How to do a groupingBy of groupingBy in java8 with a List attribute inside object

Question

I have

class Activity {
    int id;
    ActivityType activityType;
}
class ActivityType {
    int id;
}
class Tournament {
    int id;
    List<Activity> activities;
}

I have a

List<Tournament> tournaments;

And from that I need

Map<Tournament, Map<ActivityType, Map<Integer, Activity>>>

(where the Integer is the activityId)

How do I get that using Java 8?

Yes. Have edited the question.

Aswin Alagappan
– Aswin Alagappan

2017-03-14 14:10:17 +00:00
Commented Mar 14, 2017 at 14:10 — Aswin Alagappan
– Aswin Alagappan, Commented Mar 14, 2017 at 14:10

assylias · Accepted Answer · 2017-03-14 14:16:17Z

1

If I understand correctly, the Map<Integer, Activity> always contains only one element. In which case it looks more like a toMap than a groupingBy. Something like:

Map<Tournament, Map<Activity, Map<Integer, Activity>>> map = tournaments.stream()
               .collect(toMap(t -> t,
                     t -> t.getActivities().stream().collect(toMap(a -> a, a -> map(a)))));

Using this helper method:

private static Map<Integer, Activity> map(Activity a) {
  Map<Integer, Activity> m = new HashMap<> ();
  m.put(a.getId(), a);
  return m;
}

answered Mar 14, 2017 at 14:16

assylias

330k84 gold badges680 silver badges806 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Aswin Alagappan Over a year ago

I was trying to avoid the second stream. Is it possible?

Aswin Alagappan Over a year ago

And the map function can also be replaced with toMap(a -> a.id, identity())

Aswin Alagappan Over a year ago

And you can't use toMap in the second collect method, guess toMap is assuming that there are no duplicate keys, and in this case, there could be same activityType for different activities.

Eugene · Accepted Answer · 2017-03-14 14:49:04Z

1

If you have a way to get the Collectors.flatMapping from jdk-9 (I think it is present in StreamEx library), then it could look like this:

Stream.of(new Tournament()).collect(Collectors.groupingBy(Function.identity(),
            Collectors.flatMapping(t -> t.getActivities().stream(), 
                    Collectors.groupingBy(Activity::getActivityType,
                          Collectors.toMap(Activity::getId, Function.identity()))
                    )));

With StreamEx it could look like this:

 Stream.of(new Tournament()).collect(Collectors.groupingBy(Function.identity(),
            MoreCollectors.flatMapping(t -> t.getActivities().stream(),
                    Collectors.groupingBy(Activity::getActivityType,
                            Collectors.toMap(Activity::getId, Function.identity())))));

edited Mar 14, 2017 at 14:49

answered Mar 14, 2017 at 14:37

Eugene

122k17 gold badges219 silver badges335 bronze badges

5 Comments

Aswin Alagappan Over a year ago

Yeah. This would be perfect, but have to live with Java8 :/

Eugene Over a year ago

@AswinAlagappan can you bring an additional library like StreamEx for example? You might need it for a lot more then just this example...

Holger Over a year ago

@Aswin Alagappan: You don’t have to wait for Java 9 to use flatMapping. At the end of this answer is a Java 8 compatible implementation of the flatMapping collector which you can copy to your code base, as it’s quiet small. And it’s so straight-forward, that it turned out to be identical to Java 9’s implementation…

Aswin Alagappan Over a year ago

@Eugene: I will try using it. But Java 8 streams itself seems to be slow, is that the same with StreamEx?

Aswin Alagappan Over a year ago

@Holger: Cool !!

Aswin Alagappan · Accepted Answer · 2017-03-14 14:45:46Z

0

This was the best I was able to come up with java8.

Map<Tournament, Map<Activity, Map<Integer, Activity>>> map = tournaments.stream()
           .collect(toMap(identity(),
                          t -> t.getActivities().stream()
                               .collect(groupingBy(A::getActivityType(), 
                                        a -> toMap(A::getId, identity())))));

answered Mar 14, 2017 at 14:45

Aswin Alagappan

1731 gold badge1 silver badge11 bronze badges

Collectives™ on Stack Overflow

How to do a groupingBy of groupingBy in java8 with a List attribute inside object

3 Answers 3

3 Comments

5 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

5 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related