@Override
  public boolean offerLast(T t)
  {
    addLastNode(t);
    return true;
  }
@Test
  public void testOfferLastNull()
  {
    LinkedDeque<Object> q = new LinkedDeque<>();
    try
    {
      q.offerLast(null);
      Assert.fail("offerLast null should have failed");
    }
    catch (NullPointerException e)
    {
      // expected
    }
  }