How do I convert a List<Entry> to Map where value is a list using streams?

Question

How do I convert a List<Entry> to Map<Entry::getKey, List<Entry::getValue>> using streams in Java 8?

I couldn't come up with a good KeySelector for Collectors.toMap():

List<Entry<Integer, String>> list = Arrays.asList(Entry.newEntry(1, "a"), Entry.newEntry(2, "b"), Entry.newEntry(1, "c"));
        Map<Integer, List<String>> map = list.stream().collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));

What I want to get: {'1': ["a", "c"], '2': ["b"]}.

Are you sure you don't have any duplicate keys in your entry List? — Tripp Kinetics
– Tripp Kinetics, Commented Jun 8, 2018 at 14:25
@TrippKinetics there are two entries which have their keys as 1 — Ousmane D.
– Ousmane D., Commented Jun 8, 2018 at 14:28
@Aominè That appears to be one entry with the key of 1 and the value of ["a", "c"]. What I'm talking about would be something like {'1': ["a", "c"], '1': ["x", "z"] ... } — Tripp Kinetics
– Tripp Kinetics, Commented Jun 8, 2018 at 14:35

Ousmane D. · Accepted Answer · 2018-06-08 14:28:07Z

10

You can do so by using the groupingBy collector along with mapping as the downstream collector:

myList.stream()
       .collect(groupingBy(e -> e.getKey(), mapping(e -> e.getValue(), toList())));

import required:

import static java.util.stream.Collectors.*;

You can actually accomplish the same result with the toMap collector:

 myList.stream()
       .collect(toMap(e -> e.getKey(), 
                 v -> new ArrayList<>(Collections.singletonList(v.getValue())),
              (left, right) -> {left.addAll(right); return left;}));

but it's not ideal when you can use the groupingBy collector and it's less readable IMO.

edited Jun 8, 2018 at 14:28

answered Jun 8, 2018 at 14:21

Ousmane D.

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2 Comments

Alex Kornakov Over a year ago

How can I avoid static import? There's an error about lambdas if I write

.collect(                         Collectors.groupingBy(e -> e.getKey(), Collectors.mapping(e -> e.getValue(), Collectors.toList())));                 )

Alex Kornakov Over a year ago

IDEA loses the type of e, it says it's just an Object. Actually fixed it by retyping somehow, thanks.

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