exam lab gc question

[vineet walia]

exam lab gc question


1. class A{
2.
3. A a1;
4. A a2;
5.
6. public void finalize(){
7. System.out.println("-");
8. }
9.
10. public static void main(String args[]){
11. A s1=new A();
12. s1.a1=new A();
13. s1.a2=s1;
14. s1.a1.a2=new A();
15. s1.a1.a2.a2=new A();
16. s1.a1.a2.a2.a1=s1.a1;
17. s1.a2.a1.a2=null;
18. System.gc();
19. }
20.
21. }

answer is 2
what i think 4 can you explain?

1. s1.a2.a1.a2=null;
2. s1.a1.a1;
3. s1.a1.a2.a1;
4.s1.a1.a2.a1.a2;

[karthick chinnathambi]

you need to draw diagram for this......


all the members of an object are in heap.....so here a1 and a2 are in heap.....(of course all objects are in heap....)(i mentioned this for diagrammatic purpose)
all the object references are in stack..... here s1 only.....

see that at least one reference in stack is pointing to that object on heap(either directly or through chaining)
if no reference in stack is pointing to that object (either directly or through chaining) then the object has lost it's reference....
there is no way that your program can access it.....
it is eligible for garbage collection.....

[Ankit Garg]

Vineet please Use Code Tags when you post a source code. Edit your message using button and then add code tags to it...

[vineet walia]

vineet walia wrote:exam lab gc question

  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();
           }
      
      }

answer is 2
what i think 4 can you explain?

1. s1.a2.a1.a2=null;
2. s1.a1.a1;
3. s1.a1.a2.a1;
4.s1.a1.a2.a1.a2;

[Salil Vverma]

I believe, the representation in the below image would clear that only two objects would be available for garbage collection.



Regards
Salil Verma

[indra negi]

Please can someone explain me the code as I could not get it.

[Ankit Garg]

Lets see a memory map till the statement at line 16 (s1.a1.a2.a2.a1=s1.a1;)

       s1
       |                   +--------------------------------------+
       V                   V                                      |
+-------------+     +-------------+     +-------------+     +-----|-------+
|     a1----------->|     a1      |     |     a1      |     |     a1      |
|             |     |             |     |             |     |             |
|     a2      |     |     a2----------->|     a2----------->|     a2      |
+------|------+     +-------------+     +-------------+     +-------------+
   /\  |
    +--+

Then when s1.a2.a1.a2 (which is equivalent to s1.a1.a2) is set to null, then the memory map becomes

       s1
       |                   +--------------------------------------+
       V                   V                                      |
+-------------+     +-------------+     +-------------+     +-----|-------+
|     a1----------->|     a1      |     |     a1      |     |     a1      |
|             |     |             |     |             |     |             |
|     a2      |     |     a2      |     |     a2----------->|     a2      |
+------|------+     +-------------+     +-------------+     +-------------+
   /\  |                                      GC                  GC
    +--+

The two objects that I've marked as GC, are eligible for GC as they don't have any reference to them...

[Salil Vverma]

Hey Ankit,

I was looking for some tool which could help me in making the memory map of specific execution. Could you let me know the name of the tool that you used to get the memory map ?

Regards
Salil Verma

[Ankit Garg]

Salil Vverma wrote:Could you let me know the name of the tool that you used to get the memory map ?

Fingers (don't go looking for a tool named fingers, I meant I created it in notepad myself ). I don't know of any tool that can do that, maybe someone else can help...

[indra negi]

I must say Ankit you have a great understanding of GC. I understood it very well.

Thanks