import java.util.ArrayList;

public class Klasse{

    private ArrayList<Elev> elever;

    public Klasse(){
        elever = new ArrayList<>();
        addStudent();

    }

    public void addStudent(){
        elever.add(new Elev("Beate", 20));
        elever.add(new Elev("7eleven", 23));
        elever.add(new Elev("Daequan", 14));
        elever.add(new Elev("P.hd. Jeppe", 22));
        elever.add(new Elev("Freddy Flintstone", 22));
    }

    public void birthday(){
        for(Elev e : elever){
            e.birthday();
        }
    }

    public void udskrivAlle(){
        for(Elev e : elever){
            System.out.println(e.getName());
        }

    }

}

_______________________________________
(Elev klassen)

public class Elev{
    String name;
    int age;

    public Elev(){
        name = "superElev";
        age = 21;
    }

    public Elev(String name, int age){

    }

    public void birthday(){
        age++;
    }
    public String getName(){
        return name;
    }
}