Java Lambda Stream List to Map>

Question

I have to write a method which takes a List<User> and returns a Map<Role,List<User>>.

User contains a Person instance, which contains a Role.
I must create a map where Role key is mapped to a list of users (where user has this Role).

I tried the following :

public static Map<Role, List<User>> groupUsersByRole(List<User> users) {
    users.stream().collect(Collectors.toMap((u -> u.getPersonDetails().getRole()), users.stream().collect(Collectors.toList()  ) )
    users.stream().map(user -> user).filter(u -> u.).collect(Collectors.toList()) );
    //users.stream().filter(u -> u.getPersonDetails().getRole().getName().equals(u.getPersonDetails().getRole().getName())).map(user -> user.getName()).
    return null;
}

Do you really _have_ to use lambdas for that? Good old fashioned code will work and will be more legible... — fge, Dec 10 '15 at 11:56
@fge I disagree, `groupingBy` is largely more readable than the code you would write before Java 8.. — Alexis C., Dec 10 '15 at 12:04

score 0 · Accepted Answer · answered Dec 10 '15 at 11:57

0

What you are looking for is a Collectors.groupingBy, which groups the elements of the Stream by a specified property :

Map<Role, List<User>> map = users.stream().collect(Collectors.groupingBy(u->u.getPersonDetails().getRole()));

answered Dec 10 '15 at 11:57

Eran

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Java Lambda Stream List to Map>

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