@Test
public void testHashCode() {

  assertEquals(new SimpleDate(2013, 5, 10).hashCode(), new SimpleDate(2013, 5, 10).hashCode());
  assertThat(new SimpleDate(2013, 5, 10).hashCode()).isNotEqualTo(
      new SimpleDate(2014, 5, 10).hashCode());
  assertThat(new SimpleDate(2013, 5, 10).hashCode()).isNotEqualTo(
      new SimpleDate(2013, 6, 10).hashCode());
  assertThat(new SimpleDate(2013, 5, 10).hashCode()).isNotEqualTo(
      new SimpleDate(2013, 5, 11).hashCode());
}