How to convert set to map with a set as the value in java 8?

Question

I have the following class:

class A {
   private String id;
   private String name;
   private String systemid;
}

I'm getting a set of A and want to convert it to a map where the key is the system id, and the value is set of A. (Map<String, Set<A>) There can be multiple A instances with the same systemid.

Can't seem to figure out how to do it.. got till here but the identity is clearly not right

Map<String, Set<A>> sysUidToAMap = mySet.stream().collect(Collectors.toMap(A::getSystemID, Function.identity()));

can you please assist?

see here, this may help you https://stackoverflow.com/questions/16108734/convert-setmap-entryk-v-to-hashmapk-v — Sagar Gautam, Jul 20 '17 at 07:14
I want to ask one thing. As you said, you have a `Set` and you want to mapped it `Map>`, when you will mapped it, it will contain one value per key, Map is useful i think.. — Subhabrata Mondal, Jul 20 '17 at 07:17

Eran · Answer 1 · 2017-07-20T08:35:49.130

12

You can use groupingBy instead of toMap:

Map<String, Set<A>> sysUidToAMap =  
    mySet.stream()
         .collect(Collectors.groupingBy(A::getSystemID,
                                        Collectors.toSet()));

edited Jul 20 '17 at 08:35

answered Jul 20 '17 at 07:14

Eran

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If you don't really need `Set` for the groups, but can live with `List `, you can leave off the `mapping` stuff and it becomes much more concise. https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#groupingBy-java.util.function.Function- – Thilo Jul 20 '17 at 07:17
1

@Thilo That's true. I only included the mapping to a Set because that was the desired output in the question. – Eran Jul 20 '17 at 07:19
@Eran I want to ask one thing. we have a Set and we want to mapped it Map>, when you will mapped it, it will contain one value per key, Map is useful right?? – Subhabrata Mondal Jul 20 '17 at 07:20
1

@SubhabrataMondal: No. There could be multiple entries for the same systemid (we don't know what that is). But if there is "business logic" that makes sure that every systemId appears only once, then, yes, a `Map` would be more appropriate (and the conversion code should report unexpected duplicates) – Thilo Jul 20 '17 at 07:21
if my first Set contains 10 elements of A where 5 of them have sysyemid = "1" and the other 5 have systemid="b", then I wanted the map to contain 2 keys, where each value has a set of 5 elements.. so it's not one value per key – user1386966 Jul 20 '17 at 08:58

Eugene · Answer 2 · 2017-07-20T11:06:56.007

3

my 2¢: you can do it with Collectors.toMap but it's slightly more verbose:

      yourSet
      .stream()
            .collect(Collectors.toMap(
                    A::getSystemId,
                    a -> {
                        Set<A> set = new HashSet<>();
                        set.add(a);
                        return set;
                    }, (left, right) -> {
                        left.addAll(right);
                        return left;
                    }));

edited Jul 20 '17 at 11:06

answered Jul 20 '17 at 08:46

Eugene

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First, up-vote. what does `0.02$` mean? I have seen you say it several times. – holi-java Jul 20 '17 at 09:10
2

@holi-java https://en.wikipedia.org/wiki/My_two_cents it sort of mean that this is a very small addition to what has already been said – Eugene Jul 20 '17 at 09:16
1

And for whatever reason, people like to write `0.02$` instead of `2¢` and sometimes, they even omit the preceding “my”… – Holger Jul 20 '17 at 10:20

score 2 · Answer 3 · answered Jul 20 '17 at 18:01

2

As streams were not specifically requested, here's a way to do it only with the Map API:

Map<String, Set<A>> map = new HashMap<>();
mySet.forEach(a -> map.computeIfAbsent(a.getSystemId(), k -> new HashSet<>()).add(a));

answered Jul 20 '17 at 18:01

fps

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How to convert set to map with a set as the value in java 8?

3 Answers3