Generating Map from Map of Maps using Lambda in Java 8

Question

I have a Map of Maps in the format

Map < Integer, Map < String, Integer >>

If I have data in the format of

<1>, << X, 11 >, < Y, 13 >, < Z, 15 >>
<2>, << X,12 >, < A, 23 >, < L, 41 >>

How to process this using Java 8 lambda and generate the following

<1>, <[11, 13, 15]>
<2>, <[12, 23, 41]>

That is generate another Map where the key is the key of the outer map and the value is a List of the values of the inner Map. I know how to do this in the regular way but I am looking at how this using Java 8 Lambda.

Do you mean using Streams with lambdas? If not, what did you do "the regular way" and how do you hope to convert it to lambdas? — Zircon
– Zircon, Commented Jan 27, 2017 at 14:35
Java-8 has not only introduce Lambda. Stream and Method Reference can help you to this problem. — Darshit
– Darshit, Commented Jan 27, 2017 at 14:40

Aaron · Accepted Answer · 2017-01-27 14:49:21Z

6

You can use Collectors.toMap to recreate a Map<Integer, List<Integer>> :

input.entrySet().stream()
                .collect(Collectors.toMap(e->e.getKey(),
                                          e->new ArrayList(e.getValue().values())));

In this piece of code, we collect the Entry<Integer, Map<String, Integer>> into a Map<Integer, List<Integer>> by transforming them so that the value becomes the List<Integer> of values of the inner map, while the key stays the same.

You can test it here.

edited Jan 27, 2017 at 14:49

answered Jan 27, 2017 at 14:41

Aaron

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1 Comment

aquitted-mind Over a year ago

Thank you. This is what I was looking for.

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