Objects available for GC

[mohammad shaid]

How do we trace down this particular example to get to know the number of objects available for GC???
I am having problem with how to draw the sketch of the objects as its not specified in books.. can someone help.. i got an exam on tuesday

 public class Drag {
	Drag d;
	public Drag(Object d) {
		this.d=d;
	}
	public static void main(String[] args) {
		Drag d1=new Drag(new Drag(new Drag(null)));
		d1.d.d=new Drag(new Drag(null));
		Drag d2=new Drag(new Drag(new Drag(d1)));
		d2.d=d1.d;// Objects for GC-3
		System.gc();
	}
}	   

[Devaka Cooray]

Please QuoteYourSources.

[mohammad shaid]

From Devaka's ExamLabSimulator

I edited and made my own code

  public class Drag {  
    Drag d;  
    public Drag(Object d) {  
        this.d=d;  
    }  
    public static void main(String[] args) {  
        Drag d1=new Drag(new Drag(new Drag(null)));  
        d1.d.d=new Drag(new Drag(null));  
        Drag d2=new Drag(new Drag(new Drag(d1)));  
        d2.d=d1.d;// Objects for GC-3  
        System.gc();  
    }  
}           

[Prasad Kharkar]

Hello shaid
howz you?
well this code should give the compiler error because I compiled and checked
that is because in the constructor of the class we are passing the Drag object that takes the Object as parameter
that should be

   this.d = (Drag) d ; 

well I did this and will try for GC issue now
as I am not sure whether I will be able to solve it to the perfection or not so I am not posting here
I am trying it now


well
Anikit sir got the answer

[Ankit Garg]

At line 7 the memory structure looks like

    d1
     |
     V
+-----------+      +-----------+      +-----------+
|    d------------>|    d------------>|   d=null  |
+-----------+      +-----------+      +-----------+

At line 9, the memory structure looks like this

                                              d2
                                              |
                                              V
   +-----------+      +-----------+      +-----------+
   |    d      |<-----------d     |<----------d      |
   +----|------+      +-----------+      +-----------+
        |
        |
   d1   |
   |    |
   V    V                                  GC
+-----------+      +-----------+      +-----------+
|    d------------>|    d      |      |   d=null  |
+-----------+      +----|------+      +-----------+
                        V
                   +-----------+      +-----------+
                   |    d------------>|   d=null  |
                   +-----------+      +-----------+

After Line 10, the memory structure looks like this

                                              d2
                                              |
        GC                 GC                 V
   +-----------+      +-----------+      +-----------+
   |    d      |<-----------d     |      |    d      |
   +----|------+      +-----------+      +----|------+
        |                                     |
        |               +---------------------+
   d1   |               |
   |    |               |
   V    V               V                  GC
+-----------+      +-----------+      +-----------+
|    d------------>|    d      |      |   d=null  |
+-----------+      +----|------+      +-----------+
                        V
                   +-----------+      +-----------+
                   |    d------------>|   d=null  |
                   +-----------+      +-----------+

[Prasad Kharkar]

Ankit Garg Zindabad
Perfect Explanation

[Devaka Cooray]

Prasad Kharkar wrote:Zindabad

Not everyone understand Hindi. At lease, I don't

[Prasad Kharkar]

we don't need to understand Hindi
Hailing Sanath Jaysuriya and Sachin Tendulkar can make our communications better
isn't it?
Hope Devaka sir are interested in Cricket
well is the word "hail" right
I mean to say praising Sachin and Jaysuriya will make our communications better

[Devaka Cooray]

Seriously, in any case, JavaRanch is an English forum - all communication needs to be in English.
[Edit: My bad, I thought it's a Hindi word ]

[mohammad shaid]

Thanks a lot Satish for Correcting my code and Ankit sir for a very fantastic explanation.. thanks once again..

and i want to clear if the third object here is getting GCed??

                                     d2  
                                              |  
                                              V  
   +-----------+      +-----------+      +-----------+  
   |    d      |<-----------d     |<----------d      |  
   +----|------+      +-----------+      +-----------+  
        |  
        |  
   d1   |  
   |    |  
   V    V                                  GC  
+-----------+      +-----------+      +-----------+  
|    d------------>|    d      |      |   d=null  |  
+-----------+      +----|------+      +-----------+  
                        V  
                   +-----------+      +-----------+  
                   |    d------------>|   d=null  |  
                   +-----------+      +-----------+  

[mohammad shaid]

oops.. sorry.. i got it.. one object initially gets GCed.. then the rest two... Thanks Ankit bro..

[Prasad Kharkar]

@ Devaka sir
I will definitely use only English for javaranch
and "Zindabad" is a hindi word
@ Shaad
who is satish here?

[mohammad shaid]

oops.. sorry.. it was prasad... so just got lost into solving this problem.. sorry prasad.. and thanks for correcting,, hehe

[mohammad shaid]

hey prasad i gt only 2 objects for GC .. which one's the third?

Is it the d1 object?.. i mean d2.d.d was made to refer to d1.d
so is the object that initially was referring to d1.d will get GCed as the last object?

[Prasad Kharkar]

  
                                              d2
                                              |
        GC                 GC                 V
   +-----------+      +-----------+      +-----------+
   |    d      |<-----------d     |      |    d      |
   +----|------+      +-----------+      +----|------+
        |                                     |
        |               +---------------------+
   d1   |               |
   |    |               |
   V    V               V                  GC
+-----------+      +-----------+      +-----------+
|    d------------>|    d      |      |   d=null  |
+-----------+      +----|------+      +-----------+
                        V
                   +-----------+      +-----------+
                   |    d------------>|   d=null  |
                   +-----------+      +-----------+
  

[mohammad shaid]

got it.. got it.. got it.. didnt see that word GC.. Thanks prasad..

[Prasad Kharkar]

  
                                              d2
                                              |
    (2)   GC                 GC                 V
   +-----------+      +-----------+      +-----------+
   |    d      |<-----------d     |      |    d      |
   +----|------+      +-----------+      +----|------+
        |                          (1)            |
        |               +---------------------+
   d1   |               |
   |    |               |
   V    V               V                  GC (3)
+-----------+      +-----------+      +-----------+
|    d------------>|    d      |      |   d=null  |
+-----------+      +----|------+      +-----------+
                        V
                   +-----------+      +-----------+
                   |    d------------>|   d=null  |
                   +-----------+      +-----------+
  

d2.d and d1.d.d are now pointing to the same object isn't it?
thus cancelling the link between object poited by d2 and object number 2
now 2 was referring to object referred by 1 but as 2 has no external reference
1 also does not have any reference
hence the object 1, 2 are eligible for GC and 3 was done already
hope this helps now

[mohammad shaid]

yea.. got it prasad.. Thanks

[Devaka Cooray]

Prasad Kharkar wrote:"Zindabad" is a hindi word

Not an Urdu word?

[Sahil Kapoor]

Thanks to Devaka Sir

Before reading this Topic even i dint knw that "Zindabad" is an Urdu word.

[Sandy Ghosh]

Every thing is fine but line 4 must be

this.d = (Drag) d

Otherwise it won't complile....

[Ankit Garg]

Sandy Ghosh wrote:Every thing is fine but line 4 must be
this.d = (Drag) d
Otherwise it won't complile....

Yes this has been pointed out before by Prasad...

[Akil Kumar]

Ankit,

I am still not able to correlate the program with the sketch. Can you please explain? I really appreciate your help.

Thanks

[Ankit Garg]

Akila is there a specific line that's confusing you or the whole question??

[Akil Kumar]

Ankit,

I am not able to understand the whole question. While giving the exams I always get this kind of questions wrong. Sorry, you must have explained many times about this in this forum. This is what I inferred from the diagram

a. When you say d1 --> V, d2 ---> V you are referring to the instance variables
b. At line 7 it creates three new objects out of which one is passed null.
c. I am not understand the line 8
d. At line 9 again 3 objects gets created out of which the third one is passed with the reference d1.
e. At line 10 d2 is referring to d1.

I am not able to connect all these and find out how many objects are eligible for GC.

Thanks

[Martin Vanyavchich]

I thnik that at code line 8, this is the memory structure. The last in our chain of elements now points to a new Drag(null) element. Please correct me if I'm wrong.

   
   d1   
   |    
   V                                       GC
+-----------+      +-----------+      +-----------+
|    d------------>|    d      |      |   d=null  |
+-----------+      +----|------+      +-----------+
                        V
                   +-----------+      +-----------+
                   |    d------------>|   d=null  |
                   +-----------+      +-----------+

[Prasad Kharkar]

that is not V
that is downward arrow

[Ankit Garg]

Akila Agandeswaran wrote:a. When you say d1 --> V, d2 ---> V you are referring to the instance variables

Well actually that V in my diagrams is supposed to mean an arrow. So when I draw this

    a
    |
    V
+--------+
|        |
+--------+

It means that a is a reference variable. The | and V is the arrow (just like you drew ---> ). The box is the actual object to which the reference variable a points. I've drawn these types of diagrams so many times that I assume that everyone understands them. Anyways I hope you can understand the diagram better now.

Akila Agandeswaran wrote:b. At line 7 it creates three new objects out of which one is passed null.

Correct

Akila Agandeswaran wrote:c. I am not understand the line 8

Martin's diagram shows line 8 very well.

Akila Agandeswaran wrote:d. At line 9 again 3 objects gets created out of which the third one is passed with the reference d1.

Correct

Akila Agandeswaran wrote:e. At line 10 d2 is referring to d1.

No. Read it carefully. The line is d2.d = d1.d and not d2 = d1...

[Edit: beaten to the V thing]

[Akil Kumar]

Sorry for mistaking the downward arrow as V. Thanks for your response