Problem in Garbage collector

[Shashank Sharma]

    class A{
     A aob;
     public static void main(String args[]){
          A a=new A();//1
          A b=new A();//2
          A c=new A();//3
          a.aob=b;//4
          b.aob=a;//5
          c.aob=a.aob;//6
          A d=new A().aob=new A();//7
          c=b;//8
          c.aob=null;//9
          System.gc();//10
     }
}

code source:-exam lab question
//at line 9 c.aob and c.aob both are referring to null:
but the object to which B and c are referring to still can be accessed by a.aob as in line4
please help

[Vijitha Kumara]

This kind of questions are frequently asked here. You may search a little in the forum and could find many threads which discussed about this. Easiest way to solve these kinds are by drawing an object graph in a paper. Not getting your question though...

//at line 9 c.aob and c.aob both are referring to null:

You are talking about the same reference (i.e: c.aob) here?

but the object to which B and c are referring to still can be accessed by a.aob as in line4

Yes, the reference "a" is still alive (I guess you meant b, not B..).

[Shanky Sohar]

I will expect someone to comment if i am wrong.

line1:-A a=new A();
        -------------   
a------>- new A()   -
        -   aob     -- 
        -------------  
                       
line2:-A b=new A();
        ------------   
b------>- new A()   -
        -   aob     -
        -------------

Line3:- A c=new A();
        -------------   
c------>- new A()   -
        -   aob     -
        -------------  

Line4:-  a.aob=b;
        -------------                 -------------   
a------>- new A()   -       ! b------>- new A()   -
        -   aob     ---------         -   aob     -
        ------------                 -------------      
                                                        
    
          
Line5:- b.aob=a;
        -------------                 -------------   
a------>- new A()   -       ! b------>- new A()   -
 !       -   aob     ---------         -   aob     -------
 !       ------------                 -------------      !
 !                                                       !
 ---------------------------------------------------------                                                                           

line 6:- c.aob=a.aob;

        -------------   
        - new A()   -
c------>-   aob     -----
        -------------   !
                        !
-------------------------
!
!
!        -------------                 ------------   
a------>- new A()   -       ! b------>- new A()   -
 !       -   aob     ---------         -   aob     -------
 !       ------------                 -------------      !
 !                                                       !
 ---------------------------------------------------------


Line 7:-  A d=new A().aob=new A();

        -------------   
a------>- new A()   -      ------------ 
        -   aob     --------  new A() -
        -------------      -  aob     -
                           ------------ 

Line 8:-  c=b;

        --------------         
        - new A()   -        /// this link got broken and not beeen reference by any so now 1 object is eligible for gc
c      -   aob      -  
-      ------------ -  
-                       
------------------------------
                             - 
       -------------         -        ------------   
a------>- new A()   -       ! b------>- new A()   -
 !       -   aob     ---------         -   aob     -------
 !       ------------                 -------------      !
 !                                                       !
 ---------------------------------------------------------


c.aob=null;//9 

aob is a refernce variable of A.so it will not be eligible for gc. 

so answer of this Question is 1.only 1 object will be eligible for Garbage collection.

[Shanky Sohar]

Try to solve this kind of problem by drawing diagram.It will be good for you and remember the links which got broken will only be eligible for gc if they are not be refer by anyone then.

[Vinoth Kumar Kannan]

How do we draw object graphs/diagrams? someone got any link that explains it?

[Shanky Sohar]

click on this link http://cs.wellesley.edu/~cs111/fall97/lectures/ObjectDiagrams/object-diagrams.html may it will be helpful.

[Vijitha Kumara]

shanky sohar wrote:I will expect someone to comment if i am wrong.....

There is no use of reference "a" in line#7.

....so answer of this Question is 1.only 1 object will be eligible for Garbage collection.

No. Another one object from line#7 is eligible for GC (the one created in "...new A().aob...).

[Vijitha Kumara]

Same question has been discussed here.

[Shanky Sohar]

that really a tricky question,one of the great trick play is at line no 7.
now i have corrected my post because of above comment
correct answer is given below.

line1:-A a=new A();
        -------------   
a------>- new A()   -
        -   aob     -- 
        -------------  
                       
line2:-A b=new A();
        ------------   
b------>- new A()   -
        -   aob     -
        -------------

Line3:- A c=new A();
        -------------   
c------>- new A()   -
        -   aob     -
        -------------  

Line4:-  a.aob=b;
        -------------                 -------------   
a------>- new A()   -       ! b------>- new A()   -
        -   aob     ---------         -   aob     -
        ------------                 -------------      
                                                        
    
          
Line5:- b.aob=a;
        -------------                 -------------   
a------>- new A()   -       ! b------>- new A()   -
 !       -   aob     ---------         -   aob     -------
 !       ------------                 -------------      !
 !                                                       !
 ---------------------------------------------------------                                                                           

line 6:- c.aob=a.aob;

        -------------   
        - new A()   -
c------>-   aob     -----
        -------------   !
                        !
-------------------------
!
!
!        -------------                 ------------   
a------>- new A()   -       ! b------>- new A()   -
 !       -   aob     ---------         -   aob     -------
 !       ------------                 -------------      !
 !                                                       !
 ---------------------------------------------------------


Line 7:-  A d=new A().aob=new A();

        ----------------  
d------>- new A()  -
         -   aob         - 
        ----------------
then 
        ----------------  
d      - new A()      -    // this link also got broken.
!        -   aob         - 
!       ----------------
--------------  
- new A()   -
-   aob       - 
--------------
 

Line 8:-  c=b;

        --------------         
        - new A()   -        /// this link got broken and not beeen reference by any so now 1 object is eligible for gc
c      -   aob      -  
-      ------------ -  
-                       
------------------------------
                             - 
       -------------         -        ------------   
a------>- new A()   -       ! b------>- new A()   -
 !       -   aob     ---------         -   aob     -------
 !       ------------                 -------------      !
 !                                                       !
 ---------------------------------------------------------


c.aob=null;//9 

aob is a refernce variable of A.so it will not be eligible for gc. 

so now two objects are eligible for gc.