@Override
  public int hashCode()
  {
    return _key == null? 0 : _key.hashCode();
  }
@Test
  public void testHashCode()
  {
    IdResponse<Long> longResponse1 = new IdResponse<>(1L);
    IdResponse<Long> longResponse2 = new IdResponse<>(1L);
    IdResponse<Long> nullLongResponse = new IdResponse<>(null);
    IdResponse<String> nullStringResponse = new IdResponse<>(null);

    Assert.assertEquals(longResponse1.hashCode(), longResponse2.hashCode());
    Assert.assertEquals(nullLongResponse.hashCode(), nullStringResponse.hashCode());
  }