Map List of objects to Map by

Question

I want to group list of users by id() using Java Streams.

For example, I have List: new User(1L, "First"), new User(2L, "Second").

How can I group this list to get Map<Long, User>?

1L -> new User(1L, "First"),
2L -> new User(2L, "Second")

User.java

public final class User {
    final Long id;
    final String name;

    public User(final Long id, final String name) {
        this.id = id;
        this.name = name;
    }

    public Long id() {
        return this.id;
    }

    public String name() {
        return this.name;
    }
}

do you plan to have multiple users with the same ID and if yes, how would you merge them? take the first/last one in encounter order or collect them to a `List`? — Eugene, Jan 17 '18 at 11:07
in this case the answer from Eran (or the duplicate) will do. — Eugene, Jan 17 '18 at 11:42

score 2 · Accepted Answer · answered Jan 17 '18 at 11:03

2

If each ID is mapped to a single User, use Collectors.toMap:

Map<Long,User> users = list.stream().collect(Collectors.toMap(User::id,Function.identity()));

answered Jan 17 '18 at 11:03

Eran

387,369
54
702
768

Map List of objects to Map by

1 Answers1