Join 2 maps Java 8 using streams

Question

I have 2 Maps

Map<A, B> mapA
Map<B, List<C>> mapB

I want to join these maps on the values in mapA & keys in mapB the result should be

Map<A,List<C>> mapC

I am willing to know how can I do it using streams in Java8.

A,B,C for simplicty, all of these are strings in my case.

Have you tried anything? Read the documentation? What concrete problem did you face? Regardless of the syntax, what would be your strategy to achieve that? — JB Nizet
– JB Nizet, Commented Oct 25, 2018 at 20:39
map.entrySet().stream().collect(Collectors.toMap(Entry::getKey, e -> mapB.get(e.getValue()))) — shmosel
– shmosel, Commented Oct 25, 2018 at 20:41
@BillK I think your solution will neither compile, nor it is what the OP asked — Kartik
– Kartik, Commented Oct 25, 2018 at 23:33
@BillK dont think so your solution will work. I am trying to create a 3rd map from values of A matching keys of B — ayush sanghvi
– ayush sanghvi, Commented Oct 26, 2018 at 0:25
@BillK I completely agree with what you said regarding streams. But your code still won't compile. (1) addAll is in Collection interface. Map has putAll instead. (2) You are trying to add all Map<A, B> to Map<A,List<C>>, which won't work! — Kartik
– Kartik, Commented Oct 26, 2018 at 0:26

uneq95 · Accepted Answer · 2018-10-26 04:32:43Z

4

You can iterate over the map and easily construct the new map.

Map<A,List<C>> mapC = new HashMap<>();

mapA.forEach((key,value)->mapC.put(key, mapB.get(value)));

You can use this link, which compares the efficiency of different ways to iterate over the key-value pairs, to select which method you want to use.

edited Oct 26, 2018 at 4:32

answered Oct 25, 2018 at 20:50

uneq95

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Comments

Kartik · Accepted Answer · 2018-10-26 02:48:34Z

3

You could do it like this:

mapC = mapA.entrySet()
        .stream()
        .collect(Collectors.toMap(Map.Entry::getKey, e -> mapB.get(e.getValue())));

edited Oct 26, 2018 at 2:48

user10367961

answered Oct 26, 2018 at 0:47

Kartik

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6 Comments

uneq95 Over a year ago

Why create an entry set, then make a stream out of it and then finally collect them, if you cam just use Map.forEach which directly gives you the key value pair. It makes it easier and simpler to understand.

Kartik Over a year ago

@uneq95 Stream operations don't work in the sequence as you might think they do. Performance, memory consumption and readability are similar to those of your solution, if not better.

uneq95 Over a year ago

Yes, I know that it doesn't work in sequence. Neither do we require the sequence to be maintained here, as we just want to insert in a hashmap, which too doesn't maintain the sequence. I was arguing on the creation of entry set in the first place.

Kartik Over a year ago

@uneq95 I was talking about the sequence of stream operations, like conversion to stream, running filter, map, etc. and then collecting. I was not talking about the ordering of elements in the result. Try asking a new question to see which solution is better and you'll get the answer that they are similar.

shmosel Over a year ago

@uneq95 There's no "creation" of an entry set. It's just a thin view to the underlying map.

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