CODEWITHSUNDEEP
javacollectionsjava-stream
Maksim Rybalkin
Maksim Rybalkin

Reputation: 277

How to turn a Map<List<K>, List<V>> into a Map<K, List<V>>?

In a part of a code I have the situation, the type of the collection is that I need:

Map<Specialization, List<SpecialistInfo>> specialistsMap = specialistsResponse
    .getSpecialists()
    .stream()
    .collect(groupingBy(SpecialistInfo::getSpecializations));

But actually, that stream returns a Map<List<Specialization>, List<SpecialistInfo>>.

The getSpecialists() method returns List<SpecialistInfo> specialists;and every instance of SpecialistInfo contains a list too. So, the method getSpecializations returns List<Specialization> specializations

I need to group my map into a Map<Specialization, List<SpecialistInfo>>.

How it can be done via Stream API?

Upvotes: 0

Views: 79

Answers (1)

Rubydesic
Rubydesic

Reputation: 3476

So you have a List<SpecialistInfo> and each SpecialistInfo has a List<Specialization>. You need to reverse map these. Presumably, then, Specialization and SpecialistInfo are unique and there exists a many-to-many relation between Specializations and SpecialistInfo.

If you're looking for a solution not using the streams API which is thread-safe (compared to the solution of YCF_L) 

List<SpecialistInfo> specialists = getSpecialists();
Map<Specialization, List<SpecialistInfo>> specMap = new HashMap<>();
for (SpecialistInfo specInfo : specialists) { 
    for (Specialization spec : specInfo.getSpecializations()) {
        specMap.computeIfAbsent(spec, k -> new ArrayList<>()).add(specInfo);
    }
}

Upvotes: 3

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