How to get sorted a TreeMap from list using streams

Question

I have a class say Level (its just a dummy class for understanding). I want a sorted TreeMap<Level,Set<String>> based on levelId. Please find below code

import java.util.*;
import java.util.stream.Collectors;

    public class Level {
        int levelId;

        public Level(int levelId) {
            this.levelId = levelId;
        }

        public static Level getLevel(String name){
            return new Level(name.length());
        }

        public static void main(String[]args){
            Set<String> names=new HashSet<>();
            names.add("Mahesh");
            names.add("Ram");
            names.add("Rita");

            Map<Level, Set<String>> map = names.stream().collect(
                    Collectors.groupingBy(name->Level.getLevel(name),
                    Collectors.mapping(name->name,Collectors.toSet())));

        }
    }

I have also tried with Collectors.collectingAndThen().

Any help is appreciated.

Thanks in advance.

Answer 1

If you don’t want to let Level implement Comparable, you need a Comparator. Then, you have to pass a lambda expression creating a TreeMap using this separator as map factory to the groupingBy collector:

public class Level {
    int levelId;

    public Level(int levelId) {
        this.levelId = levelId;
    }

    public static Level getLevel(String name){
        return new Level(name.length());
    }

    public int getLevelId() {
        return levelId;
    }

    public static void main(String[]args){
        Set<String> names=new HashSet<>();
        names.add("Mahesh");
        names.add("Ram");
        names.add("Rita");

        Comparator<Level> c = Comparator.comparingInt(Level::getLevelId);
        Map<Level, Set<String>> map = names.stream()
            .collect(Collectors.groupingBy(
                         Level::getLevel, () -> new TreeMap<>(c), Collectors.toSet()));
    }
}