Java, is it possible to 'convert' object from subclass to object from superclass

Question

I have two classes Student and Tutor. Tutor is basically a student (Tutor extends Student) who has facultyID. Once his contract is complete, he returns to being just a student. So can I somehow convert him back to his "previous" roll of student?

Answer 1

What you really want to do here is use composition and not inheritance. Keep all your objects as type Student, and then temporarily assign the behaviour of a TutorRole as it is required to each instance of Student.

With this design your Student class will contain a property (member variable) of type TutorRole that you can add or remove at runtime. Adding an isTutor() method will allow you to detemine whether a Student is a Tutor at runtime in a clear and concise manner.

The TutorRole class will encapsulate the behaviour (i.e. methods) of being a Tutor.

/*
 * The TutorRole can be set at runtime
 */
public class Student {

    private String facultyId;

    private TutorRole tutorRole = null;

    public boolean isTutor() {
        return !(tutorRole == null);
    }

    public void doTutorStuff() {
        if(isTutor()) {
            tutorRole.doTutorStuff();
        }
        else {
            throw new NotTutorException();
        }
    }

    public void setTutorRole(TutorRole tutorRole) {
        this.tutorRole = tutorRole;
    }
}

/*
 * Ideally this class should implement a generic interface, but I'll keep this simple
 */
public class TutorRole {

    public void doTutorStuff() {
        // implementation here
    }
}

/*
 * Now let's use our classes...
 */
Student st = new Student(); // not a tutor
st.setTutorRole(new TutorRole()); // now a tutor
if(st.isTutor()) {
    st.doTutorStuff();
}
st.setTutorRole(null); // not a tutor anymore

An alternative approach is to have a Tutor class contain a reference to a Student object, but it depends on how you are going to be interacting with the Student and Tutor objects on which way around you want to code this.