private Synchronized() {}

  static class SynchronizedObject implements Serializable {
    final Object delegate;
    final Object mutex;

    SynchronizedObject(Object delegate, @CheckForNull Object mutex) {
      this.delegate = checkNotNull(delegate);
      this.mutex = (mutex == null) ? this : mutex;
    }

    Object delegate() {
      return delegate;
    }

    // No equals and hashCode; see ForwardingObject for details.