public void test_hashCode_unique() {

    Set<Integer> uniques = new HashSet<Integer>(201 * 12);
    for (int i = 1900; i <= 2100; i++) {
      for (int j = 1; j <= 12; j++) {
        assertTrue(uniques.add(YearMonth.of(i, j).hashCode()));
      }
    }
  }