Java 8: how to 'JOIN' two Maps having the same Keys?

Question

I have two Maps, both sharing the same keys, .

Map<Long/*JOIN.ID*/, Long/*Temp ID*/> tempIDsMap;
Map<Long/*JOIN.ID*/, Long/*Real ID*/> realIDsMap;

What I want to get (maybe using Java 8 Stream API and avoiding loops) is the JOIN of these maps on the JOIN.ID keys, to get a new Map like below:

Map<Long/*Temp ID*/. Long/*Real ID*/> realIDsByTempMap;

I don't know of any given framework to do that for you, so you have to create your own logic to complete this task. What have you tried so far? Can you provide the code, you have tried? Can you point out specific issues or problems? — Korashen, Aug 08 '18 at 08:28
what do you mean by join by key? your maps are `Map`. So which value is to select in final map(as value can be different for same key)? Do you want to create `Map>` ? — Shubhendu Pramanik, Aug 08 '18 at 08:38

score 4 · Accepted Answer · answered Aug 08 '18 at 08:29

4

Use Collectors.toMap:

Map<Long,Long> realIDsByTempMap = 
    tempIDsMap.entrySet()
              .stream()
              .collect(Collectors.toMap(Map.Entry::getValue,
                       e -> realIDsMap.get(e.getKey())));

answered Aug 08 '18 at 08:29

Eran

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1

Nice, but I bet there is a duplicate for that somewhere. I am just too lazy searching today ;-) – GhostCat Aug 08 '18 at 08:34
1

Here it is: https://stackoverflow.com/a/24018167/4390212 – DimaSan Aug 08 '18 at 08:39
1

@DimaSan Here we close ;-) – GhostCat Aug 08 '18 at 08:47

Java 8: how to 'JOIN' two Maps having the same Keys?

1 Answers1