CODEWITHSUNDEEP
javajava-8groupingjava-streamcollectors
hotkey
hotkey

Reputation: 147901

Java Streams: get values grouped by inner map key

I have Map<A, Map<B, C>> and I want to get Map<B, List<C>> from it using Java Streams.

I try to do it as follows:

public <A, B, C> Map<B, List<C>> groupsByInnerKey(Map<A, Map<B, C>> input) {
    return input.values()
            .stream()
            .flatMap(it -> it.entrySet().stream())
            .collect(Collectors.groupingBy(Map.Entry::getKey));
}

What I expect:

But it doesn't compile, literally:

Non-static method cannot be referenced from a static context

at Map.Entry::getKey in the last line.

Can someone explain what is wrong or what is the right way to achieve what I want?

Upvotes: 15

Views: 6039

Answers (2)

Tunaki
Tunaki

Reputation: 137064

Your Stream is composed of Map.Entry objects but want you want to collect is actually the value of the entry, not the entry itself. With your current code, you would be obtaining a Map<B, List<Map.Entry<B, C>>>.

As such, you are just missing a call to Collectors.mapping. This collector will map the Stream element with the given mapper function and collect that result into the downstream container. In this case, the mapper is Map.Entry::getValue (so returning the value from the map entry) and the downstream collector collects into a List.

public <A, B, C> Map<B, List<C>> groupsByInnerKey(Map<A, Map<B, C>> input) {
    return input.values()
            .stream()
            .flatMap(it -> it.entrySet().stream())
            .collect(Collectors.groupingBy(
                 Map.Entry::getKey,
                 Collectors.mapping(Map.Entry::getValue, Collectors.toList())
            ));
}

Upvotes: 14

Eran
Eran

Reputation: 393771

Your stream pipeline returns a Map<B, List<Map.Entry<B,C>>>, not a Map<B, List<C>>.

To get what a Map<B, List<C>>, you need to add a mapping that would map Map.Entry<B,C> to C :

return input.entrySet()
        .stream()
        .flatMap(it -> it.getValue().entrySet().stream())
        .collect(Collectors.groupingBy(Map.Entry::getKey,Collectors.mapping(Map.Entry::getValue,Collectors.toList())));

Upvotes: 8

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