Question

I have four classes. Class Person, and three more, Student, Professor, Tutor, each of which extends class Person. Class Person has 2 variables studentID and staffID. However, only student can have studentID != null, and Tutors and Professors can have staffID != null. Now when creating new object Student, how can I make sure that no matter what staffID always stays null? staffID must remain in class Person, so no moving it around.

Answer 1

Create interfaces to restrict operations:

package main;

public interface IsStaff {

    public void setStaffId(Integer staffId);

    public Integer getStaffId();

}



package main;

public interface IsStudent {

    public void setStudentId(Integer studentId);

    public Integer getStudentId();

}

Create your Person class:

package main;

public class Person {

    protected Integer studentId = null;

    protected Integer staffId = null;

}

Create your Student and Staff subclasses with the Interfaces to define the only allowed operations:

package main;

public class Staff extends Person implements IsStaff {

    @Override
    public void setStaffId(Integer staffId) {
        this.staffId = staffId;
    }

    @Override
    public Integer getStaffId() {
        return staffId;
    }

}



package main;

public class Student extends Person implements IsStudent {

    @Override
    public void setStudentId(Integer studentId) {
        this.studentId = studentId;
    }

    @Override
    public Integer getStudentId() {
        return studentId;
    }

}

And now create your Tutors and Professors:

package main;

public class Tutor extends Staff {

}


package main;

public class Professor extends Staff {

}

Objects of the Student class don’t have operations that can affect the staffId, and objects of the Staff class (including the Tutor and Professor subclasses) don’t have operations that can affect the studentId. Add other operations as necessary (common ones can go into Person directly).

As a bonus, you can use the interfaces to better define methods, like this:

public void assignParkingSpaceTo(IsStaff staffMember);

public void issueLateSlipTo(IsStudent student);