preserve Rows.objectToStrings behavior of translating null into "null…

…" inside of lists and arrays (#15190)

processing/src/main/java/org/apache/druid/data/input/Rows.java

@@ -25,7 +25,6 @@
 import org.apache.druid.common.config.NullHandling;
 import org.apache.druid.java.util.common.StringUtils;
 import org.apache.druid.java.util.common.parsers.ParseException;
-import org.apache.druid.math.expr.Evals;
 
 import javax.annotation.Nullable;
 import java.util.Arrays;
@@ -60,7 +59,11 @@ public static List<Object> toGroupKey(long timeStamp, InputRow inputRow)
   }
 
   /**
-   * Convert an object to a list of strings.
+   * Convert an object to a list of strings. This function translates single value nulls into an empty list, and any
+   * nulls inside of a list or array into the string "null". Do not use this method if you don't want this behavior,
+   * but note that many implementations of {@link InputRow#getDimension(String)} do use this method, so it is
+   * recommended to use {@link InputRow#getRaw(String)} if you want the actual value without this coercion. For legacy
+   * reasons, some stuff counts on this incorrect behavior, (such as {@link Rows#toGroupKey(long, InputRow)}).
    */
   public static List<String> objectToStrings(final Object inputValue)
   {
@@ -73,7 +76,7 @@ public static List<String> objectToStrings(final Object inputValue)
       // convert byte[] to base64 encoded string
       return Collections.singletonList(StringUtils.encodeBase64String((byte[]) inputValue));
     } else if (inputValue instanceof Object[]) {
-      return Arrays.stream((Object[]) inputValue).map(Evals::asString).collect(Collectors.toList());
+      return Arrays.stream((Object[]) inputValue).map(String::valueOf).collect(Collectors.toList());
     } else {
       return Collections.singletonList(String.valueOf(inputValue));
     }

processing/src/test/java/org/apache/druid/segment/transform/TransformerTest.java

@@ -389,7 +389,7 @@ public void testTransformWithArrayLongInputs()
     Assert.assertNotNull(actual);
     Assert.assertEquals(ImmutableList.of("dim"), actual.getDimensions());
     Assert.assertArrayEquals(new Object[]{1L, 2L, null, 3L}, (Object[]) actual.getRaw("dim"));
-    Assert.assertArrayEquals(new String[]{"1", "2", null, "3"}, actual.getDimension("dim").toArray());
+    Assert.assertEquals(ImmutableList.of("1", "2", "null", "3"), actual.getDimension("dim"));
     Assert.assertEquals(row.getTimestamp(), actual.getTimestamp());
   }
 
@@ -416,9 +416,9 @@ public void testTransformWithArrayFloatInputs()
     Assert.assertEquals(2.3, (Double) raw[1], 0.00001);
     Assert.assertNull(raw[2]);
     Assert.assertEquals(3.4, (Double) raw[3], 0.00001);
-    Assert.assertArrayEquals(
-        new String[]{"1.2000000476837158", "2.299999952316284", null, "3.4000000953674316"},
-        actual.getDimension("dim").toArray()
+    Assert.assertEquals(
+        ImmutableList.of("1.2000000476837158", "2.299999952316284", "null", "3.4000000953674316"),
+        actual.getDimension("dim")
     );
     Assert.assertEquals(row.getTimestamp(), actual.getTimestamp());
   }
@@ -445,12 +445,12 @@ public void testTransformWithArrayDoubleInputs()
     Assert.assertEquals(2.3, (Double) raw[1], 0.0);
     Assert.assertNull(raw[2]);
     Assert.assertEquals(3.4, (Double) raw[3], 0.0);
-    Assert.assertArrayEquals(new String[]{"1.2", "2.3", null, "3.4"}, actual.getDimension("dim").toArray());
+    Assert.assertEquals(ImmutableList.of("1.2", "2.3", "null", "3.4"), actual.getDimension("dim"));
     Assert.assertEquals(row.getTimestamp(), actual.getTimestamp());
   }
 
   @Test
-  public void testTransformWithExpr()
+  public void testTransformWithArrayExpr()
   {
     final Transformer transformer = new Transformer(
         new TransformSpec(
@@ -517,6 +517,6 @@ public int compareTo(Row o)
     });
     Assert.assertEquals(actualTranformedRow.getDimension("dim"), dimList.subList(0, 5));
     Assert.assertArrayEquals(dimList.subList(0, 5).toArray(), (Object[]) actualTranformedRow.getRaw("dim"));
-    Assert.assertArrayEquals(new Object[]{"a"}, actualTranformedRow.getDimension("dim1").toArray());
+    Assert.assertEquals(ImmutableList.of("a"), actualTranformedRow.getDimension("dim1"));
   }
 }