Java 8 follow-up changes (#2747)

* Java 8 follow-up changes

* Fix Error Prone warnings

Note that for the `final` variable warnings, it seems some of those variables
were made `final` intentionally, without the compiler requiring it.
But for consistency I removed the `final` from them as well.

* Disable `UnnecessaryAnonymousClass` Error Prone pattern

Is disabled by default as well.

It seems what this pattern does is suggest to replace anonymous classes
stored in constants with a corresponding method (with the intention that
the developer then uses a method reference to that method). But it even
does this when the anonymous class implements a custom functional type,
and not a generic type such as `Function`.

For example it suggested to replace
```
private static final FieldNamingStrategy CUSTOM_FIELD_NAMING_STRATEGY =
  new FieldNamingStrategy() {
    @OverRide
    public String translateName(Field f) {
      return "foo";
    }
  };
```

with a method
```
private static String customFieldNamingStrategy(Field f) {
  return "foo";
}
```

Which is probably not that helpful, and might not actually increase
readability.

* Revert some changes for API not available yet in target Android version

.github/workflows/build.yml

@@ -10,12 +10,7 @@ jobs:
     name: "Build on JDK ${{ matrix.java }}"
     strategy:
       matrix:
-        java: [ 11, 17 ]
-        # Custom JDK 21 configuration
-        include:
-          - java: 21
-            # Disable Enforcer check which (intentionally) prevents using JDK 21 for building
-            extra-mvn-args: -Denforcer.fail=false
+        java: [ 11, 17, 21 ]
     runs-on: ubuntu-latest
 
     steps:
@@ -28,7 +23,7 @@ jobs:
           cache: 'maven'
       - name: Build with Maven
         # This also runs javadoc:jar to detect any issues with the Javadoc generated during release
-        run: mvn --batch-mode --no-transfer-progress verify javadoc:jar ${{ matrix.extra-mvn-args || '' }}
+        run: mvn --batch-mode --no-transfer-progress verify javadoc:jar
 
   native-image-test:
     name: "GraalVM Native Image test"

README.md

@@ -89,7 +89,7 @@ Gson uses Maven to build the project:
 mvn clean verify
 ```
 
-JDK 11 or newer is required for building, JDK 17 is recommended. Newer JDKs are currently not supported for building (but are supported when _using_ Gson).
+JDK 11 or newer is required for building, JDK 17 or 21 is recommended. Newer JDKs are currently not supported for building (but are supported when _using_ Gson).
 
 ### Contributing
 

extras/src/main/java/com/google/gson/graph/GraphAdapterBuilder.java

@@ -52,7 +52,7 @@ public GraphAdapterBuilder() {
   }
 
   public GraphAdapterBuilder addType(Type type) {
-    final ObjectConstructor<?> objectConstructor = constructorConstructor.get(TypeToken.get(type));
+    ObjectConstructor<?> objectConstructor = constructorConstructor.get(TypeToken.get(type));
     InstanceCreator<Object> instanceCreator =
         new InstanceCreator<Object>() {
           @Override
@@ -95,8 +95,8 @@ public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return null;
       }
 
-      final TypeAdapter<T> typeAdapter = gson.getDelegateAdapter(this, type);
-      final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+      TypeAdapter<T> typeAdapter = gson.getDelegateAdapter(this, type);
+      TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
       return new TypeAdapter<T>() {
         @Override
         public void write(JsonWriter out, T value) throws IOException {

extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java

@@ -252,9 +252,9 @@ public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
       return null;
     }
 
-    final TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
-    final Map<String, TypeAdapter<?>> labelToDelegate = new LinkedHashMap<>();
-    final Map<Class<?>, TypeAdapter<?>> subtypeToDelegate = new LinkedHashMap<>();
+    TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
+    Map<String, TypeAdapter<?>> labelToDelegate = new LinkedHashMap<>();
+    Map<Class<?>, TypeAdapter<?>> subtypeToDelegate = new LinkedHashMap<>();
     for (Map.Entry<String, Class<?>> entry : labelToSubtype.entrySet()) {
       TypeAdapter<?> delegate = gson.getDelegateAdapter(this, TypeToken.get(entry.getValue()));
       labelToDelegate.put(entry.getKey(), delegate);

extras/src/test/java/com/google/gson/typeadapters/PostConstructAdapterFactoryTest.java

@@ -88,7 +88,7 @@ public boolean equals(Object o) {
       if (!(o instanceof Sandwich)) {
         return false;
       }
-      final Sandwich other = (Sandwich) o;
+      Sandwich other = (Sandwich) o;
       if (this.bread == null ? other.bread != null : !this.bread.equals(other.bread)) {
         return false;
       }
@@ -115,7 +115,7 @@ public boolean equals(Object o) {
       if (!(o instanceof MultipleSandwiches)) {
         return false;
       }
-      final MultipleSandwiches other = (MultipleSandwiches) o;
+      MultipleSandwiches other = (MultipleSandwiches) o;
       if (this.sandwiches == null
           ? other.sandwiches != null
           : !this.sandwiches.equals(other.sandwiches)) {

gson/src/main/java/com/google/gson/Gson.java

@@ -538,7 +538,7 @@ public void write(JsonWriter out, Number value) throws IOException {
     };
   }
 
-  private static TypeAdapter<AtomicLong> atomicLongAdapter(final TypeAdapter<Number> longAdapter) {
+  private static TypeAdapter<AtomicLong> atomicLongAdapter(TypeAdapter<Number> longAdapter) {
     return new TypeAdapter<AtomicLong>() {
       @Override
       public void write(JsonWriter out, AtomicLong value) throws IOException {
@@ -554,7 +554,7 @@ public AtomicLong read(JsonReader in) throws IOException {
   }
 
   private static TypeAdapter<AtomicLongArray> atomicLongArrayAdapter(
-      final TypeAdapter<Number> longAdapter) {
+      TypeAdapter<Number> longAdapter) {
     return new TypeAdapter<AtomicLongArray>() {
       @Override
       public void write(JsonWriter out, AtomicLongArray value) throws IOException {
@@ -682,7 +682,7 @@ public <T> TypeAdapter<T> getAdapter(Class<T> type) {
    *   public int numReads = 0;
    *   public int numWrites = 0;
    *   public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
-   *     final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
+   *     TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
    *     return new TypeAdapter<T>() {
    *       public void write(JsonWriter out, T value) throws IOException {
    *         ++numWrites;

gson/src/main/java/com/google/gson/TypeAdapterFactory.java

@@ -117,7 +117,7 @@
  *   }
  *
  *   private <E> TypeAdapter<Multiset<E>> newMultisetAdapter(
- *       final TypeAdapter<E> elementAdapter) {
+ *       TypeAdapter<E> elementAdapter) {
  *     return new TypeAdapter<Multiset<E>>() {
  *       public void write(JsonWriter out, Multiset<E> value) throws IOException {
  *         if (value == null) {

gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java

@@ -95,13 +95,13 @@ static String checkInstantiable(Class<?> c) {
   }
 
   public <T> ObjectConstructor<T> get(TypeToken<T> typeToken) {
-    final Type type = typeToken.getType();
-    final Class<? super T> rawType = typeToken.getRawType();
+    Type type = typeToken.getType();
+    Class<? super T> rawType = typeToken.getRawType();
 
     // first try an instance creator
 
     @SuppressWarnings("unchecked") // types must agree
-    final InstanceCreator<T> typeCreator = (InstanceCreator<T>) instanceCreators.get(type);
+    InstanceCreator<T> typeCreator = (InstanceCreator<T>) instanceCreators.get(type);
     if (typeCreator != null) {
       return new ObjectConstructor<T>() {
         @Override
@@ -113,7 +113,7 @@ public T construct() {
 
     // Next try raw type match for instance creators
     @SuppressWarnings("unchecked") // types must agree
-    final InstanceCreator<T> rawTypeCreator = (InstanceCreator<T>) instanceCreators.get(rawType);
+    InstanceCreator<T> rawTypeCreator = (InstanceCreator<T>) instanceCreators.get(rawType);
     if (rawTypeCreator != null) {
       return new ObjectConstructor<T>() {
         @Override
@@ -145,7 +145,7 @@ public T construct() {
 
     // Check whether type is instantiable; otherwise ReflectionAccessFilter recommendation
     // of adjusting filter suggested below is irrelevant since it would not solve the problem
-    final String exceptionMessage = checkInstantiable(rawType);
+    String exceptionMessage = checkInstantiable(rawType);
     if (exceptionMessage != null) {
       return new ObjectConstructor<T>() {
         @Override
@@ -161,7 +161,7 @@ public T construct() {
       // finally try unsafe
       return newUnsafeAllocator(rawType);
     } else {
-      final String message =
+      String message =
           "Unable to create instance of "
               + rawType
               + "; ReflectionAccessFilter does not permit using reflection or Unsafe. Register an"
@@ -181,7 +181,7 @@ public T construct() {
    * constructor.
    */
   private static <T> ObjectConstructor<T> newSpecialCollectionConstructor(
-      final Type type, Class<? super T> rawType) {
+      Type type, Class<? super T> rawType) {
     if (EnumSet.class.isAssignableFrom(rawType)) {
       return new ObjectConstructor<T>() {
         @Override
@@ -233,7 +233,7 @@ private static <T> ObjectConstructor<T> newDefaultConstructor(
       return null;
     }
 
-    final Constructor<? super T> constructor;
+    Constructor<? super T> constructor;
     try {
       constructor = rawType.getDeclaredConstructor();
     } catch (NoSuchMethodException e) {
@@ -249,7 +249,7 @@ private static <T> ObjectConstructor<T> newDefaultConstructor(
                     || Modifier.isPublic(constructor.getModifiers())));
 
     if (!canAccess) {
-      final String message =
+      String message =
           "Unable to invoke no-args constructor of "
               + rawType
               + ";"
@@ -267,7 +267,7 @@ public T construct() {
     // Only try to make accessible if allowed; in all other cases checks above should
     // have verified that constructor is accessible
     if (filterResult == FilterResult.ALLOW) {
-      final String exceptionMessage = ReflectionHelper.tryMakeAccessible(constructor);
+      String exceptionMessage = ReflectionHelper.tryMakeAccessible(constructor);
       if (exceptionMessage != null) {
         /*
          * Create ObjectConstructor which throws exception.
@@ -323,7 +323,7 @@ public T construct() {
   /** Constructors for common interface types like Map and List and their subtypes. */
   @SuppressWarnings("unchecked") // use runtime checks to guarantee that 'T' is what it is
   private static <T> ObjectConstructor<T> newDefaultImplementationConstructor(
-      final Type type, Class<? super T> rawType) {
+      Type type, Class<? super T> rawType) {
 
     /*
      * IMPORTANT: Must only create instances for classes with public no-args constructor.
@@ -409,7 +409,7 @@ public T construct() {
     return null;
   }
 
-  private <T> ObjectConstructor<T> newUnsafeAllocator(final Class<? super T> rawType) {
+  private <T> ObjectConstructor<T> newUnsafeAllocator(Class<? super T> rawType) {
     if (useJdkUnsafe) {
       return new ObjectConstructor<T>() {
         @Override
@@ -444,8 +444,8 @@ public T construct() {
             " Or adjust your R8 configuration to keep the no-args constructor of the class.";
       }
 
-      // Explicit final variable to allow usage in the anonymous class below
-      final String exceptionMessageF = exceptionMessage;
+      // Separate effectively final variable to allow usage in the anonymous class below
+      String exceptionMessageF = exceptionMessage;
 
       return new ObjectConstructor<T>() {
         @Override

gson/src/main/java/com/google/gson/internal/Excluder.java

@@ -108,11 +108,11 @@ public Excluder withExclusionStrategy(
   }
 
   @Override
-  public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> type) {
+  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     Class<?> rawType = type.getRawType();
 
-    final boolean skipSerialize = excludeClass(rawType, true);
-    final boolean skipDeserialize = excludeClass(rawType, false);
+    boolean skipSerialize = excludeClass(rawType, true);
+    boolean skipDeserialize = excludeClass(rawType, false);
 
     if (!skipSerialize && !skipDeserialize) {
       return null;

gson/src/main/java/com/google/gson/internal/NonNullElementWrapperList.java

@@ -127,5 +127,6 @@ public int hashCode() {
     return delegate.hashCode();
   }
 
-  // TODO: Once Gson targets Java 8 also override List.sort
+  // Maybe also delegate List#sort and List#spliterator in the future, but that
+  // requires Android API level 24
 }

gson/src/main/java/com/google/gson/internal/ReflectionAccessFilterHelper.java

@@ -85,7 +85,7 @@ private abstract static class AccessChecker {
       // TODO: Ideally should use Multi-Release JAR for this version specific code
       if (JavaVersion.isJava9OrLater()) {
         try {
-          final Method canAccessMethod =
+          Method canAccessMethod =
               AccessibleObject.class.getDeclaredMethod("canAccess", Object.class);
           accessChecker =
               new AccessChecker() {

gson/src/main/java/com/google/gson/internal/UnsafeAllocator.java

@@ -54,8 +54,8 @@ private static UnsafeAllocator create() {
       Class<?> unsafeClass = Class.forName("sun.misc.Unsafe");
       Field f = unsafeClass.getDeclaredField("theUnsafe");
       f.setAccessible(true);
-      final Object unsafe = f.get(null);
-      final Method allocateInstance = unsafeClass.getMethod("allocateInstance", Class.class);
+      Object unsafe = f.get(null);
+      Method allocateInstance = unsafeClass.getMethod("allocateInstance", Class.class);
       return new UnsafeAllocator() {
         @Override
         @SuppressWarnings("unchecked")
@@ -77,8 +77,8 @@ public <T> T newInstance(Class<T> c) throws Exception {
       Method getConstructorId =
           ObjectStreamClass.class.getDeclaredMethod("getConstructorId", Class.class);
       getConstructorId.setAccessible(true);
-      final int constructorId = (Integer) getConstructorId.invoke(null, Object.class);
-      final Method newInstance =
+      int constructorId = (Integer) getConstructorId.invoke(null, Object.class);
+      Method newInstance =
           ObjectStreamClass.class.getDeclaredMethod("newInstance", Class.class, int.class);
       newInstance.setAccessible(true);
       return new UnsafeAllocator() {
@@ -99,7 +99,7 @@ public <T> T newInstance(Class<T> c) throws Exception {
     //     Class<?> instantiationClass, Class<?> constructorClass);
     // }
     try {
-      final Method newInstance =
+      Method newInstance =
           ObjectInputStream.class.getDeclaredMethod("newInstance", Class.class, Class.class);
       newInstance.setAccessible(true);
       return new UnsafeAllocator() {

gson/src/main/java/com/google/gson/internal/bind/EnumTypeAdapter.java

@@ -54,7 +54,7 @@ public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
   private final Map<String, T> stringToConstant = new HashMap<>();
   private final Map<T, String> constantToName = new HashMap<>();
 
-  private EnumTypeAdapter(final Class<T> classOfT) {
+  private EnumTypeAdapter(Class<T> classOfT) {
     try {
       // Uses reflection to find enum constants to work around name mismatches for obfuscated
       // classes

gson/src/main/java/com/google/gson/internal/bind/JsonTreeReader.java

@@ -282,14 +282,14 @@ public int nextInt() throws IOException {
   }
 
   JsonElement nextJsonElement() throws IOException {
-    final JsonToken peeked = peek();
+    JsonToken peeked = peek();
     if (peeked == JsonToken.NAME
         || peeked == JsonToken.END_ARRAY
         || peeked == JsonToken.END_OBJECT
         || peeked == JsonToken.END_DOCUMENT) {
       throw new IllegalStateException("Unexpected " + peeked + " when reading a JsonElement.");
     }
-    final JsonElement element = (JsonElement) peekStack();
+    JsonElement element = (JsonElement) peekStack();
     skipValue();
     return element;
   }

gson/src/main/java/com/google/gson/internal/bind/NumberTypeAdapter.java

@@ -41,7 +41,7 @@ private NumberTypeAdapter(ToNumberStrategy toNumberStrategy) {
   }
 
   private static TypeAdapterFactory newFactory(ToNumberStrategy toNumberStrategy) {
-    final NumberTypeAdapter adapter = new NumberTypeAdapter(toNumberStrategy);
+    NumberTypeAdapter adapter = new NumberTypeAdapter(toNumberStrategy);
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked")
       @Override

gson/src/main/java/com/google/gson/internal/bind/ObjectTypeAdapter.java

@@ -49,7 +49,7 @@ private ObjectTypeAdapter(Gson gson, ToNumberStrategy toNumberStrategy) {
     this.toNumberStrategy = toNumberStrategy;
   }
 
-  private static TypeAdapterFactory newFactory(final ToNumberStrategy toNumberStrategy) {
+  private static TypeAdapterFactory newFactory(ToNumberStrategy toNumberStrategy) {
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked")
       @Override

gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java

@@ -103,7 +103,7 @@ private List<String> getFieldNames(Field f) {
   }
 
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, final TypeToken<T> type) {
+  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     Class<? super T> raw = type.getRawType();
 
     if (!Object.class.isAssignableFrom(raw)) {
@@ -175,18 +175,18 @@ private static <M extends AccessibleObject & Member> void checkAccessible(
   }
 
   private BoundField createBoundField(
-      final Gson context,
-      final Field field,
-      final Method accessor,
-      final String serializedName,
-      final TypeToken<?> fieldType,
-      final boolean serialize,
-      final boolean blockInaccessible) {
+      Gson context,
+      Field field,
+      Method accessor,
+      String serializedName,
+      TypeToken<?> fieldType,
+      boolean serialize,
+      boolean blockInaccessible) {
 
-    final boolean isPrimitive = Primitives.isPrimitive(fieldType.getRawType());
+    boolean isPrimitive = Primitives.isPrimitive(fieldType.getRawType());
 
     int modifiers = field.getModifiers();
-    final boolean isStaticFinalField = Modifier.isStatic(modifiers) && Modifier.isFinal(modifiers);
+    boolean isStaticFinalField = Modifier.isStatic(modifiers) && Modifier.isFinal(modifiers);
 
     JsonAdapter annotation = field.getAnnotation(JsonAdapter.class);
     TypeAdapter<?> mapped = null;
@@ -196,14 +196,14 @@ private BoundField createBoundField(
           jsonAdapterFactory.getTypeAdapter(
               constructorConstructor, context, fieldType, annotation, false);
     }
-    final boolean jsonAdapterPresent = mapped != null;
+    boolean jsonAdapterPresent = mapped != null;
     if (mapped == null) {
       mapped = context.getAdapter(fieldType);
     }
 
     @SuppressWarnings("unchecked")
-    final TypeAdapter<Object> typeAdapter = (TypeAdapter<Object>) mapped;
-    final TypeAdapter<Object> writeTypeAdapter;
+    TypeAdapter<Object> typeAdapter = (TypeAdapter<Object>) mapped;
+    TypeAdapter<Object> writeTypeAdapter;
     if (serialize) {
       writeTypeAdapter =
           jsonAdapterPresent