public T pollFirst() {
    if (head == null) {
      return null;
    }
    T first = head.element;
    this.remove(first);
    return first;
  }
@Test
  public void testPollOneElement() {
    LOG.info("Test poll one element");
    set.add(list.get(0));
    assertEquals(list.get(0), set.pollFirst());
    assertNull(set.pollFirst());
    LOG.info("Test poll one element - DONE");
  }