How to convert Object list into a map of <String, List<String>>?

Question

I have a list of users

Collection<User> userList = groupManager.getUserList(groupName);

Not knowing the Java-Streams API that well, I want to map them into a map as follows: the key would be the user key, and the value would be a list the User object's other properties.

Map<String, List<String>> userDsiplayData = userList.stream()
     .collect(Collectors.toMap(User::getKey, **.......**))

I tried to replace **..... ** with Arrays.asList(User::getUsername(), User::getDisplayName(), User::getEmail()) or similar constructs but I can't seem to get it to work.

Andrew Vershinin · Accepted Answer · 2021-02-01 12:32:24Z

6

Just use a lambda:

Map<String, List<String>> userDsiplayData = userList.stream()
     .collect(Collectors.toMap(User::getKey, 
         user -> List.of(user.getUsername(), user.getDisplayName(), user.getEmail()))

And by the way, your Arrays.asList(User::getUsername(), User::getDisplayName(), User::getEmail()) piece is wrong - you try to return a list of method handles, rather than a mapping function (besides the fact that it's not syntactically correct - parentheses do not belong there).

edited Feb 1, 2021 at 12:32

answered Feb 1, 2021 at 9:00

Andrew Vershinin

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1 Comment

Mark Rotteveel Over a year ago

With Java 9 or higher, you can also use List.of(...) instead of Arrays.asList(..), with the added benefit that the list is immutable (instead of only fixed size).

Youcef LAIDANI · Accepted Answer · 2021-02-01 10:06:13Z

2

In your case the collect should be like this:

.collect(Collectors.toMap(
        User::getKey, 
        u -> Arrays.asList(u.getUsername(), u.getDisplayName(), u.getEmail())));

You can also create a custom method which return the expected List of fields in User class:

public class User {
    private String key;
    private String username;
    private String displayName;
    private String email;

    // getters setters

    public List<String> getUserInfo() {
        return Arrays.asList(username, displayName, email)));
    }
}

and then you can call it by using method reference like this:

.collect(Collectors.toMap(User::getKey, User::getUserInfo));

edited Feb 1, 2021 at 10:06

answered Feb 1, 2021 at 9:06

Youcef LAIDANI

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