Collector groupingby not to return entire Object

August 8, 2022

I have FootTeam object .I am trying to find which teams belongs to Which League.

Expected O/p is

> EPL : { MAN_UTD,MAN_CITY,CHELSEA} BUNDESLIGA : {
> BAYERN,DORTMUND,STUTTGART}

=============

public class FootBallTeam {
    
    private String name; //team name
    
    private String league;

Below code works fine but it gives me the complete FootBallTeam object.I just want the name of FootTeams i.e FootBallTeam.name

Map<String, List<FootBallTeam>> total = list
        .stream()
        .collect(
                Collectors.groupingBy(
                        FootBallTeam::getLeague
                        ));

Sample objetcs in the list. EPL and BBUNDESLIGA are 2 leagues

FootBallTeam f1 = new FootBallTeam("MAN_UTD", "EPL");
        FootBallTeam f2= new FootBallTeam("CHELSEA", "EPL");
        FootBallTeam f3 = new FootBallTeam("MAN_CITY", "EPL");
        
        FootBallTeam f4 = new FootBallTeam("BAYERN", "BUNDESLIGA");
        FootBallTeam f5 = new FootBallTeam("DORTMUND", "BUNDESLIGA");
        FootBallTeam f6 = new FootBallTeam("STUTTGART", "BUNDESLIGA")

>Solution :

I just want the names of FootTeams i.e. FootBallTeam.name

You need to apply additional collector mapping() as a downstream collector of groupingBy()

Map<String, List<String>> teamNamesByLeague = list.stream()
    .collect(Collectors.groupingBy(
        FootBallTeam::getLeague,
        Collectors.mapping(FootBallTeam::getName,
            Collectors.toList())
    ));