// "Use 'Collectors.toMap()' collector" "true"
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;

public class Test {
    public static void main(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);
    }
}