How to figure out what object is in a abstract list?

user11749216 :

I have an abstract class called sessions. Lectures and tutorials extend sessions. Then I have a class called enrollment which holds a list of sessions (Lectures & tutorials). How can I loop through the session list in Enrolment and return a list of Lectures only from the session list?

My next question is should I instead store 2 lists. One list of Lectures and one list of Tutorials, instead of 1 session list? This is because the sessions list is useless to me and I have to loop through it each time to get information about lectures and tutorials. Is there a way I am missing to get all the lectures objects? I am new to java.

public class Enrolment {

    private List<Session> sessions;

    public Enrolment() {
        this.sessions = new ArrayList<>();
    }

    public addSession(Session session) {
        this.sessions.add(session);
    }
}

---

public class Session {

    private int time;

    public Session(int time) {
        this.time = time;
    }
}

---

public class Lecture extends Session {

    private String lecturer;

    public Lecture(int time, String lecturer) {
        super(time);
        this.lecturer = lecturer;
    }
}

---

public class Tutorial extends Session {

    private String tutor;
    private int tutorScore;

    public Tutorial(int time, String tutor, int tutorScore) {
        super(time);
        this.tutor = tutor;
        this.tutorScore = tutorScore;
    }

}

---

public class test {
    public static void main(String[] args) {
        Enrolment newEnrolment = new Enrolment();

        Lecture morningLec = new Lecture(900, "Dr. Mike");
        newEnrolment.addSession(morningLec);

        Tutorial afternoonTut = new Tutorial(1400, "John Smith", 3);
        newEnrolment.addSession(afternoonTut);

        Lecture middayLec = new Lecture(1200, "Mr. Micheals");
        newEnrolment.addSession(middayLec);

        Tutorial NightTut = new Tutorial(1900, "Harry Pauls", 4);
        newEnrolment.addSession(NightTut);
    }
}

Deadpool :

Stream the sessions list and use instanceof to filter the Lectures type objects

List<Lecture> l = sessions.stream()
                           .filter(Lecture.class::isInstance) 
                           .map(Lecture.class::cast)                               
                           .collect(Collectors.toList());

By using for loop use two different lists for each type

List<Lecture> l = new ArrayList<>();
List<Tutorial> t = new ArrayList<>();
for (Session s : sessions) {
    if (s instanceof Lecture) {
        l.add((Lecture) s);
    }
      else if(s instanceof Tutorial) {
        t.add((Tutorial) s);
    }
}