How to initialize to “intertwined” objects in Java at the same time?

Question

I'm currently working on a Java school project aimed at linking a Java application to a database. We have to use the typical example of a school modelisation, composed of 3 classes: -Teacher -Student -Subject

Each class a it's number of arguments, but the problem that I have is that there is an object of the Subject class in a Teacher (called speciality), and an object of the Teacher class in a Subject(called inCharge), knowing that a Subject is the speciality of is inCharge Teacher.

I have to initialize an ArrayList of Teacher, and I'm trying to do it as follows:

while (rset.next()){
    Prof **toInsert** = new Teacher(rset.getInt("NUM_PROF"),
                             rset.getString("NOM_PROF"),
                             rset.getString("PRENOM_PROF"),
                             rset.getString("ADR_PROF"),
                             rset.getString("CP_PROF"),
                             rset.getString("VILLE_PROF"),
                             **new Subject**(rset.getString("CODE"),
                                        rset.getString("LIBELLE"),
                                        rset.getInt("H_COURS-PREV"),
                                        rset.getInt("H_COURS_REA"),
                                        rset.getInt("H_TP_PREV"),
                                        rset.getInt("H_TP_REA"),
                                        rset.getString("DISCIPLINE"),
                                        rset.getInt("COEFF_CC"),
                                        rset.getInt("COEFF_TEST"),
                                        **toInsert**);
    ListeProf.add(i, toInsert);
    ++i;
}

My Teacher class constructor looks like this:

public Teacher(..., Subject speciality){
    ...
    m_speciality = speciality;
}

My Subject class constructor looks like this:

public Subject(..., Teacher inCharge){
    ...
    m_inCharge = inCharge;
}

As you can see, my problem here is that the constructor of the Subject I want to create to be the speciality of the toInsert Teacher needs to use this said Teacher. Is there a way to do both initialization of the objects at the same time, or should I create both objects separately, then bind them to each other using setters?

Sorry for French parts in the code, I tried to translate only the part where help is needed, and thanks in advance for yout answers ^^

Answer 1

Cyclical dependencies between instances should be avoided - not least because you can't actually create the instances, as you have found.

If you really need to have the teacher as a property of the subject, one way you can do it is to have a setter on the Subject class:

  while (rset.next()) {
    Subject subject = new Subject(... /* don't specify Teacher as parameter */);

    Teacher teacher = new Teacher(..., subject);

    subject.setTeacher(teacher);
  }

But you should revisit your design to see if you can get rid of them.

The thing which is jumping out here is that you are making the Teacher a property of the Subject . This doesn't seem correct to me: for one thing, lots of teachers might teach the same subject. (And each teacher might teach multiple subjects, of course).

The relationship that you're trying to model here looks like:

• Teacher teaches Subject
• Subject is taught by Teacher

I would construct the "is taught by" relationship as something outside the Teacher and Subject classes, for example:

// If only one teacher teaches a subject.
Map<Subject, Teacher> isTaughtBy;

So then you do something like:

  Map<Subject, Teacher> isTaughtBy;
  while (rset.next()) {
    Subject subject = new Subject(... /* don't specify Teacher as parameter */);

    Teacher teacher = new Teacher(..., subject);

    isTaughtBy.put(subject, teacher);
  }