return delegate().offerLast(e);
  }

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

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

  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  @Override
  @CheckForNull
  public E pollFirst() {
    return delegate().pollFirst();
  }

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

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

    @Override
    public boolean removeFirstOccurrence(Object o) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.removeFirstOccurrence(o);