public T poll() {
    T highPriority = _highPriority.pollFirst();
    return (highPriority != null) ? highPriority : _lowPriority.pollFirst();
  }
@Test
  public void testPollOnEmpty() {
    final LIFOBiPriorityQueue<?> queue = new LIFOBiPriorityQueue<>();
    assertNull(queue.poll());
  }