@Override
    public E getLast() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.getLast();
    }

    @Override
    public @Nullable E peekFirst() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.peekFirst();
    }

    @Override
    public @Nullable E peekLast() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.peekLast();
    }

 *       size. In many cases {@link Ordering#leastOf} may work for your use case with significantly
 *       improved (and asymptotically superior) performance.
 *   <li>The retrieval operations {@link #peek}, {@link #peekFirst}, {@link #peekLast}, {@link
 *       #element}, and {@link #size} are constant-time.
 *   <li>The enqueuing and dequeuing operations ({@link #offer}, {@link #add}, and all the forms of

 *       size. In many cases {@link Ordering#leastOf} may work for your use case with significantly
 *       improved (and asymptotically superior) performance.
 *   <li>The retrieval operations {@link #peek}, {@link #peekFirst}, {@link #peekLast}, {@link
 *       #element}, and {@link #size} are constant-time.
 *   <li>The enqueuing and dequeuing operations ({@link #offer}, {@link #add}, and all the forms of

    public E getLast() {
      synchronized (mutex) {
        return delegate().getLast();
      }
    }

    @Override
    @CheckForNull
    public E peekFirst() {
      synchronized (mutex) {
        return delegate().peekFirst();
      }
    }

    @Override
    @CheckForNull
    public E peekLast() {
      synchronized (mutex) {
        return delegate().peekLast();

    public E getLast() {
      synchronized (mutex) {
        return delegate().getLast();
      }
    }

    @Override
    @CheckForNull
    public E peekFirst() {
      synchronized (mutex) {
        return delegate().peekFirst();
      }
    }

    @Override
    @CheckForNull
    public E peekLast() {
      synchronized (mutex) {
        return delegate().peekLast();