* ForwardingMap#clear}, {@link ForwardingMap#containsKey}, {@link ForwardingMap#isEmpty}, {@link
   * ForwardingMap#remove}, {@link ForwardingMap#size}, and the {@link Set#iterator} method of
   * {@link ForwardingMap#entrySet}. In many cases, you may wish to override {@link
   * ForwardingMap#keySet} to forward to this implementation or a subclass thereof.
   *

   * ForwardingMap#clear}, {@link ForwardingMap#containsKey}, {@link ForwardingMap#isEmpty}, {@link
   * ForwardingMap#remove}, {@link ForwardingMap#size}, and the {@link Set#iterator} method of
   * {@link ForwardingMap#entrySet}. In many cases, you may wish to override {@link
   * ForwardingMap#keySet} to forward to this implementation or a subclass thereof.
   *

    StandardImplForwardingMap<String, String> forwardingMap =
        new StandardImplForwardingMap<>(new HashMap<>());
    forwardingMap.put("foo", "bar");
    forwardingMap.put("baz", null);

    assertThat(forwardingMap.toString()).isEqualTo(hashmap.toString());
  }

  private static <K, V> Map<K, V> wrap(Map<K, V> delegate) {
    return new ForwardingMap<K, V>() {
      @Override

  // to a superclass.

  public MapsTransformValuesUnmodifiableIteratorTest() {
    super(true, true, false /*supportsPut*/, true, true, false);
  }

  private static class UnmodifiableIteratorMap<K, V> extends ForwardingMap<K, V> {
    final Map<K, V> delegate;

    UnmodifiableIteratorMap(Map<K, V> delegate) {
      this.delegate = delegate;
    }

    @Override
    protected Map<K, V> delegate() {
      return delegate;

  // to a superclass.

  public MapsTransformValuesUnmodifiableIteratorTest() {
    super(true, true, false /*supportsPut*/, true, true, false);
  }

  private static class UnmodifiableIteratorMap<K, V> extends ForwardingMap<K, V> {
    final Map<K, V> delegate;

    UnmodifiableIteratorMap(Map<K, V> delegate) {
      this.delegate = delegate;
    }

    @Override
    protected Map<K, V> delegate() {
      return delegate;

 * IdentityHashMap} backing maps.
 *
 * @author Kevin Bourrillion
 * @author Mike Bostock
 */
@GwtCompatible
abstract class AbstractBiMap<K extends @Nullable Object, V extends @Nullable Object>
    extends ForwardingMap<K, V> implements BiMap<K, V>, Serializable {

  private transient Map<K, V> delegate;

  @RetainedWith private transient AbstractBiMap<V, K> inverse;

 * IdentityHashMap} backing maps.
 *
 * @author Kevin Bourrillion
 * @author Mike Bostock
 */
@GwtCompatible
abstract class AbstractBiMap<K extends @Nullable Object, V extends @Nullable Object>
    extends ForwardingMap<K, V> implements BiMap<K, V>, Serializable {

  private transient Map<K, V> delegate;

  @RetainedWith private transient AbstractBiMap<V, K> inverse;