@Override
  public T peekLast()
  {
    if (_tail == null)
    {
      return null;
    }
    return _tail._value;
  }
@Test
  public void testEmptyPeekLast()
  {
    LinkedDeque<Object> q = new LinkedDeque<>();
    Assert.assertNull(q.peekLast(), "peekLast on empty queue should return null");
  }