Aggregation

// ========== Aggregation: Team has Players ==========

// Part class - exists independently of Team
class Player {
    private String name;
    private int jerseyNumber;

    public Player(String name, int jerseyNumber) {
        this.name = name;
        this.jerseyNumber = jerseyNumber;
    }

    public String getName() { return name; }
    public int getJerseyNumber() { return jerseyNumber; }

    public void play() {
        System.out.println(name + " (#" + jerseyNumber + ") is playing");
    }
}

// Whole class - has Players but doesn't own them
class Team {
    private String name;
    private List<Player> players;  // Aggregation - Team has Players

    public Team(String name) {
        this.name = name;
        this.players = new ArrayList<>();
    }

    // Players are PASSED IN, not created by Team
    public void addPlayer(Player player) {
        players.add(player);
        System.out.println(player.getName() + " joined " + name);
    }

    public void removePlayer(Player player) {
        players.remove(player);
        System.out.println(player.getName() + " left " + name);
    }

    public void playMatch() {
        System.out.println(name + " is playing a match!");
        for (Player player : players) {
            player.play();
        }
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        // Players exist independently
        Player messi = new Player("Messi", 10);
        Player ronaldo = new Player("Ronaldo", 7);

        // Team aggregates (references) players
        Team barcelona = new Team("Barcelona");
        barcelona.addPlayer(messi);
        barcelona.playMatch();

        // Team is destroyed, but players still exist
        barcelona = null;
        
        // Messi can join another team
        Team psg = new Team("PSG");
        psg.addPlayer(messi);  // Messi still exists!
        psg.addPlayer(ronaldo);
        psg.playMatch();
    }
}