return (queue instanceof SynchronizedQueue) ? queue : new SynchronizedQueue<E>(queue, mutex);
  }

  static class SynchronizedQueue<E extends @Nullable Object> extends SynchronizedCollection<E>
      implements Queue<E> {

    SynchronizedQueue(Queue<E> delegate, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

   *
   * <p>It is imperative that the user manually synchronize on the returned queue when accessing the
   * queue's iterator:
   *
   * <pre>{@code
   * Queue<E> queue = Queues.synchronizedQueue(MinMaxPriorityQueue.<E>create());
   * ...
   * queue.add(element);  // Needn't be in synchronized block
   * ...
   * synchronized (queue) {  // Must synchronize on queue!