@Override
    public Stream<E> stream() {
      synchronized (mutex) {
        return delegate().stream();
      }
    }

    @Override
    public Stream<E> parallelStream() {
      synchronized (mutex) {
        return delegate().parallelStream();
      }
    }

    @Override
    public void forEach(Consumer<? super E> action) {
      synchronized (mutex) {
        delegate().forEach(action);
      }