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