How to collect object using Collectors.toMap if the key is inside a list of child property?

Question

class MyObject {
  private List<MyChildObject> children;
}
class MyChildObject {
  private String key;
}

My goal is to transfor a list of MyObject to a Map<String, MyObject> which String is MyChildObject.key.

My attempt stopped at myObjectList.stream().collect(Collectors.toMap(//how to extract key here?, Function.identity()));

Thanks.

Looks strange to my, because `MyObject` contains a list children where each child has it's own `key`. — Oleg Cherednik, Oct 08 '20 at 20:09
Which key of the list? The first, the last, an arbitrary one, a random one? What happens if the list is empty? — knittl, Oct 08 '20 at 20:09
I need to map the parent object to each of its children's key. — LunaticJape, Oct 08 '20 at 20:47

Eklavya · Accepted Answer · 2020-10-08T20:30:45.077

4

You can use flatMap to flatten the Children to make Children's key and MyObject pair then collect as Map using Collectors.toMap

myObjectList.stream()
            .flatMap(e -> e.getChildren()
                           .stream()
                           .map(c -> new AbstractMap.SimpleEntry<>(c.getKey(), e)))
            .collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));

edited Oct 08 '20 at 20:30

answered Oct 08 '20 at 20:14

Eklavya

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Great, thanks. I think this is what I was looking for. – LunaticJape Oct 08 '20 at 20:58

How to collect object using Collectors.toMap if the key is inside a list of child property?

1 Answers1