@Override
  protected int compareNonNull(T o1, T o2) {
    return Comparator.<T> naturalOrder().compare(o1, o2);
  }
@Test
  public void should_compare_instances_according_to_their_natural_order() {
    // GIVEN
    String s1 = "aaa";
    String s2 = "bbb";
    // WHEN
    int less = NATURAL_ORDER_COMPARATOR.compareNonNull(s1, s2);
    int equal = NATURAL_ORDER_COMPARATOR.compareNonNull(s1, s1);
    int greater = NATURAL_ORDER_COMPARATOR.compareNonNull(s2, s1);
    // THEN
    then(less).isNegative();
    then(equal).isZero();
    then(greater).isPositive();
  }