Optional<String> z = Optional.of("z");

    Comparator<Optional<String>> comparator = emptiesFirst(comparing(String::length));
    testComparator(comparator, empty, z, abc);

    // Just demonstrate that no explicit type parameter is required
    Comparator<Optional<String>> unused = emptiesFirst(naturalOrder());
  }

  public void testEmptiesLast() {
    Optional<String> empty = Optional.empty();

    Optional<String> z = Optional.of("z");

    Comparator<Optional<String>> comparator = emptiesFirst(comparing(String::length));
    testComparator(comparator, empty, z, abc);

    // Just demonstrate that no explicit type parameter is required
    Comparator<Optional<String>> unused = emptiesFirst(naturalOrder());
  }

  public void testEmptiesLast() {
    Optional<String> empty = Optional.empty();

    return Comparator.<Optional<T>, @Nullable T>comparing(
        o -> orElseNull(o), Comparator.nullsLast(valueComparator));
  }

  @IgnoreJRERequirement // helper for emptiesFirst+emptiesLast
  /*
   * If we make these calls inline inside the lambda inside emptiesFirst()/emptiesLast(), we get an
   * Animal Sniffer error, despite the @IgnoreJRERequirement annotation there. For details, see
   * ImmutableSortedMultiset.
   */

   * value.
   *
   * @since 22.0 (but only since 33.4.0 in the Android flavor)
   */
  public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<? super T> valueComparator) {
    checkNotNull(valueComparator);
    return Comparator.<Optional<T>, @Nullable T>comparing(
        o -> orElseNull(o), Comparator.nullsFirst(valueComparator));
  }