@Test
void compare() {

    final Pair<String, String> pair1 = new Pair<>(null, null);
    final Pair<String, String> pair2 = new Pair<>("FIRST", null);
    final Pair<String, String> pair3 = new Pair<>(null, "FIRST");
    final Pair<String, String> pair4 = new Pair<>("SECOND", "SECOND");
    assertEquals(0, instance.compare(pair1.getFirst(), pair1.getSecond()));
    assertEquals(-1, instance.compare(pair2.getFirst(), pair2.getSecond()));
    assertEquals(1, instance.compare(pair3.getFirst(), pair3.getSecond()));
    assertEquals(0, instance.compare(pair4.getFirst(), pair4.getSecond()));
}