multimap.putAll("bar", asList(2, 3));
    multimap.put("foo", 1);
    assertEquals("{bar=[3, 2, 1], foo=[4, 3, 2, 1, -1]}", multimap.toString());
  }

  public void testGetComparator() {
    TreeMultimap<@Nullable String, @Nullable Integer> multimap = createPopulate();
    assertEquals(StringLength.COMPARATOR, multimap.keyComparator());