Garbage Collection question

How to work on Garbage Collector questions? I never get my answers right. Is there any way to find out which of the objects are eligible for Garbage Collection?
Can anyone of you please tell me how to find out how many objects are eligible for GC?

Following is the code(This question is from Examlab) :

I use the following method:

Take a piece of paper and draw the objects on the right and the references on the left.
Use lines to connect them. Then go through the code and erase and draw new lines to
update the references.

Exactly. You do that and going wrong is unlikely.

Can't make out the diagram for the above code

When line 0 executes,
---------ob1--------ob2---------ob3
gb1->grab(){g->grab(){g->grab(){g->null}}}
gb1.g---------------^
gb1.g.g---------------------------^
gb1.g.g.g------------------------------------^
-------------------------------------------ob4---------ob5
So when line1 executes- gb1.g.g->grab(){g->grab(){g->null}}, when gb1.g.g points to a new object, ob3 has nothing accessing it.

When line2 executes-
---------ob6--------ob7----------ob8
gb2->grab(){g->grab(){g->grab(){g->gb1}}}
gb2.g----------------^
gb2.g.g-----------------------^
gb2.g.g.g-> the same object pointed by gb1;

When line3 executes-
gb2.g-> the same object pointed by gb1.g, ob7 has nothing accessing it.

Totally 8 objects are created.

I think 2 objects ob3 and ob7 are eligible for Garbage collection. Please let me know if I'm wrong.

Rohit Ramachandran wrote: public class Grab{ Grab g; public Grab(Object g){ this.g=(Grab) g; } public static void main(String[] args){ Grab gb1=new Grab(new Grab(new Grab(null)));//line0 gb1.g.g=new Grab(new Grab(null));//line1 Grab gb2=new Grab(new Grab(new Grab(gb1)));//line2 gb2.g=gb1.g;//line3 System.gc(); } }
When line 0 executes,
---------ob1--------ob2---------ob3
gb1->grab(){g->grab(){g->grab(){g->null}}}
gb1.g---------------^
gb1.g.g---------------------------^
gb1.g.g.g------------------------------------^
-------------------------------------------ob4---------ob5
So when line1 executes- gb1.g.g->grab(){g->grab(){g->null}}, when gb1.g.g points to a new object, ob3 has nothing accessing it.

When line2 executes-
---------ob6--------ob7----------ob8
gb2->grab(){g->grab(){g->grab(){g->gb1}}}
gb2.g----------------^
gb2.g.g-----------------------^
gb2.g.g.g-> the same object pointed by gb1;

When line3 executes-
gb2.g-> the same object pointed by gb1.g, ob7 has nothing accessing it.

Totally 8 objects are created.

I think 2 objects ob3 and ob7 are eligible for Garbage collection. Please let me know if I'm wrong.

In the ExamLab Answer is- 3 objects are eligible for GC.

Hold on a bit. I'm taking out a paper.

Whether gb1.g.g.g will also be eligible to GC? As it is pointing to null..? I am not sure

Yes yes yes.

3 Objects eligible for garbage collection. As I've mentioned ob3 and ob7. Since ob8 can be reached only through an ob7.g, since ob7 can't be accessed even ob8 can't be accessed.

Hence 3 objects eligible are ob3,ob7 and ob8. You should understand it now. If you don't read it and try.

Saumya Srivastava wrote:Whether gb1.g.g.g will also be eligible to GC? As it is pointing to null..? I am not sure

No no no. references aren't eligible for garbage collection. Only Objects are.

Rohit Ramachandran wrote:Yes yes yes.

3 Objects eligible for garbage collection. As I've mentioned ob3 and ob7. Since ob8 can be reached only through an ob7.g, since ob7 can't be accessed even ob8 can't be accessed.

Hence 3 objects eligible are ob3,ob7 and ob8. You should understand it now. If you don't read it and try.

Thank you Rohit. You explained it very well

My pleasure. What's a greenhorn btw? I'm a Ranch hand, you seem to be a greenhorn.

I m sorry..I don't know what is the purpose of that... how it came and what it means...Do you know?

Saumya Srivastava wrote:I m sorry..I don't know what is the purpose of that... how it came and what it means...Do you know?

A unrelated question, but easy to answer...

http://faq.javaranch.com/java/SaloonTitles

Henry

If it helps, I draw diagrams like this for GC questions (objects marked as GC in the diagram are eligible for garbage collection)

public class Grab{
   Grab g;
   public Grab(Object g){
       this.g=(Grab) g;
   }
   public static void main(String[] args){
           Grab gb1=new Grab(new Grab(new Grab(null))); //line1
           gb1.g.g=new Grab(new Grab(null));            //line2
           Grab gb2=new Grab(new Grab(new Grab(gb1)));  //line3
           gb2.g=gb1.g;                                 //line4
           System.gc();
   }
}

//after line 1
   gb1
┌───────┐  ┌───────┐  ┌───────┐
│   g───┼─>│   g───┼─>│   g   │
└───────┘  └───────┘  └───────┘

//after line 2
   gb1
┌───────┐  ┌───────┐  ┌───────┐
│   g───┼─>│   g   │  │   g   │GC
└───────┘  └───┼───┘  └───────┘
               ▼
           ┌───────┐  ┌───────┐
           │   g───┼─>│   g   │
           └───────┘  └───────┘

//after line 3
                         gb2
┌───────┐  ┌───────┐  ┌───────┐
│g      │<─┼── g   │<─┼── g   │
└┼──────┘  └───────┘  └───────┘
 │    gb1
 │ ┌───────┐  ┌───────┐  ┌───────┐
 └>│   g───┼─>│   g   │  │   g   │GC
   └───────┘  └───┼───┘  └───────┘
                  ▼
              ┌───────┐  ┌───────┐
              │   g───┼─>│   g   │
              └───────┘  └───────┘

//after line 4
   GC         GC         gb2
┌───────┐  ┌───────┐  ┌───────┐
│g      │<─┼── g   │  │   g   │
└┼──────┘  └───────┘  └───┼───┘
 │                ┌───────┘
 │    gb1         ▼
 │ ┌───────┐  ┌───────┐  ┌───────┐
 └>│   g───┼─>│   g   │  │   g   │GC
   └───────┘  └───┼───┘  └───────┘
                  ▼
              ┌───────┐  ┌───────┐
              │   g───┼─>│   g   │
              └───────┘  └───────┘

Thanks Ankit. Very nice explanation.

this question is explained in earlier thread gc thread

Dude, I don't get it. How will anything be eligible for garbage collection at line 3?

rohit see my above thread it has bitmap image of the above garbage collection problem

Dude, I don't get it. How will anything be eligible for garbage collection at line 3?

If this statement was meant for me, then I would say a new object was not eligible for GC. Its the same object that became eligible for GC at line 2. I didn't remove it from the diagram so that in the last diagram you can clearly see that total 3 objects are eligible for GC...

Ankit Garg wrote:If it helps, I draw diagrams like this for GC questions (objects marked as GC in the diagram are eligible for garbage collection)
public class Grab{ Grab g; public Grab(Object g){ this.g=(Grab) g; } public static void main(String[] args){ Grab gb1=new Grab(new Grab(new Grab(null))); //line1 gb1.g.g=new Grab(new Grab(null)); //line2 Grab gb2=new Grab(new Grab(new Grab(gb1))); //line3 gb2.g=gb1.g; //line4 System.gc(); } } //after line 1 gb1 ┌───────┐ ┌───────┐ ┌───────┐ │ g───┼─>│ g───┼─>│ g │ └───────┘ └───────┘ └───────┘ //after line 2 gb1 ┌───────┐ ┌───────┐ ┌───────┐ │ g───┼─>│ g │ │ g │GC └───────┘ └───┼───┘ └───────┘ ▼ ┌───────┐ ┌───────┐ │ g───┼─>│ g │ └───────┘ └───────┘ //after line 3 gb2 ┌───────┐ ┌───────┐ ┌───────┐ │g │<─┼── g │<─┼── g │ └┼──────┘ └───────┘ └───────┘ │ gb1 │ ┌───────┐ ┌───────┐ ┌───────┐ └>│ g───┼─>│ g │ │ g │GC └───────┘ └───┼───┘ └───────┘ ▼ ┌───────┐ ┌───────┐ │ g───┼─>│ g │ └───────┘ └───────┘ //after line 4 GC GC gb2 ┌───────┐ ┌───────┐ ┌───────┐ │g │<─┼── g │ │ g │ └┼──────┘ └───────┘ └───┼───┘ │ ┌───────┘ │ gb1 ▼ │ ┌───────┐ ┌───────┐ ┌───────┐ └>│ g───┼─>│ g │ │ g │GC └───────┘ └───┼───┘ └───────┘ ▼ ┌───────┐ ┌───────┐ │ g───┼─>│ g │ └───────┘ └───────┘

Got it !! Very easily understood... thank u...

Dude I'm not sure exactly which 3 objects are eligible for garbage collection. Could you please look at my diagram above and tell me if it's ob3, ob7 and ob8 eligible?

Hey Thanks for your diagrams.But i would like to see how the diagrams change when you have a static instance variable in the class Grab apart from g.Please explain that too.Thanks in advance