Convert list of Object to Map<Key, List<Object>> with java 8 stream

Question

So I have a list of users with several information including company uid, this value can be found in several users, as it defines to what company the user belongs to. I need to create a Map of userList where the map key is company uid. I have managed to do so using this code:

HashMap<String, List<LDAPUser>> allUsers = new HashMap<>();
    userService.findAllUsers().forEach(u -> {
        Optional.ofNullable(allUsers.putIfAbsent(u.getCompanyUid(),
                new ArrayList<>(Collections.singletonList(u))))
                .ifPresent(list -> list.add(u));
    });

Even if it works fine, I think there must be a cleaner approach using flatMap, map or collect method from stream, but I can't get it to work basically because I don't see how I can create a list containing all users.

Answer 1

There's a collector to group things, consider the following example

Application.java

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Application {

    private static class User {

        private final String name;

        private final Integer companyId;

        public User(String name, Integer companyId) {
            this.name = name;
            this.companyId = companyId;
        }

        public String getName() {
            return name;
        }

        public Integer getCompanyId() {
            return companyId;
        }

        @Override
        public String toString() {
            return "User{" +
                    "name='" + name + '\'' +
                    ", companyId=" + companyId +
                    '}';
        }
    }

    public static void main(String[] args) {
        final List<User> users = Arrays.asList(new User("A", 1), new User("B", 1), new User("C", 2));
        final Map<Integer, List<User>> byCompanyId = users.stream()
                .collect(Collectors.groupingBy(User::getCompanyId));
        System.out.println(byCompanyId);
    }
}

that will print

{1=[User{name='A', companyId=1}, User{name='B', companyId=1}], 2=[User{name='C', companyId=2}]}