Garbage collection

[Preethi Dev]

Hi, this question is from Devaka's practice exam3
class A{

A a1;
A a2;

public void finalize(){
System.out.println("-");
}

public static void main(String args[]){
A s1=new A();
s1.a1=new A();
s1.a2=s1;
s1.a1.a2=new A();
s1.a1.a2.a2=new A();
s1.a1.a2.a2.a1=s1.a1;
s1.a2.a1.a2=null;
System.gc();
}

}

Please explain me a good way to draw picture for this type of question.
answer is 2 obbjects are eligible.

Thanks in advance
Preetha

[jean-gobert de coster]

Hum... I can only find one... First thing you can notice is that there is only one place in the code where a reference is modified:

s1.a2.a1.a2=null

so when you look at what was referenced by this, you'll see that it was a new A()

and also, there is later no reference made to s1.a2.a1.a2 so that instance of A becomes GC-able. But I don't see any other instance of A not being referenced any more.

[Henry Wong]

The answer is correct -- two objects are eligible. And it's not too difficult to draw it out, just give it a try.

Henry

[Treimin Clark]

I found TWO objects eligible. But actually I don't know how to explain this. :roll:

[James Tharakan]

@ Preetha Arun ,

I was wondering how to post a image in the forum.
So ,i have send you a mail(yahoo).Check your mail.
Hope that makes things clear.
If anybody know how to post image ,please let me know.(Where the image is in my home pC).

[Ankit Garg]

James you cannot include images on javaranch from your hard disk. You will have to upload the image somewhere. I use flickr to upload the images and put them here...

[James Tharakan]

Hmm , thanks Ankit.
Next time i wll try that.

[Punit Singh]

        2(s1.a2)
           |--| 
           |  |         
s1------> |a1|a2|  
           |    
           |------->|a1|a2|                   
           1(s1.a1) |   |6(s1.a2.a1.a2=null)   
                    |   |----->|a1|a2|                  (eligible 1)
                    |3(s1.a1.a2)    | 
                    |               |---------> |a1|a2|  (eligible 2)
                    |         4(s1.a1.a2.a2)       |                
                    |<--------------------------|  
                       5(s1.a1.a2.a2.a1) 
 

[ December 16, 2008: Message edited by: Punit Singh ]
[ December 16, 2008: Message edited by: Punit Singh ]

[jean-gobert de coster]

oh yes, I got confused by the fact that s1.a2.a1.a2 was actually the same thing as s1.a1.a2

Indeed, not doing this with the picture makes things more complicated.

I hate those kind of questions that make you waste precious time on the exam.

[Preethi Dev]

Hi, i am just trying to draw the logic here....


1.A s1=new A();

s1--------------->OA1object

2.s1.a1=new A();

s1.a1------------>OA2object(i.e OA1object.a1)

3.s1.a2=s1;

s1--------------->OA1object
^
|
s1.a2----------------

4.s1.a1.a2=new A();

s1.a1.a2---------->OA3object(i.e OA2object.a2)

5.s1.a1.a2.a2=new A();

s1.a1.a2.a2-------->OA4object(i.e OA3object.a2)

6.s1.a1.a2.a2.a1=s1.a1;



7.s1.a1.a2.a2.a1=s1.a1;


s1.a1-------------->OA2object
^
|
s1.a1.a2.a2.a1----------

8.s1.a2.a1.a2=null

s1.a2=s1;

s1.a1.a2=null(i.e OA3object reference is null,so OA3object is eligible for GC)

OA4object is created by invoking OA3object ,since OA3object lost it's reference oA4object is also eligible for Gc


This is what i understood from this program. Please anyone tell me ,is this correct or not?! :roll:

Thanks in advance
Preetha

[ December 16, 2008: Message edited by: Preetha Arun ]
[ December 16, 2008: Message edited by: Preetha Arun ]

[Preethi Dev]

Hi James,

Thanks for taking such an effort to clear my question.
I am getting clear with it.

thanks to all.

Preetha

[Abhi vijay]

Preetha, I was calculating the no of objects eligible for GC the same way you have tried in the above post, so can you please tell me which is the 2nd object for GC?

[Preethi Dev]

Hi,

the second object is OA4object.

[Abhi vijay]

Thanks..Preetha.