@Override
  public T pollFirst()
  {
    if (_head == null)
    {
      return null;
    }
    return removeNode(_head);
  }
@Test
  public void testEmptyPollFirst()
  {
    LinkedDeque<Object> q = new LinkedDeque<>();
    Assert.assertNull(q.pollFirst(), "pollFirst on empty queue should return null");
  }