Replace asserts with proper exceptions for parameter validation

Parameter validation is done by using asserts, and throws AssertionError
when validation fails.

Asserts may not always be enabled. Furthermore, specific exceptions
such as NullPointerException should be thrown to make the error more
explicit.

Let's replace the asserts with checkNotNull() and checkArgument(),
which throws an appropriate exception when validation fails.

src/main/java/seedu/address/commons/core/UnmodifiableObservableList.java

@@ -1,5 +1,7 @@
 package seedu.address.commons.core;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.text.Collator;
 import java.util.Collection;
 import java.util.Collections;
@@ -28,7 +30,7 @@ public class UnmodifiableObservableList<E> implements ObservableList<E> {
     private final ObservableList<? extends E> backingList;
 
     public UnmodifiableObservableList(ObservableList<? extends E> backingList) {
-        assert backingList != null;
+        checkNotNull(backingList);
         this.backingList = backingList;
     }
 

src/main/java/seedu/address/commons/util/AppUtil.java

@@ -1,5 +1,7 @@
 package seedu.address.commons.util;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import javafx.scene.image.Image;
 import seedu.address.MainApp;
 
@@ -9,7 +11,7 @@
 public class AppUtil {
 
     public static Image getImage(String imagePath) {
-        assert imagePath != null;
+        checkNotNull(imagePath);
         return new Image(MainApp.class.getResourceAsStream(imagePath));
     }
 

src/main/java/seedu/address/commons/util/FileUtil.java

@@ -1,5 +1,8 @@
 package seedu.address.commons.util;
 
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.io.File;
 import java.io.IOException;
 import java.nio.file.Files;
@@ -80,8 +83,8 @@ public static void writeToFile(File file, String content) throws IOException {
      * @return {@code pathWithForwardSlash} but '/' replaced with {@code File.separator}
      */
     public static String getPath(String pathWithForwardSlash) {
-        assert pathWithForwardSlash != null;
-        assert pathWithForwardSlash.contains("/");
+        checkNotNull(pathWithForwardSlash);
+        checkArgument(pathWithForwardSlash.contains("/"));
         return pathWithForwardSlash.replace("/", File.separator);
     }
 

src/main/java/seedu/address/commons/util/JsonUtil.java

@@ -1,5 +1,7 @@
 package seedu.address.commons.util;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.io.File;
 import java.io.IOException;
 import java.util.Optional;
@@ -55,7 +57,7 @@ static <T> T deserializeObjectFromJsonFile(File jsonFile, Class<T> classOfObject
     public static <T> Optional<T> readJsonFile(
             String filePath, Class<T> classOfObjectToDeserialize) throws DataConversionException {
 
-        assert filePath != null;
+        checkNotNull(filePath);
 
         File file = new File(filePath);
 
@@ -84,8 +86,8 @@ public static <T> Optional<T> readJsonFile(
      * @throws IOException if there was an error during writing to the file
      */
     public static <T> void saveJsonFile(T jsonFile, String filePath) throws IOException {
-        assert jsonFile != null;
-        assert filePath != null;
+        checkNotNull(jsonFile);
+        checkNotNull(filePath);
 
         serializeObjectToJsonFile(new File(filePath), jsonFile);
     }

src/main/java/seedu/address/commons/util/StringUtil.java

@@ -1,5 +1,8 @@
 package seedu.address.commons.util;
 
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.io.PrintWriter;
 import java.io.StringWriter;
 
@@ -20,12 +23,12 @@ public class StringUtil {
      * @param word cannot be null, cannot be empty, must be a single word
      */
     public static boolean containsWordIgnoreCase(String sentence, String word) {
-        assert word != null : "Word parameter cannot be null";
-        assert sentence != null : "Sentence parameter cannot be null";
+        checkNotNull(sentence);
+        checkNotNull(word);
 
         String preppedWord = word.trim();
-        assert !preppedWord.isEmpty() : "Word parameter cannot be empty";
-        assert preppedWord.split("\\s+").length == 1 : "Word parameter should be a single word";
+        checkArgument(!preppedWord.isEmpty(), "Word parameter cannot be empty");
+        checkArgument(preppedWord.split("\\s+").length == 1, "Word parameter should be a single word");
 
         String preppedSentence = sentence;
         String[] wordsInPreppedSentence = preppedSentence.split("\\s+");
@@ -42,7 +45,7 @@ public static boolean containsWordIgnoreCase(String sentence, String word) {
      * Returns a detailed message of the t, including the stack trace.
      */
     public static String getDetails(Throwable t) {
-        assert t != null;
+        checkNotNull(t);
         StringWriter sw = new StringWriter();
         t.printStackTrace(new PrintWriter(sw));
         return t.getMessage() + "\n" + sw.toString();

src/main/java/seedu/address/commons/util/XmlUtil.java

@@ -1,5 +1,7 @@
 package seedu.address.commons.util;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.io.File;
 import java.io.FileNotFoundException;
 
@@ -27,8 +29,8 @@ public class XmlUtil {
     public static <T> T getDataFromFile(File file, Class<T> classToConvert)
             throws FileNotFoundException, JAXBException {
 
-        assert file != null;
-        assert classToConvert != null;
+        checkNotNull(file);
+        checkNotNull(classToConvert);
 
         if (!FileUtil.isFileExists(file)) {
             throw new FileNotFoundException("File not found : " + file.getAbsolutePath());
@@ -51,8 +53,8 @@ public static <T> T getDataFromFile(File file, Class<T> classToConvert)
      */
     public static <T> void saveDataToFile(File file, T data) throws FileNotFoundException, JAXBException {
 
-        assert file != null;
-        assert data != null;
+        checkNotNull(file);
+        checkNotNull(data);
 
         if (!file.exists()) {
             throw new FileNotFoundException("File not found : " + file.getAbsolutePath());

src/main/java/seedu/address/logic/commands/AddCommand.java

@@ -1,5 +1,6 @@
 package seedu.address.logic.commands;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_ADDRESS;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_EMAIL;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_PHONE;
@@ -44,7 +45,7 @@ public AddCommand(ReadOnlyPerson person) {
 
     @Override
     public CommandResult execute() throws CommandException {
-        assert model != null;
+        checkNotNull(model);
         try {
             model.addPerson(toAdd);
             return new CommandResult(String.format(MESSAGE_SUCCESS, toAdd));

src/main/java/seedu/address/logic/commands/ClearCommand.java

@@ -1,5 +1,7 @@
 package seedu.address.logic.commands;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import seedu.address.model.AddressBook;
 
 /**
@@ -13,7 +15,7 @@ public class ClearCommand extends Command {
 
     @Override
     public CommandResult execute() {
-        assert model != null;
+        checkNotNull(model);
         model.resetData(new AddressBook());
         return new CommandResult(MESSAGE_SUCCESS);
     }

src/main/java/seedu/address/logic/commands/CommandResult.java

@@ -1,5 +1,7 @@
 package seedu.address.logic.commands;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 /**
  * Represents the result of a command execution.
  */
@@ -8,7 +10,7 @@ public class CommandResult {
     public final String feedbackToUser;
 
     public CommandResult(String feedbackToUser) {
-        assert feedbackToUser != null;
+        checkNotNull(feedbackToUser);
         this.feedbackToUser = feedbackToUser;
     }
 

src/main/java/seedu/address/logic/commands/EditCommand.java

@@ -1,5 +1,7 @@
 package seedu.address.logic.commands;
 
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_ADDRESS;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_EMAIL;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_PHONE;
@@ -53,8 +55,8 @@ public class EditCommand extends Command {
      * @param editPersonDescriptor details to edit the person with
      */
     public EditCommand(int filteredPersonListIndex, EditPersonDescriptor editPersonDescriptor) {
-        assert filteredPersonListIndex > 0;
-        assert editPersonDescriptor != null;
+        checkArgument(filteredPersonListIndex > 0);
+        checkNotNull(editPersonDescriptor);
 
         this.filteredPersonListIndex = IndexUtil.oneToZeroIndex(filteredPersonListIndex);
         this.editPersonDescriptor = new EditPersonDescriptor(editPersonDescriptor);
@@ -126,7 +128,7 @@ public boolean isAnyFieldEdited() {
         }
 
         public void setName(Optional<Name> name) {
-            assert name != null;
+            checkNotNull(name);
             this.name = name;
         }
 
@@ -135,7 +137,7 @@ public Optional<Name> getName() {
         }
 
         public void setPhone(Optional<Phone> phone) {
-            assert phone != null;
+            checkNotNull(phone);
             this.phone = phone;
         }
 
@@ -144,7 +146,7 @@ public Optional<Phone> getPhone() {
         }
 
         public void setEmail(Optional<Email> email) {
-            assert email != null;
+            checkNotNull(email);
             this.email = email;
         }
 
@@ -153,7 +155,7 @@ public Optional<Email> getEmail() {
         }
 
         public void setAddress(Optional<Address> address) {
-            assert address != null;
+            checkNotNull(address);
             this.address = address;
         }
 
@@ -162,7 +164,7 @@ public Optional<Address> getAddress() {
         }
 
         public void setTags(Optional<Set<Tag>> tags) {
-            assert tags != null;
+            checkNotNull(tags);
             this.tags = tags;
         }
 

src/main/java/seedu/address/logic/parser/EditCommandParser.java

@@ -1,5 +1,6 @@
 package seedu.address.logic.parser;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import static seedu.address.commons.core.Messages.MESSAGE_INVALID_COMMAND_FORMAT;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_ADDRESS;
 import static seedu.address.logic.parser.CliSyntax.PREFIX_EMAIL;
@@ -29,7 +30,7 @@ public class EditCommandParser {
      * and returns an EditCommand object for execution.
      */
     public Command parse(String args) {
-        assert args != null;
+        checkNotNull(args);
         ArgumentMultimap argMultimap =
                 ArgumentTokenizer.tokenize(args, PREFIX_PHONE, PREFIX_EMAIL, PREFIX_ADDRESS, PREFIX_TAG);
         List<Optional<String>> preambleFields = ParserUtil.splitPreamble(argMultimap.getPreamble(), 2);

src/main/java/seedu/address/logic/parser/ParserUtil.java

@@ -1,5 +1,7 @@
 package seedu.address.logic.parser;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashSet;
@@ -49,39 +51,39 @@ public static List<Optional<String>> splitPreamble(String preamble, int numField
      * Parses a {@code Optional<String> name} into an {@code Optional<Name>} if {@code name} is present.
      */
     public static Optional<Name> parseName(Optional<String> name) throws IllegalValueException {
-        assert name != null;
+        checkNotNull(name);
         return name.isPresent() ? Optional.of(new Name(name.get())) : Optional.empty();
     }
 
     /**
      * Parses a {@code Optional<String> phone} into an {@code Optional<Phone>} if {@code phone} is present.
      */
     public static Optional<Phone> parsePhone(Optional<String> phone) throws IllegalValueException {
-        assert phone != null;
+        checkNotNull(phone);
         return phone.isPresent() ? Optional.of(new Phone(phone.get())) : Optional.empty();
     }
 
     /**
      * Parses a {@code Optional<String> address} into an {@code Optional<Address>} if {@code address} is present.
      */
     public static Optional<Address> parseAddress(Optional<String> address) throws IllegalValueException {
-        assert address != null;
+        checkNotNull(address);
         return address.isPresent() ? Optional.of(new Address(address.get())) : Optional.empty();
     }
 
     /**
      * Parses a {@code Optional<String> email} into an {@code Optional<Email>} if {@code email} is present.
      */
     public static Optional<Email> parseEmail(Optional<String> email) throws IllegalValueException {
-        assert email != null;
+        checkNotNull(email);
         return email.isPresent() ? Optional.of(new Email(email.get())) : Optional.empty();
     }
 
     /**
      * Parses {@code Collection<String> tags} into a {@code Set<Tag>}.
      */
     public static Set<Tag> parseTags(Collection<String> tags) throws IllegalValueException {
-        assert tags != null;
+        checkNotNull(tags);
         final Set<Tag> tagSet = new HashSet<>();
         for (String tagName : tags) {
             tagSet.add(new Tag(tagName));

src/main/java/seedu/address/model/AddressBook.java

@@ -1,5 +1,7 @@
 package seedu.address.model;
 
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -60,7 +62,7 @@ public void setTags(Collection<Tag> tags) throws UniqueTagList.DuplicateTagExcep
     }
 
     public void resetData(ReadOnlyAddressBook newData) {
-        assert newData != null;
+        checkNotNull(newData);
         try {
             setPersons(newData.getPersonList());
         } catch (UniquePersonList.DuplicatePersonException e) {
@@ -100,7 +102,7 @@ public void addPerson(Person p) throws UniquePersonList.DuplicatePersonException
      */
     public void updatePerson(int index, ReadOnlyPerson editedReadOnlyPerson)
             throws UniquePersonList.DuplicatePersonException {
-        assert editedReadOnlyPerson != null;
+        checkNotNull(editedReadOnlyPerson);
 
         Person editedPerson = new Person(editedReadOnlyPerson);
         syncMasterTagListWith(editedPerson);

src/main/java/seedu/address/model/ModelManager.java

@@ -1,5 +1,8 @@
 package seedu.address.model;
 
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
 import java.util.Set;
 import java.util.logging.Logger;
 
@@ -30,7 +33,7 @@ public class ModelManager extends ComponentManager implements Model {
      */
     public ModelManager(ReadOnlyAddressBook addressBook, UserPrefs userPrefs) {
         super();
-        assert !CollectionUtil.isAnyNull(addressBook, userPrefs);
+        checkArgument(!CollectionUtil.isAnyNull(addressBook, userPrefs));
 
         logger.fine("Initializing with address book: " + addressBook + " and user prefs " + userPrefs);
 
@@ -74,7 +77,7 @@ public synchronized void addPerson(Person person) throws UniquePersonList.Duplic
     @Override
     public void updatePerson(int filteredPersonListIndex, ReadOnlyPerson editedPerson)
             throws UniquePersonList.DuplicatePersonException {
-        assert editedPerson != null;
+        checkNotNull(editedPerson);
 
         int addressBookIndex = filteredPersons.getSourceIndex(filteredPersonListIndex);
         addressBook.updatePerson(addressBookIndex, editedPerson);