Objects eligible for GC

[Rekha Srinath]

Source: Get Certified Exam Lab, by Devaka Cooray

class A{
     A aob;
     public static void main(String args[]){
          A a=new A();
          A b=new A();
          A c=new A();
          a.aob=b;
          b.aob=a;
          c.aob=a.aob;
          A d=new A().aob=new A();
          c=b;
          c.aob=null;
          System.gc();
     }
}

Question: How many objects are eligible for GC after the c.aob = null call?

Answer given: 2 objects

But, I thought the answer is 1 object, which is the object that was originally referenced by the variable c...

How does it turn out to be 2?
[ November 15, 2008: Message edited by: Rekha Srinath ]

[mani vannan]

Rekha,
My suggestion would be, you have to plot the mapping in a graph and add/remove references as you go reading the code, when solving GC related problem. It's quickest way and you wont missout anything. I've written an article Object's eligibility factors for Garbage Collection in Java]Object's eligibility factors for Garbage Collection in Java for the same, hope it might hep you.

[Rekha Srinath]

Yes Manivannan. I did that. Based on that only, I decided that only one object is eligible.

My doubt is in this:
There is an A object referenced by d. And this object's aob reference refers another A object. (which is done by the statement A d=new A().aob=new A();

I assume that this is valid, and that both these objects are not eligible for GC, which finally leaves me with just one object eligible for GC
[ November 15, 2008: Message edited by: Rekha Srinath ]

[Kenneth Lomvey]

Rekha, this question is already discussed in the following topic:
https://coderanch.com/t/417827/java-programmer-SCJP/certification/GC

[Rekha Srinath]

Oh thanks Kenneth for the link.

[Henry Wong]

Hmmmm.... This would be an interesting one to draw out.

Let's name the objects OA, OB, OC, etc., as they are being created. This is to distinguish them from the references. Also, let's follow the aob reference of the objects, in order to get the full graph. So...

---------------------------
After:

A a=new A();
A b=new A();
A c=new A();

We look like this...

a --> OA --> null
b --> OB --> null
c --> OC --> null

---------------------------
After:

a.aob=b;
b.aob=a;
c.aob=a.aob;

We look like this...

c --> OC --|
           |
          \/
b ------> OB ----|
           ^     |
           |     |
a --> OA --|     |     (OA and OB forms a little island) 
      ^          |
      |----------|

---------------------------
After:

A d=new A().aob=new A();

We look like this...

c --> OC --|
           |
          \/
b ------> OB ----|
           ^     |
           |     |
a --> OA --|     | 
      ^          |
      |----------|
 
 
      OD --> OE --> null
             ^
             |
d -----------| 

note: object OD is not reachable after this line.

---------------------------
After:

c=b;
c.aob=null;

We look like this...

c ---------|
           |
          \/
b ------> OB --> null
           ^   
           |  
a --> OA --|   
 
d --> OE --> null

So... We lost access to objects OC and OD. Those two objects are no longer reachable.

Hope this helps, and please correct me if I missed anything...
Henry
[ November 15, 2008: Message edited by: Henry Wong ]

[Kenneth Lomvey]

Hi Henry,

Thanks for your great explanation.

[Ken Truitt]

I got 1 object as well, Rekha. I think the trick is in
the line A d=new A().aob=new A(); which Henry Wong's graph
shows actually results in an anonymous A being created, which,
unlike d, gets it's object field initialized with another A,
and THIS is what d points to. So the trick is that after
that line, the anonymous A object is already eligable for
gc, without anything having to be nulled.

Is that right?

[Rekha Srinath]

Yes thats the catch Ken..

[Devaka Cooray]

Rekha,

I think that the problem is now solved. Thanks for Henry, he gave a nice explanation.

I'm sorry, that I didn't include any explanation for GC questions in my simulator. I think I should include them in the next version.

Thanks for discussing this question.


Cheers