Garbage collector vs circular reference

[Sylvain Bouchard]

Hi Ranchers,

I came across code at work, and I try to figured it out how the garbage collector will handle this case.
In fact, it looks like a goof tricky question for the exam !

Object A have an attribute Object B
Object B have an attribute Object A

Object A{
  Object B objectB;
  public A(){
     this.objectB = new B(this);	 
  }
}

Object B{
  Object A objectA;
  public B(B objA){
     this.objectA = objA;
  }
}


TestClass{
   public static void main(String[] args){
		Object A objA = new A();
		objA = null;
		// Garbage Collector  
   }
}

objA will obviously be eligible for GC !
But what about objectB and objectA? They still reference each other.

I'm tempted to say that they will not be GC !!

What do you think?

Sylvain

[Paul Clapham]

The garbage collector doesn't have a problem with identifying those objects as being available for garbage collection. The fact that you don't know how it does that doesn't prevent it from doing its work.

[Sylvain Bouchard]

Hi Paul,

So you're saying All three objects (two references) will be GC ?

[Paul Clapham]

I only see two objects. There's an object of type A, and an object of type B, and they contain references to each other.

And no, I'm not saying those two objects will be garbage-collected. I'm saying they are available to be garbage-collected. Whether the garbage collector actually collects them depends on whether it's necessary to do so, and that's a decision which is up to the garbage collector.

[Sylvain Bouchard]

Sorry, I've misused the word GC, I meant "candidate for GC" !

For the three object:
- objA in the main function
- objectB attribute in Class A
- objectA attribute in Class B, which is the same reference as the object in the main function

Three object, two references...

So, If i get back to my original question and your explanation: all objects will be candidate for GC.

[Paul Clapham]

No, you have that backwards. There are three variables which at some times contain references to one of the two objects. It's the objects which get garbage-collected, not the variables. So discussions which talk about "variable X being garbage-collected" are misleading, although you'll find they are extremely common in this forum.

[Sylvain Bouchard]

I see your point ! You're absolutely right.
I think in terms of variables, when I should think object "creation"

Object Reference
A ---> objA
---> objectA

B ---> objectB

--------------------------------------
2 Objects candidate to be GC

[Paul Clapham]

Yes, I think you have it now. Although when making a diagram like that, I would have the arrows (which represent references) going from the variables to the objects which they reference. You have the arrows backwards.

Note that it's possible for several variables to contain references to a single object, and in fact that is the case in this example for part of the time. However a variable can only refer to one object at a time, unless it contains null in which case it doesn't refer to any object.

[Sylvain Bouchard]

Thank you Paul, for your help!
I got the picture and will surely score higher in this objective !