return new SynchronizedMap<>(map, mutex);
  }

  private static class SynchronizedMap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedObject implements Map<K, V> {
    transient @Nullable Set<K> keySet;
    transient @Nullable Collection<V> values;
    transient @Nullable Set<Map.Entry<K, V>> entrySet;

    SynchronizedMap(Map<K, V> delegate, @Nullable Object mutex) {

  }

  @VisibleForTesting
  static <K extends @Nullable Object, V extends @Nullable Object> Map<K, V> map(
      Map<K, V> map, @Nullable Object mutex) {
    return new SynchronizedMap<>(map, mutex);
  }

  private static class SynchronizedMap<K extends @Nullable Object, V extends @Nullable Object>
      extends SynchronizedObject implements Map<K, V> {
    transient @Nullable Set<K> keySet;