@Override
  public String toString()
  {
    return (this._key == null)? "" : this._key.toString();
  }
@Test
  public void testToString()
  {
    IdResponse<Long> longIdResponse = new IdResponse<>(6L);
    longIdResponse.toString();

    IdResponse<Long> nullIdResponse = new IdResponse<>(null);
    nullIdResponse.toString();
  }