map.put("a", 1);
    assertThat(map.firstKey()).isNotNull();
  }

  public void testFloorEntry() {
    assertThat(create().floorEntry("a")).isNull();
  }

  public void testFloorKey() {
    assertThat(create().floorKey("a")).isNull();
  }

  public void testHeadMap_k() {
    NavigableMap<String, Integer> map = create();
    SortedMap<String, Integer> headMap = map.headMap("a");