garbage collection

[Kefe Abalov]

given:
12.void start(){
13.A a = new a();
14.B b = new b();
15.a.s(b);
16.b=null;
17.a=null;
18.System.out.println("start is over");
}
when the object B ,created in the line 14,eligible for garbage collection?
A.after the line 16.
B.after the line 17.
C.after the line 18
D.none of above
answer:C
who can tell me why?

[Andres Gonzalez]

An object is eligible for GC when is not reachable by any existing thread.
If I am not mistaken, the method a.s(b) receives as a parameter b, but we don't know what's gonna happen with that reference inside that method. It can be assigned to another reference, etc. So even if we execute line 16, we don't know for sure what really happened inside method s(). It is only eligible when the method is finished, which is out of scope.
Please correct me if I'm wrong
[ July 20, 2003: Message edited by: Andres Gonzalez ]

[Alex Radomski]

I wonder if there's missing code in this question?
I think Andres you're correct.
We don't know for certain what will happen to the b reference once s() method is called.
It might be reassigned to another variable of B type?
Alex

[Anupam Sinha]

If the functioning of the a.s(b) is not provided then answer c can not be considered correct. I guess its a wrong question or you can a correct question with wrong answers.
[ July 20, 2003: Message edited by: Anupam Sinha ]

[Valentin Crettaz]

kef,
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[Sudhakar Krishnamurthy]

correct me if i am wrong....
No matter what happens in the method a.s(b), once b=null doesn't it mean all the variables pointing to this reference becomes null and so shouldn't the correct answer be 'after line 16'???

[Priyanka Chopda]

hello,
no matter what happens to b inside the method s(). but as long as object a is pointing to objcet b thru s(), only after a is set to null b is free for GC.
If Wrong...plz correct me!
JC

[Anupam Sinha]

Hi all
Well the reason for my earlier answer that it is not possible to know for sure whether b is elligible for GC or not is that the method a.s() may store the reference of b in some varibale. Consider this code for a.s()

class a
{
    public void s(Object o)
    {
         AnotherClass ac = new AnotherClass();
         ac.anotherMethod(o);
    }
}

Now in here, the object reference contained in o(b in the orignal code) is being transfered to a method in AnotherClass. This will prevent the varibale b from being GCed. Remember object references are GCed and not variables.
[ July 22, 2003: Message edited by: Anupam Sinha ]