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 Mar 14 '17 at 14:10 — Aswin Alagappan, Mar 14 '17 at 14:10

score 1 · Answer 1 · answered Mar 14 '17 at 14:16

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 '17 at 14:16

assylias

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I was trying to avoid the second stream. Is it possible? – Aswin Alagappan Mar 14 '17 at 14:25
And the map function can also be replaced with `toMap(a -> a.id, identity())` – Aswin Alagappan Mar 14 '17 at 14:26
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. – Aswin Alagappan Mar 14 '17 at 14:39

Eugene · Answer 2 · 2017-03-14T14:49:04.273

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 '17 at 14:49

answered Mar 14 '17 at 14:37

Eugene

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Yeah. This would be perfect, but have to live with Java8 :/ – Aswin Alagappan Mar 14 '17 at 14:40
1

@AswinAlagappan can you bring an additional library like `StreamEx` for example? You might need it for a lot more then just this example... – Eugene Mar 14 '17 at 14:48
3

@Aswin Alagappan: You don’t have to wait for Java 9 to use `flatMapping`. At the end of [this answer](http://stackoverflow.com/a/39131049/2711488) 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… – Holger Mar 14 '17 at 16:18
@Eugene: I will try using it. But Java 8 streams itself seems to be slow, is that the same with StreamEx? – Aswin Alagappan Mar 14 '17 at 17:19
@Holger: Cool !! – Aswin Alagappan Mar 14 '17 at 17:23

score 0 · Accepted Answer · answered Mar 14 '17 at 14:45

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())))));

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

3 Answers3