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

Issue

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.

Solution

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()));

Answered By - Eklavya
Answer Checked By - Cary Denson (JavaFixing Admin)