class User {
    public Integer id;
    public String username;
    
    public User(Integer id, String username) {
        this.id = id;
        this.username = username;
    }
}

public static void main (String[] args) {
    List<User> userList = new ArrayList<>();
    userList.add(new User(1, "admin"));
    userList.add(new User(2, "guest"));
    userList.add(new User(3, null));

    Map<Integer, String> idsUsernamesMap = 
        userList.stream().collect(Collectors.toMap(user -> user.id, user -> user.username));
                
    idsUsernamesMap.forEach( (k,v) -> System.out.println("id: " + k + "; username: " + v));
}