GC problem

[vineet walia]

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. }

how many object are eligible for GC???

[Balraj Momi]

I think four objects are eligible for GC.

[Anastasia Sirotenko]

3 objects:

     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))); // gb1 -> [Garb].g-> [Garb].g -> [Garb]
     8.           gb1.g.g=new Garb(new Garb(null)); //gb1 -> [Garb].g-> [Garb].g -X-> [Garb]  // here we get 1 object for GC, X is for link broken
                                                    //gb1 -> [Garb].g-> [Garb].g -> [Garb].g-> [Garb] 
     9.           Garb gb2=new Garb(new Garb(new Garb(gb1))); //gb2 -> [Garb].g-> [Garb].g -> gb1
     10.          gb2.g=gb1.g; //gb2 -X-> [Garb].g-> [Garb].g -> gb1 // here we get 2 objects for GC, X is for link broken
                               //gb2 -> gb1
     11.          System.gc();
     12.     }
     13. }

[Balraj Momi]

Anastasia is right, there are 3 objects eligible.
It will be 4 If gb1 is assigned to null.

[Nitish Bangera]

Vineet walia says
how many object are eligible for GC???

I guess you should draw a diagram and first give your answer so that you can verify with the ranchers.

[Himanshu Kansal]

Number of objects actually created - Number of objects still having references assigned

[Ankit Garg]

vineet please Quote Your Sources when you post a question. You've posted multiple questions without a proper source. Your questions will be deleted if you don't give the source properly...

[Nitish Bangera]

This is a famous examlab question....

[Ankit Garg]

Nitish Bangera wrote:This is a famous examlab question....

Its good that it is from a legal mock exam, but generally we expect the topic creator himself/herself to specify the source. That way its easier for everyone. Anyways its good that you are making efforts

[S Ali]

3 objects ,right?

[Ankit Garg]

Its easy to find the number of object eligible for GC using a diagram

At line 7, the memory map looks like

     gb1
      |
      V
+------------+
|     g      |
+-----|------+
      |
      V
+------------+
|     g      |
+-----|------+
      |
      V
+------------+
|     g      |
+------------+

At line 8 it becomes

     gb1                       
      |
      V
+------------+
|     g      |
+-----|------+
      |
      V
+------------+    +------------+
|     g---------->|     g      |
+------------+    +-----|------+
                        |
                        V
+------------+    +------------+
|     G      |    |     g      |
+------------+    +------------+

at line 9 it becomes

                                           +------------+     +------------+
     gb1                                   |     g      |<----------g      |<----gb2
      |                                    +-----|------+     +------------+
      V                                          V                  
+------------+                             +------------+
|     g      |<----------------------------------g      |
+-----|------+                             +------------+
      |                                   
      V
+------------+    +------------+
|     g---------->|     g      |
+------------+    +-----|------+
                        |
                        V
+------------+    +------------+
|     G      |    |     g      |
+------------+    +------------+

and at line 10 it is

                                           +------------+     +------------+
     gb1                                   |     G      |     |     g      |<----gb2
      |                                    +-----|------+     +-----|------+
      V                                          V                  |
+------------+                             +------------+           |
|     g      |<----------------------------------G      |           |
+-----|------+                             +------------+           |
      |   +---------------------------------------------------------+
      |   |
      V   V
+------------+    +------------+
|     g---------->|     g      |
+------------+    +-----|------+
                        |
                        V
+------------+    +------------+
|     G      |    |     g      |
+------------+    +------------+

So 3 objects are eligible for GC (The one with the capital G in them)...

[vineet walia]

Anastasia Sirotenko wrote:3 objects:

     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))); // gb1 -> [Garb].g-> [Garb].g -> [Garb]
     8.           gb1.g.g=new Garb(new Garb(null)); //gb1 -> [Garb].g-> [Garb].g -X-> [Garb]  // here we get 1 object for GC, X is for link broken
                                                    //gb1 -> [Garb].g-> [Garb].g -> [Garb].g-> [Garb] 
     9.           Garb gb2=new Garb(new Garb(new Garb(gb1))); //gb2 -> [Garb].g-> [Garb].g -> gb1
     10.          gb2.g=gb1.g; //gb2 -X-> [Garb].g-> [Garb].g -> gb1 // here we get 2 objects for GC, X is for link broken
                               //gb2 -> gb1
     11.          System.gc();
     12.     }
     13. }

bro not geting your handy notation can you exp lain little bit??

[vineet walia]

Ankit Garg wrote:Its easy to find the number of object eligible for GC using a diagram

thank Ankit Garg i thnk i will help me out

[Edit AG: Removed long quotes which were looking messy]

[Ankit Garg]

@vineet, you can use the @<NAME> to direct your message to someone specific instead of quoting whole posts. Like what you said in the last two posts can be done like -

@Anastasia, bro not geting your handy notation can you explain little bit??

@Ankit, thanks I thnk I will help me out

That way its easier to read as quoting long posts can look very messy...

[Tanya Shetty]

Can somebody tell me why this line No. 7

 Garb gb1=new Garb(new Garb(new Garb(null)));   

does not give a null pointer exception, since, ultimately it does try to cast a null type to GARB type in the constructor..

I have tries this code and it runs fine!

Also, if i replace line no. 7 by,

 Garb gb1=new Garb(null);   

I DO get the null pointer exception , why is this ???

[Anastasia Sirotenko]

NullPointer comes from line 8, where you try to instantiate gb1.g.g , tho gb1.g is null after line 7

[Ankit Garg]

Tanya this is from the JLS

The null reference can always be cast to any reference type.

[Himanshu Kansal]

Just check where exatly you get the exception, and you shall know

[Fritz Guerilus]

Ankit that was a GREAT diagram.
Absolutely wonderful. I wonder how long it took you to draw it out.

The first time I answered this question I drew the diagram but I still got the answer wrong. I drew an extra object becuase I was confused.

Then I reviewed/drew the diagram again before looking in the forums for more explaination.

When I reviewed it for the 2nd time, I got the answer right. and double checked the forum.

The trick with all of these GC questions is that the class always HAS-A instance variable of some object type, and that can be confusing.

The best thing you can do is keep practicing and draw it out.