Object reference case study

Question

When person 1 become partner with person 3, person 2 should no longer have person 1 as partner and person 4 should no longer have person 3 as partner. How should I solve this?

public class Person {

    private String name;
    private Person partner;

    public Person(String name){
        this.name = name;
    }

    public void setPartner(Person partner){
        this.partner = partner;
        partner.partner = this; 
    }

    public static void main(String[] args) {

        Person one = new Person("1");           
        Person two = new Person("2");
        Person three = new Person("3");
        Person four = new Person("4");

        one.setPartner(two);
        three.setPartner(four);

        one.setPartner(three);

        //Person two is still partner with person 1
        //and person four is still partner with person 3 
   }

Answer 1

I think putting this as the first line in setPartner should work: this.partner.partner = null;

Of course you must check if this.partner is null or not.

Answer 2

public void setPartner(Person partner){
    if (this.partner != null) {
        this.partner.partner = null; // Reset the partner of the old partner.
    }
    this.partner = partner;          // Assign new partner.
    this.partner.partner = this;     // Set the partner of the new partner.
}

Answer 3

change setPartner to:

public void setPartner(Person partner){
    if(this.partner != null)
        this.partner.partner = null;
    this.partner = partner;
    partner.partner = this; 
}

Answer 4

I would suggest that you set up a setRelationship method, which would action setPartner on the current Person, and action a new removePartner on the old partner, if not null.

The new setRelationship method would be in place so that there would be no confusion as to what setPartner does - there would be no side-effects that might be missed by the unsuspecting programmer.

Answer 5

Here is my Code:

public void setPartner(Person partner) {
    if (this.partner != null)
       this.partner.partner = null;
    this.partner = partner;
    if (partner.partner != null)
       partner.partner.partner = null;
    partner.partner = this;
}

Answer 6

public void setPartner(Partner b) {
  // Special case, otherwise we'll have troubles
  // when this.partner is already b.
  if (this.partner == b) return;

  if (this.partner != null) {
    this.partner.partner = null;
  }

  this.partner = b;

  // Make sure that the new partner has the right partner.
  // This will make sure the original b.partner has its
  // partner field nullified.
  // Note that if we don't have the special case above,
  // this will be an infinite recursion.
  b.setPartner(this);
}

Answer 7

class Person {

  String name;
  Partnership partnership;
  void setPartnership(Partnership p) {
    partnership=p;
  }
}
class Partnership {
  Person partner1;
  Person partner2;
  public setPartners(Person p1,Person p2) {
    p1.setPartnership(this);
    p2.setPartnership(this);
}

Ideally you would want a way to prevent setPartnership being called from anywhere other than Partnership.