public void testHigherEntry() {
    assertThat(create().higherEntry("a")).isNull();
  }

  public void testHigherKey() {
    assertThat(create().higherKey("a")).isNull();
  }

  public void testLastEntry() {
    assertThat(create().lastEntry()).isNull();
  }

  public void testLastKey() {
    NavigableMap<String, Integer> map = create();
    map.put("a", 1);
    assertThat(map.lastKey()).isNotNull();