@Test
public void testHigherKey() {

    IncrementalSortMap<Integer, String> q = map5();
    Object e1 = q.higherKey(three);
    assertEquals(four, e1);
    Object e2 = q.higherKey(zero);
    assertEquals(one, e2);
    Object e3 = q.higherKey(five);
    assertNull(e3);
    Object e4 = q.higherKey(six);
    assertNull(e4);
}