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Garbage Collection question

 
Ranch Hand
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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) :

 
Bartender
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IntelliJ IDE Opera
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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.
 
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Exactly. You do that and going wrong is unlikely.
 
Saumya Srivastava
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Can't make out the diagram for the above code
 
Rohit Ramachandran
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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.
 
Saumya Srivastava
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Rohit Ramachandran wrote:
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.
 
Rohit Ramachandran
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Hold on a bit. I'm taking out a paper.
 
Saumya Srivastava
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Whether gb1.g.g.g will also be eligible to GC? As it is pointing to null..? I am not sure
 
Rohit Ramachandran
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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.
 
Rohit Ramachandran
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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.
 
Saumya Srivastava
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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
 
Rohit Ramachandran
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My pleasure. What's a greenhorn btw? I'm a Ranch hand, you seem to be a greenhorn.
 
Saumya Srivastava
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I m sorry..I don't know what is the purpose of that... how it came and what it means...Do you know?
 
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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
 
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If it helps, I draw diagrams like this for GC questions (objects marked as GC in the diagram are eligible for garbage collection)
 
Greenhorn
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Thanks Ankit. Very nice explanation.
 
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this question is explained in earlier thread gc thread
 
Rohit Ramachandran
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Dude, I don't get it. How will anything be eligible for garbage collection at line 3?
 
sumit kothalikar
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rohit see my above thread it has bitmap image of the above garbage collection problem
 
Ankit Garg
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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...
 
Saumya Srivastava
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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)



Got it !! Very easily understood... thank u...
 
Rohit Ramachandran
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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?
 
Greenhorn
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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
 
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