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);
}
}