void (String[] args) { List<String> input = Arrays.asList("a", "bbb", "cc", "ddd", " x", "ee"); // group by length // <= reduce to max /*need to unwrap optional*/ Map<Integer, String> map2 = input.stream().collect( Collectors.toMap(String::length, Function.identity(), (s1, s2) -> s1.compareTo(s2) > 0 ? /*find max*/ s1 : s2)); System.out.println(map2); }