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

    TestMap<K, V> inner = new TestMap<>(new HashMap<K, V>(), mutex);
    Map<K, V> outer = Synchronized.map(inner, mutex);
    return outer;
  }

  static class TestMap<K, V> extends ForwardingMap<K, V> implements Serializable {
    public final Object mutex;
    private final Map<K, V> delegate;

    public TestMap(Map<K, V> delegate, Object mutex) {
      checkNotNull(mutex);
      this.delegate = delegate;

    TestMap<K, V> inner = new TestMap<>(new HashMap<K, V>(), mutex);
    Map<K, V> outer = Synchronized.map(inner, mutex);
    return outer;
  }

  static class TestMap<K, V> extends ForwardingMap<K, V> implements Serializable {
    public final Object mutex;
    private final Map<K, V> delegate;

    public TestMap(Map<K, V> delegate, Object mutex) {
      checkNotNull(mutex);
      this.delegate = 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;

  // 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;

  }

  /**
   * @see Maps#unmodifiableBiMap(BiMap)
   */
  private static final class UnmodifiableBiMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingMap<K, V> implements BiMap<K, V>, Serializable {
    final Map<K, V> unmodifiableMap;
    final BiMap<? extends K, ? extends V> delegate;
    @LazyInit @RetainedWith @Nullable BiMap<V, K> inverse;

  }

  /**
   * @see Maps#unmodifiableBiMap(BiMap)
   */
  private static final class UnmodifiableBiMap<
          K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingMap<K, V> implements BiMap<K, V>, Serializable {
    final Map<K, V> unmodifiableMap;
    final BiMap<? extends K, ? extends V> delegate;
    @LazyInit @RetainedWith @Nullable BiMap<V, K> inverse;