Collection<E> collection, @CheckForNull Object mutex) {
    return new SynchronizedCollection<>(collection, mutex);
  }

  @VisibleForTesting
  static class SynchronizedCollection<E extends @Nullable Object> extends SynchronizedObject
      implements Collection<E> {
    private SynchronizedCollection(Collection<E> delegate, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }