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