/** 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;
  }

  @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());

  @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)

  @MapFeature.Require(value = SUPPORTS_PUT, absent = ALLOWS_NULL_VALUES)
  @CollectionSize.Require(absent = ZERO)
  public void testReplaceEntry_presentNullValueUnsupported() {
    assertThrows(NullPointerException.class, () -> getMap().replace(k0(), v0(), null));
    expectUnchanged();
  }

  @MapFeature.Require(value = SUPPORTS_PUT, absent = ALLOWS_NULL_VALUE_QUERIES)
  @CollectionSize.Require(absent = ZERO)

  }

  @MapFeature.Require(absent = SUPPORTS_PUT)
  @CollectionSize.Require(absent = ZERO)
  public void testPut_unsupportedPresentExistingValue() {
    try {
      assertEquals("put(present, existingValue) should return present or throw", v0(), put(e0()));
    } catch (UnsupportedOperationException tolerated) {
    }
    expectUnchanged();
  }

  @MapFeature.Require(absent = SUPPORTS_PUT)

    expectUnchanged();
  }

  @MapFeature.Require(absent = SUPPORTS_PUT)
  @CollectionSize.Require(absent = ZERO)
  public void testPutIfAbsent_unsupportedPresentDifferentValue() {
    try {
      getMap().putIfAbsent(k0(), v3());
    } catch (UnsupportedOperationException tolerated) {
    }
    expectUnchanged();
  }

  @MapFeature.Require(value = SUPPORTS_PUT, absent = ALLOWS_NULL_KEYS)

    assertFalse(multimap().containsEntry(null, null));
  }

  @MapFeature.Require(absent = ALLOWS_NULL_KEY_QUERIES)
  public void testContainsEntryNullDisallowedBecauseKeyQueriesDisallowed() {
    assertThrows(NullPointerException.class, () -> multimap().containsEntry(null, v3()));
  }

  @MapFeature.Require(absent = ALLOWS_NULL_VALUE_QUERIES)
  public void testContainsEntryNullDisallowedBecauseValueQueriesDisallowed() {

  }

  @CollectionFeature.Require(absent = ALLOWS_NULL_VALUES)
  public void testFind_nullNotContainedAndUnsupported() {
    try {
      assertEquals(getMethodName() + "(nullNotPresent) should return -1 or throw", -1, find(null));
    } catch (NullPointerException tolerated) {
    }
  }

  @CollectionFeature.Require(ALLOWS_NULL_VALUES)
  @CollectionSize.Require(absent = ZERO)