Question about Garbage collection from ExamLab diagnostic Test

[PujaR Agarwal]

Class A{

A a;
public A(A a){
this.a=a;
}
public static void main(String args[])
{
         A a1=null;
         A a2=new A(new A(null));
         A a3=new A(a2);
         a1=a3;
         a1.a=new A(null);
         a2.a=null   //line 14
        System.gc();
}
}

How many objects will be eligible for garbage collection after executing line 14. Please provide solution with explanation.

[Ankit Garg]

Hi Puja, welcome to javaranch.

Can you tell us which line is confusing you?? Usually you should try to prepare a memory map that represents the objects created by the program.

Class A{

   A a;
   public A(A a){
      this.a=a;
   }
   public static void main(String args[])
   {
      A a1=null;
      A a2=new A(new A(null));
      A a3=new A(a2);
      a1=a3;
      a1.a=new A(null);
      a2.a=null //line 14
      System.gc();
   }
} 

For example the memory map of the program at line 12 would look like this

       a1  a3                           a2
         \ /                            |
          V                             V
+-----------------+               +-----------------+               +-----------------+
|       a---------+-------------->|       a---------+-------------->|       a         |
+-----------------+               +-----------------+               +-----------------+

[PujaR Agarwal]

Thanks Ankit,


I too made a memory map diagram for the same and I conclude that because of a2.a=null and a1=a3 just one object that is a2 is eligible for garbage collection because as a1 and a3 are still being referenced so they are not eligible . am I understanding it correctly?


Thanks
Puja

[deepesh soni]

i think one object will be eligible for GC. .......also a1, a2, a3 are references not objects ...so you cannot say

a2 is eligible for garbage collection

. Please correct me if i am wrong.

[PujaR Agarwal]

Thanks

but I am still not sure for the correct explanation. Can anybody give the correct solution with explanation ?

[Ankit Garg]

If you continue with my diagram, then after line 14, the memory map would look like this

       a1  a3                           a2
         \ /                            |
          V                             V                                  GC eligible
+-----------------+               +-----------------+               +-----------------+
|       a         +               |       a         +               |       a         |
+-------+---------+               +-----------------+               +-----------------+
        |
        V
+-----------------+
|       a         +
+-----------------+

[PujaR Agarwal]

hello Ankit,

Can you please tell more specifically which object is eligible for garbage collection and why because by diagram I am able to understand that one object is there which is eligible for it but which one . I guessed is a2. Please clear it.


Thanks
Puja

[deepesh soni]

On line 10
A a2=new A(new A(null));
the object which is created by calling "new A(null)" does not have any reference. So this object will be garbage collected.

[Akanksha Joy]

Yes only one object is eligible for garbage collection and thats the one referenced by instance member a of object a2 which is afterwards made to null.

[PujaR Agarwal]

Thanks all.

This is another example of garbage collection from ExamLab

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

How many objects are eligible for garbage collection and which are they ?


I tried to solve this question and conclude that object c and d will be eligible for garbage collection. Please correct me if I am wrong.

Thanks
Puja

[deepesh soni]

IMO Only the object initially referenced by c will be garbage collected.

[PujaR Agarwal]

but examlab answer is 2 objects are eligible.

[Devaka Cooray]

IMO Only the object initially referenced by c will be garbage collected.

but examlab answer is 2 objects are eligible.

For this second question, there are 2 objects eligible. It was previously discussed.
The first question was discussed here.

Devaka.

[Devaka Cooray]

PujaR Agarwal wrote:This is another example of garbage collection from ExamLab

Please start a new topic for new questions, that would be helpful to both you and others.

[PujaR Agarwal]

Thanks all for helping me.