Java 8 grouping by from one-to-many

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);
    }


}

In case someone looking for a working example, the below should be useful.

import java.util.AbstractMap;
import java.util.AbstractMap.SimpleEntry;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class GroupSubject {
    public static void main(String[] args) {

        List<ContentItem> items = Arrays.asList(new ContentItem("maths"), new ContentItem("science"),
                new ContentItem("social"), new ContentItem("chemistry"), new ContentItem("maths"));

        Map<String, List<ContentItem>> x = (Map<String, List<ContentItem>>) items.stream()
                .flatMap(item -> item.getSubjects().stream().map(subject -> pair(subject, item)))
                .collect(Collectors.groupingBy(e -> ((SimpleEntry<String, ContentItem>) e).getKey(), Collectors
                        .mapping(e -> ((SimpleEntry<String, ContentItem>) e).getValue(), Collectors.toList())));

        System.out.println(x);

    }

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

}

class ContentItem {

    private List<String> subjects = new ArrayList<String>();

    public ContentItem(String string) {
        subjects.add(string);
    }

    public List<String> getSubjects() {
        return subjects;
    }

    public void setSubjects(List<String> subjects) {
        this.subjects = subjects;
    }

}