Java 8 grouping by from one-to-many

Question

I want to learn how to use the Java 8 syntax with streams and got a bit stuck.

It's easy enough to groupingBy when you have one key for every value. But what if I have a List of keys for every value and still want to categorise them with groupingBy? Do I have to break it into several statements or is there possibly a little stream magic that can be done to make it simpler.

This is the basic code:

List<Album> albums = new ArrayList<>();
Map<Artist, List<Album>> map = albums.stream().collect(Collectors.groupingBy(this::getArtist));

It works great if there is only one Artist for every Album. But I must return a List since an Album can have many Artists. Album and Artist are used for illustration of course, I have real-world types..

There's probably a simple solution but I haven't found it in a while so I'm calling on the collective brain this site represents to solve it. :) A complex solution is also welcome in case a simple one doesn't exist.

In Album class or as an utility method taking an Album as argument:

Artist getArtist(); // ok

List<Artist> getArtist(); // Not ok, since we now have many "keys" for every Album

Cheers, Mikael Grev

Answer 1

I think you are after Collectors.mapping which can be passed as a second argument to groupingBy

Complete example

import java.util.AbstractMap;
import java.util.List;
import java.util.Map;

import static java.util.Arrays.asList;
import static java.util.Map.Entry;
import static java.util.stream.Collectors.*;

public class SO {

    public static void main(String... args) {

        List<Album> albums = asList(
                new Album(
                        asList(
                                new Artist("bob"),
                                new Artist("tom")
                        )
                ),
                new Album(asList(new Artist("bill")))
        );

        Map<Artist, List<Album>> x = albums.stream()
                .flatMap(album -> album.getArtist().stream().map(artist -> pair(artist, album)))
                .collect(groupingBy(Entry::getKey, mapping(Entry::getValue, toList())));

        x.entrySet().stream().forEach(System.out::println);
    }

    static class Artist {
        private final String name;

        Artist(String name) {
            this.name = name;
        }

        public String toString() {return name;}

    }

    static class Album {
        private List<Artist> artist;

        Album(List<Artist> artist) {
            this.artist = artist;
        }

        List<Artist> getArtist() {
            return artist;
        }

    }

    private static <T,U> AbstractMap.SimpleEntry<T,U> pair(T t, U u) {
        return new AbstractMap.SimpleEntry<T,U>(t,u);
    }


}