/** Implementation of an {@link Optional} not containing a reference. */
@GwtCompatible
final class Absent<T> extends Optional<T> {
  static final Absent<Object> INSTANCE = new Absent<>();

  @SuppressWarnings("unchecked") // implementation is "fully variant"
  static <T> Optional<T> withType() {
    return (Optional<T>) INSTANCE;
  }

  private Absent() {}

  @Override
  public boolean isPresent() {
    return false;
  }

/** Implementation of an {@link Optional} not containing a reference. */
@GwtCompatible
final class Absent<T> extends Optional<T> {
  static final Absent<Object> INSTANCE = new Absent<>();

  @SuppressWarnings("unchecked") // implementation is "fully variant"
  static <T> Optional<T> withType() {
    return (Optional<T>) INSTANCE;
  }

  private Absent() {}

  @Override
  public boolean isPresent() {
    return false;
  }

    expectUnchanged();
  }

  @CollectionFeature.Require(absent = SUPPORTS_REMOVE)
  @CollectionSize.Require(ZERO)
  public void testRetainAll_emptyPreviouslyEmptyUnsupported() {
    expectReturnsFalseOrThrows(empty);
    expectUnchanged();
  }

  @CollectionFeature.Require(SUPPORTS_REMOVE)
  @CollectionSize.Require(absent = ZERO)
  public void testRetainAll_emptyPreviouslyNonEmpty() {

    expectUnchanged();
  }

  @MapFeature.Require(value = SUPPORTS_PUT, absent = ALLOWS_NULL_VALUES)
  @CollectionSize.Require(absent = ZERO)
  public void testReplace_presentNullValueUnsupported() {
    assertThrows(NullPointerException.class, () -> getMap().replace(k0(), null));
    expectUnchanged();
  }

  @MapFeature.Require(value = SUPPORTS_PUT, absent = ALLOWS_NULL_VALUE_QUERIES)
  public void testReplace_absentNullValueUnsupported() {

  @MapFeature.Require(SUPPORTS_REMOVE)
  @CollectionSize.Require(absent = ZERO)
  public void testRemoveKeyRemovesFromInverse() {
    getMap().remove(k0());
    expectMissing(e0());
  }

  @MapFeature.Require(SUPPORTS_REMOVE)
  @CollectionSize.Require(absent = ZERO)
  public void testRemoveKeyFromKeySetRemovesFromInverse() {
    getMap().keySet().remove(k0());

  }

  @CollectionFeature.Require(absent = SUPPORTS_ADD)
  public void testAddAll_unsupportedNonePresent() {
    assertThrows(
        UnsupportedOperationException.class, () -> collection.addAll(createDisjointCollection()));
    expectUnchanged();
    expectMissing(e3(), e4());
  }

  @CollectionFeature.Require(SUPPORTS_ADD)
  @CollectionSize.Require(absent = ZERO)
  public void testAddAll_supportedSomePresent() {

  @ListFeature.Require(SUPPORTS_ADD_WITH_INDEX)
  @CollectionSize.Require(absent = ZERO)
  public void testAddAtIndex_supportedPresent() {
    getList().add(0, e0());
    expectAdded(0, e0());
  }

  @ListFeature.Require(absent = SUPPORTS_ADD_WITH_INDEX)
  @CollectionSize.Require(absent = ZERO)
  /*
   * absent = ZERO isn't required, since unmodList.add() must

    }
    expectUnchanged();
  }

  @MapFeature.Require(absent = SUPPORTS_REMOVE)
  @CollectionSize.Require(absent = ZERO)
  public void testRemove_unsupportedPresent() {
    assertThrows(UnsupportedOperationException.class, () -> getMap().remove(k0(), v0()));
    expectUnchanged();
  }

  @MapFeature.Require(absent = SUPPORTS_REMOVE)
  public void testRemove_unsupportedAbsent() {
    try {

  @CollectionSize.Require(absent = ZERO)
  @MapFeature.Require(SUPPORTS_PUT)
  public void testPutAllNonEmptyIterableOnPresentKey() {
    assertTrue(multimap().putAll(k0(), newArrayList(v3(), v4())::iterator));
    assertGet(k0(), v0(), v3(), v4());
  }

  @CollectionSize.Require(absent = ZERO)
  @MapFeature.Require(SUPPORTS_PUT)

@DoNotMock("Use Optional.of(value) or Optional.absent()")
@GwtCompatible
public abstract class Optional<T> implements Serializable {
  /**
   * Returns an {@code Optional} instance with no contained reference.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
   * {@code Optional.empty}.
   */
  public static <T> Optional<T> absent() {
    return Absent.withType();
  }

  /**