@Override
    public void addLast(E e) {
      synchronized (mutex) {
        delegate().addLast(e);
      }
    }

    @Override
    public boolean offerFirst(E e) {
      synchronized (mutex) {
        return delegate().offerFirst(e);
      }
    }

    @Override
    public boolean offerLast(E e) {
      synchronized (mutex) {
        return delegate().offerLast(e);
      }

    @Override
    public void addLast(E e) {
      synchronized (mutex) {
        delegate().addLast(e);
      }
    }

    @Override
    public boolean offerFirst(E e) {
      synchronized (mutex) {
        return delegate().offerFirst(e);
      }
    }

    @Override
    public boolean offerLast(E e) {
      synchronized (mutex) {
        return delegate().offerLast(e);
      }