about object reference ?

[emily li]

i saw a class with the following structure:

class Line {
    Color color;
    Point start;
    Point end;
    ... ...
}
class Point {
    int x, y;
    Line l; // which this point is belonged to
    void aMethod() {
        ... ...
        System.out.println(l.color);
    }
}

in the real application, they are more complicated. but they do have the structure like above: Line has Point as member, and Point has Line as member as well to make some operation easier.

my question is: is this kind of structure good? or proper? any performance, or potential problem there?
it really confuses me... as it looks like a kind of infinite reference: Line -> Point -> Line -> Point -> ...
thanks,

[Christophe Verré]

It all depends on the relationship between your classes. Two classes having a one-to-one relationship may refer to each other. Imagine that a person can only have one dog, and a dog can only have one loving master.

class Person {
	private Dog dog;
}

class Dog { 
	private Person master;
}

A Person called "emily" may have a Dog called "wiz". "emily" knows about "wiz" and "wiz" knows about "emily".

(I'm not discussing whether or not the design of your Point and Line classes is good or not)

[emily li]

thanks for your reply. but i understand "it depends on the relationship between them". maybe my sample is bad.

what i want to know is: is this design correct, or good?

[David Newton]

It's impossible to answer that question out of context. But there are lots of reasons why that might be a reasonable design decision:

class Person {
    List<Dog> dogsThisPersonKnows;
}

class Dog {
    List<Person> peopleThisDogKnows;
}

[Paul Clapham]

We don't know what these Points and Lines are for in your application, nor do we know how they should be related.

Your design makes it so that a Point can only be in one Line. There are lots of situations where that would be wrong -- for example if a Point could be at the intersection of two Lines. But maybe that doesn't apply in your application. We can't tell.

[emily li]

sorry, my bad example.

just forget the line & point. let me say just 2 objects.

class A {
    B aMemberOfA;
    // ... some other members
}
class B {
    A aMemberOfB;
    // ... some other members
}

A & B have some relationship. i know this design make the usage later much much easier. i can just use B.aMemberOfB (while inside B) to access the member/method of class A. but is that a good design?
does aMemberOfB keep the reference of A or a whole copy? will this design cause any performance problem?
i worked on C for a while. i know this kind of structure is not allowed in C (or maybe just hard to implement in C). that's why i comes up this question.

sorry, i didn't make myself clear. hope this help to clarify my question/concern.

thanks,

[Esmaeil Ashrafi]

emily li wrote:...
... but is that a good design?
does aMemberOfB keep the reference of A or a whole copy? will this design cause any performance problem?
i worked on C for a while. i know this kind of structure is not allowed in C (or maybe just hard to implement in C). that's why i comes up this question.

sorry, i didn't make myself clear. hope this help to clarify my question/concern.

thanks,

This post was more clear, i think. it just keeps a reference to the object instantiated from class A ( or made of A ! ) and not only cause any problem, but makes sense to interaction between objects: that's the way they can call each other!
You can just figure it out by answering the question:

how to call a method of class A from within class B ?
of course you need to have a reference to A to point to it from B (or anywhere outside A)

Look at another example i think will make more sense even:

 
class GUI implements ActionListener   // this class has a text area that notifies activity of another class is being made
               // or the result of calculations of that other class to the user 
{
   JTextArea messageBoard = new JTextArea(); 

  // if you think it's needed to have complete example codes, tell me ;)
  // below is the method should be called in other class
   public void updateMessageBoard(String newMessage) {
       messageBoard.append(newMessage);
   }

   public void actionPerformed(ActionEvent e) { // assume some button fired this action
         new CalculatorClass(this).start();
   }   

}


class CalculatorClass extends Thread   // [b]this class should have a reference to GUI to report the new messages to it[/b]
       GUI reporter;   // that's what needed!
       public CalculatorClass (GUI ui) {
               this.reporter = ui;
       }
// overriden method called automatically upon calling start()
       public void run() {
          for (;;) {
          // calculations...
          String message = "some calculations done!";
          reporter.updateMessageBoard(message);
          }
       }
}

[emily li]

I see. thank you

Esmaeil Ashrafi wrote:
... ...
This post was more clear, i think. it just keeps a reference to the object instantiated from class A ( or made of A ! ) and not only cause any problem, but makes sense to interaction between objects: that's the way they can call each other!
You can just figure it out by answering the question:

how to call a method of class A from within class B ?
of course you need to have a reference to A to point to it from B (or anywhere outside A)
... ...