Garbage Collection question

[Kamila Jolan]

Hi,

The following question ask how many object are eligible for GC after line 10.

   
     1. public class Garb{
     2.      Garb g;
     3.      public Garb(Object g){
     4.           this.g=(Garb)g;
     5.      }
     6.      public static void main(String args[]){
     7.           Garb gb1=new Garb(new Garb(new Garb(null)));
     8.           gb1.g.g=new Garb(new Garb(null));
     9.           Garb gb2=new Garb(new Garb(new Garb(gb1)));
     10.          gb2.g=gb1.g;
     11.          System.gc();
     12.     }
     13. }

The answer is 3 but I don't know which one is the third object. To clarify it I was trying to drew the picture what is happening(but I am not sure is it correct)

line: 7. Garb gb1=new Garb(new Garb(new Garb(null))); // 3 objects created:
gb1-->|_|
gb1.g-->|_|
gb1.g.g->|_|

line: 8. gb1.g.g=new Garb(new Garb(null)); // 2 objects created, 1 eligible to GC:
gb1-->|_|
gb1.g-->|_|
gb1.g.g->|_| |_| //1st object eligible for GC
gab1.g.g.g->|_|

line: 9. Garb gb2=new Garb(new Garb(new Garb(gb1))); // Another 3 objects created:
gb1-->|_|<--gb2.g.g.g
gb1.g-->|_|
gb1.g.g->|_| |_|//1st object eligible to GC
gab1.g.g.g->|_|
gb2-->|_|
gb2.g-->|_|
gb2.g.g-->|_|

line: 10. gb2.g=gb1.g;// 2nd object eligile to GC
gb1-->|_|<--gb2.g.g.g
gb1.g-->|_|<--gb2
gb1.g.g->|_| |_|//1st object eligible to GC
gab1.g.g.g->|_|
|_|//2nd
gb2.g-->|_|
gb2.g.g-->|_|

Thus, I am not sure which is the third object available for GC.

Any help will be appreciated.

Kamila

[Kacper Jonczyk]

Hi Kamila,

You're almost there. At line 10 there are 2 additional objects eligible for GC instead of 1.

Gb2.g refers to the object which has a reference to another object. After changing gb2.g reference, both of them will be eligible for GC, because neither of them can be accessed any longer.

Hope this helps. Please correct me if I'm wrong. It's bit twisted example although

[Abimaran Kugathasan]

One is in the 7th line and the other is in the 10th line!

[Shanky Sohar]

Welcome to JavaRanch

Could you please QUOTEYOURSORCES from where you get this question.

[Kamila Jolan]

Kacper, Abimaran thanks for the answer
Kacper I think you are correct.
Shanky, the question is from Deveka ExamLab simulator.

Regards,
Kamila

[Bert Bates]

Hi Guys,

This question will definitely toughen you up for the real exam, but to be clear, it's a little bit "over the top". Questions on the real exam won't be this twisty-turny.

[sumit kothalikar]

hi kamila
i am trying to draw this diagram have look

[Mehdi Ben Larbi]

Excellent diagram,it helps thanks.

[sumit kothalikar]

you are welcome

[Divyya Joshi]

Sumit can you please tell why g.gb1 set to null ?

[Ankit Garg]

This question has been asked before like here and here. You might find the explanations there helpful as well...