shrinkable;
 	private final Class<? extends Throwable>[] exceptionTypes;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/LazyOfShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/LazyOfShrinkable.java
index f4731ad73..497c11246 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/LazyOfShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/LazyOfShrinkable.java
@@ -6,7 +6,9 @@
 
 import net.jqwik.api.*;
 
-public class LazyOfShrinkable<T> implements Shrinkable<T> {
+import org.jspecify.annotations.*;
+
+public class LazyOfShrinkable<T extends @Nullable Object> implements Shrinkable<T> {
 	public final Shrinkable<T> current;
 	public final int depth;
 	public final Set<LazyOfShrinkable<T>> parts;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/SampleShrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/SampleShrinkable.java
index 2b16fe9de..a097af29a 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/SampleShrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/SampleShrinkable.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class SampleShrinkable<T> extends AbstractValueShrinkable<T> {
+import org.jspecify.annotations.*;
+
+public class SampleShrinkable<T extends @Nullable Object> extends AbstractValueShrinkable<T> {
 
 	private final List<T> samples;
 	private final int index;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java
index b2bbacb74..04d2317c4 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableList.java
@@ -7,9 +7,11 @@
 import net.jqwik.engine.properties.*;
 import net.jqwik.engine.support.*;
 
+import org.jspecify.annotations.*;
+
 import static net.jqwik.engine.properties.UniquenessChecker.*;
 
-public class ShrinkableList<E> extends ShrinkableContainer<List<E>, E> {
+public class ShrinkableList<E extends @Nullable Object> extends ShrinkableContainer<List<E>, E> {
 
 	// Only used in tests
 	ShrinkableList(List<Shrinkable<E>> elements, int minSize, int maxSize) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java
index aaacd69b8..d47e6abeb 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/ShrinkableSet.java
@@ -8,7 +8,7 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.properties.*;
 
-public class ShrinkableSet<E> extends ShrinkableContainer<Set<E>, E> {
+public class ShrinkableSet<E extends @Nullable Object> extends ShrinkableContainer<Set<E>, E> {
 
 	public ShrinkableSet(
 		Collection<? extends Shrinkable<E>> elements,
diff --git a/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java b/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java
index fb8644003..beabd71fd 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/shrinking/Unshrinkable.java
@@ -7,7 +7,9 @@
 import net.jqwik.api.*;
 import net.jqwik.engine.support.*;
 
-public class Unshrinkable<T> implements Shrinkable<T> {
+import org.jspecify.annotations.*;
+
+public class Unshrinkable<T extends @Nullable Object> implements Shrinkable<T> {
 	private final ShrinkingDistance distance;
 	private final Supplier<? extends T> valueSupplier;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/DefaultActionChainArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/state/DefaultActionChainArbitrary.java
index 06b430c14..3d5052093 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/DefaultActionChainArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/DefaultActionChainArbitrary.java
@@ -4,6 +4,7 @@
 import java.util.*;
 import java.util.function.*;
 
+import org.jspecify.annotations.*;
 import org.junit.platform.commons.support.*;
 
 import net.jqwik.api.*;
@@ -11,7 +12,7 @@
 import net.jqwik.api.state.*;
 import net.jqwik.engine.support.*;
 
-public class DefaultActionChainArbitrary<T> extends ArbitraryDecorator<ActionChain<T>> implements ActionChainArbitrary<T> {
+public class DefaultActionChainArbitrary<T extends @Nullable Object> extends ArbitraryDecorator<ActionChain<T>> implements ActionChainArbitrary<T> {
 
 	private ChainArbitrary<T> chainArbitrary;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java
index 5c15ae319..713529e79 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/DefaultChainArbitrary.java
@@ -8,7 +8,9 @@
 import net.jqwik.engine.properties.arbitraries.*;
 import net.jqwik.engine.support.*;
 
-public class DefaultChainArbitrary<T> extends TypedCloneable implements ChainArbitrary<T> {
+import org.jspecify.annotations.*;
+
+public class DefaultChainArbitrary<T extends @Nullable Object> extends TypedCloneable implements ChainArbitrary<T> {
 
 	private int maxTransformations = Integer.MIN_VALUE;
 	private Supplier<? extends ChangeDetector<? super T>> changeDetectorSupplier = ChangeDetector::alwaysTrue;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java b/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java
index 3103e257a..10fa78042 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/state/ShrinkableChain.java
@@ -15,7 +15,7 @@
 import net.jqwik.api.state.*;
 import net.jqwik.engine.*;
 
-public class ShrinkableChain<T> implements Shrinkable<Chain<T>> {
+public class ShrinkableChain<T extends @Nullable Object> implements Shrinkable<Chain<T>> {
 
 	public static final int MAX_TRANSFORMER_TRIES = 1000;
 	private final long randomSeed;
@@ -127,9 +127,9 @@ private class ChainIterator implements Iterator<T> {
 
 		private final Random random = SourceOfRandomness.newRandom(randomSeed);
 		private int steps = 0;
-		private T current;
+		private @Nullable T current;
 		private boolean initialSupplied = false;
-		private Transformer<T> nextTransformer = null;
+		private @Nullable Transformer<T> nextTransformer = null;
 
 		private ChainIterator(T initial) {
 			this.current = initial;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionSequenceGenerator.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionSequenceGenerator.java
index e36f79cc8..c451becf3 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionSequenceGenerator.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/ActionSequenceGenerator.java
@@ -5,7 +5,9 @@
 import net.jqwik.api.*;
 import net.jqwik.api.stateful.*;
 
-class ActionSequenceGenerator<M> implements RandomGenerator<ActionSequence<M>> {
+import org.jspecify.annotations.*;
+
+class ActionSequenceGenerator<M extends @Nullable Object> implements RandomGenerator<ActionSequence<M>> {
 	private final int genSize;
 	private final int maxSize;
 	private final Arbitrary<Action<M>> actionArbitrary;
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/DefaultActionSequenceArbitrary.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/DefaultActionSequenceArbitrary.java
index 3bda6b677..ad614037e 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/DefaultActionSequenceArbitrary.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/DefaultActionSequenceArbitrary.java
@@ -4,7 +4,9 @@
 import net.jqwik.api.stateful.*;
 import net.jqwik.engine.properties.arbitraries.*;
 
-public class DefaultActionSequenceArbitrary<M> extends TypedCloneable implements ActionSequenceArbitrary<M> {
+import org.jspecify.annotations.*;
+
+public class DefaultActionSequenceArbitrary<M extends @Nullable Object> extends TypedCloneable implements ActionSequenceArbitrary<M> {
 
 	private final Arbitrary<Action<M>> actionArbitrary;
 
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/SequentialActionSequence.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/SequentialActionSequence.java
index 005d676bd..f6a2a7186 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/SequentialActionSequence.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/SequentialActionSequence.java
@@ -4,6 +4,7 @@
 import java.util.function.*;
 import java.util.stream.*;
 
+import org.jspecify.annotations.*;
 import org.opentest4j.*;
 
 import net.jqwik.api.*;
@@ -11,7 +12,7 @@
 import net.jqwik.api.stateful.*;
 import net.jqwik.engine.support.*;
 
-class SequentialActionSequence<M> implements ActionSequence<M> {
+class SequentialActionSequence<M extends @Nullable Object> implements ActionSequence<M> {
 
 	protected ActionGenerator<M> actionGenerator;
 	protected int intendedSize;
@@ -20,7 +21,7 @@ class SequentialActionSequence<M> implements ActionSequence<M> {
 	private final List<Consumer<M>> peekers = new ArrayList<>();
 
 	protected RunState runState = RunState.NOT_RUN;
-	private M currentModel = null;
+	private @Nullable M currentModel = null;
 
 	SequentialActionSequence(ActionGenerator<M> actionGenerator, int intendedSize) {
 		if (intendedSize < 1) {
diff --git a/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkableActionSequence.java b/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkableActionSequence.java
index 5c1735977..d6347a3f2 100644
--- a/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkableActionSequence.java
+++ b/engine/src/main/java/net/jqwik/engine/properties/stateful/ShrinkableActionSequence.java
@@ -7,13 +7,15 @@
 import net.jqwik.api.stateful.*;
 import net.jqwik.engine.support.*;
 
-class ShrinkableActionSequence<T> implements Shrinkable<ActionSequence<T>> {
+import org.jspecify.annotations.*;
+
+class ShrinkableActionSequence<T extends @Nullable Object> implements Shrinkable<ActionSequence<T>> {
 
 	private final ActionGenerator<T> actionGenerator;
 	private final int maxSize;
 	private final ShrinkingDistance distance;
 
-	private SequentialActionSequence<T> generatedSequence = null;
+	private @Nullable SequentialActionSequence<T> generatedSequence = null;
 
 	ShrinkableActionSequence(ActionGenerator<T> actionGenerator, int maxSize, ShrinkingDistance distance) {
 		this.actionGenerator = actionGenerator;
diff --git a/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java b/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
index 1b1faf86e..6e44bfebb 100644
--- a/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
+++ b/engine/src/main/java/net/jqwik/engine/support/ChooseRandomlyByFrequency.java
@@ -5,7 +5,9 @@
 
 import net.jqwik.api.*;
 
-public class ChooseRandomlyByFrequency<T> implements Function<Random, T> {
+import org.jspecify.annotations.*;
+
+public class ChooseRandomlyByFrequency<T extends @Nullable Object> implements Function<Random, T> {
 
 	private int[] upperBounds;
 	private int size = 0;
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java
index a621463ef..af85f398f 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/CombinedIterator.java
@@ -1,9 +1,11 @@
 package net.jqwik.engine.support.combinatorics;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 import java.util.stream.*;
 
-public class CombinedIterator<T> implements Iterator<List<T>> {
+public class CombinedIterator<T extends @Nullable Object> implements Iterator<List<T>> {
 	private final List<? extends Iterable<? extends T>> iterables;
 	private final List<Iterator<? extends T>> iterators;
 	private final List<T> elements;
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java
index b7d742fa1..b30a0a79e 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/ConcatIterator.java
@@ -1,13 +1,15 @@
 package net.jqwik.engine.support.combinatorics;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 import java.util.concurrent.atomic.*;
 
-public class ConcatIterator<T> implements Iterator<T> {
+public class ConcatIterator<T extends @Nullable Object> implements Iterator<T> {
 
 	private final List<? extends Iterator<? extends T>> iterators;
 	private final AtomicInteger position;
-	private Iterator<? extends T> next;
+	private @Nullable Iterator<? extends T> next;
 
 	public ConcatIterator(List<? extends Iterator<? extends T>> iterators) {
 		this.iterators = iterators;
@@ -17,7 +19,7 @@ public ConcatIterator(List<? extends Iterator<? extends T>> iterators) {
 		}
 	}
 
-	private Iterator<? extends T> findNext() {
+	private @Nullable Iterator<? extends T> findNext() {
 		while (!iterators.get(position.get()).hasNext()) {
 			if (position.get() >= iterators.size() -1)
 				return null;
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java
index 560979c3b..d874d8da0 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/PermutationIterator.java
@@ -1,12 +1,14 @@
 package net.jqwik.engine.support.combinatorics;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 
 // Taken and adapted from https://codereview.stackexchange.com/questions/119969/
-public class PermutationIterator<T> implements Iterator<List<T>> {
+public class PermutationIterator<T extends @Nullable Object> implements Iterator<List<T>> {
 	private final List<T> values = new ArrayList<>();
-	private List<T> next;
-	private int[] indices;
+	private @Nullable List<T> next;
+	private int @Nullable [] indices;
 
 	public PermutationIterator(List<? extends T> values) {
 		this.values.addAll(values);
diff --git a/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java b/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java
index 3779206e4..f73410154 100644
--- a/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java
+++ b/engine/src/main/java/net/jqwik/engine/support/combinatorics/SetIterator.java
@@ -1,8 +1,10 @@
 package net.jqwik.engine.support.combinatorics;
 
+import org.jspecify.annotations.*;
+
 import java.util.*;
 
-public class SetIterator<T> implements Iterator<Set<T>> {
+public class SetIterator<T extends @Nullable Object> implements Iterator<Set<T>> {
 
 	private final Iterator<List<T>> combinedListIterator;
 	private final Set<Set<T>> generatedSets = new LinkedHashSet<>();