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

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

    // Just demonstrate that no explicit type parameter is required
    comparator = Comparators.emptiesFirst(naturalOrder());
  }

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

   * than all other values, and orders the rest using {@code valueComparator} on the contained
   * value.
   *
   * @since 22.0
   */
  public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<? super T> valueComparator) {
    checkNotNull(valueComparator);
    return Comparator.<Optional<T>, @Nullable T>comparing(
        o -> o.orElse(null), Comparator.nullsFirst(valueComparator));
  }