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;

 */
@J2ktIncompatible
@GwtIncompatible
@SuppressWarnings("serial") // using writeReplace instead of standard serialization
public final class MutableClassToInstanceMap<B extends @Nullable Object>
    extends ForwardingMap<Class<? extends @NonNull B>, B>
    implements ClassToInstanceMap<B>, Serializable {

  /**
   * Returns a new {@code MutableClassToInstanceMap} instance backed by a {@link HashMap} using the

 * 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;

  }

  /**
   * @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;