Q about UML Distilled: transient relationship?

[Gian Franco]

Hi,

Martin Fowler says the following:

Whether a property is implemented as a field or as calculated value, it represents something an object can always provide. You shouldn�t use a property to model a transient relationship, such as an object that is passed as a parameter during a method call and used only within the confiness of that interaction.

I can�t really understand what he wants to say in the second sentence: what is the problem with transient relationships?

Cheers,

Gian Franco

[Edwin Keeton]

I don't think he's saying there's anything bad about transient relationships. What he means is you don't try to model them (transient relationships) with a property. (He gives the example of an object parameter to a method.) It might look something like this:

ClassA {
public static void main(String[] args) {
    ClassA a = new ClassA();    
    a.doSomething();
    // do some more stuff
    System.out.println("I don't know anything about ClassB"); 
}
 
void doSomething() {
    ClassB b = new ClassB();
    doSomethingWithB(b);
    System.out.println("A");
    // do something else
}
 
void doSomethingWithB(ClassB b) {
        b.doSomething();
    }
}
 
ClassB {
    void doSomething(ClassB b) {
       System.out.println("B");
    }
}
 

    
 
OTOH, you would model an association type of relationship with a property.  For example, ClassA is related to ClassB by assocation:
 
[CODE]ClassA {
    ClassB b;
 
    ClassB getB() {
        if (b == null) {
             b = new ClassB();
        }
        return b;
    }
 
    void setB(ClassB b) {
        this.b = b;
    }
 
    public static void main(String[] args) {
        ClassA a = new ClassA();
        // We can reference 'b' from anywhere in the class.
        a.doSomethingWithB(b);
    }
  

Hope this helps.

Ed

[Axel Janssen]

A class diagram is a static view on the interfaces, classes and the relationships among them (association, implementation and inheritance).
If you put transient relationships in the model, reader of model is overwhelmed with information.
Also transient stuff is inherently dynamic. An instance member is allways "there" as long as the class does exist, though it might be null. A transient variable has a much shorter life-cycle.

Transient stuff gets focus in types of uml models which show dynamic behaviour like interaction diagrams.

[Gian Franco]

Thank you, Ed and Axel, the combination
of your replies makes it all clear

Cheers,

Gian Franco Casula