/** 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;
  }

    assertOneToOne();
  }

  @CollectionSize.Require(absent = ZERO)
  @CollectionFeature.Require(SUPPORTS_REMOVE)
  public void testSetCount_oneToOne_removeSupported() {
    assertOneToOne();
  }

  @CollectionSize.Require(absent = ZERO)
  @CollectionFeature.Require(absent = {SUPPORTS_ADD, SUPPORTS_REMOVE})
  public void testSetCount_oneToOne_unsupported() {
    try {
      assertOneToOne();

  @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)

    assertNull(
        "computeIfAbsent(absent, returnsNull) should return null",
        getMap()
            .computeIfAbsent(
                k3(),
                k -> {
                  assertEquals(k3(), k);
                  return null;
                }));
    expectUnchanged();
  }

  @MapFeature.Require({SUPPORTS_PUT, ALLOWS_NULL_VALUES})
  @CollectionSize.Require(absent = ZERO)

    assertFalse(result.remove(null));
  }

  @CollectionSize.Require(absent = ZERO)
  @MapFeature.Require(value = SUPPORTS_PUT, absent = ALLOWS_NULL_VALUES)
  public void testAddNullValueUnsupported() {
    Collection<V> result = multimap().asMap().get(k0());
    assertThrows(NullPointerException.class, () -> result.add(null));
  }

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

        });
  }

  @MapFeature.Require(absent = SUPPORTS_PUT)
  @CollectionSize.Require(absent = ZERO)
  public void testPutAll_unsupportedSomePresent() {
    assertThrows(
        UnsupportedOperationException.class, () -> putAll(MinimalCollection.of(e3(), e0())));
    expectUnchanged();
  }

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

  }

  @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_SET)
  @CollectionSize.Require(absent = ZERO)
  public void testSet() {
    doTestSet(e3());
  }

  @CollectionSize.Require(absent = ZERO)
  @CollectionFeature.Require(ALLOWS_NULL_VALUES)
  @ListFeature.Require(SUPPORTS_SET)
  public void testSet_null() {
    doTestSet(null);
  }

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