garbage collection in java

[Vidya sagari]

class CardBoard {
	Short Story = 200;

	CardBoard go(CardBoard cb) {
		cb = null;
		return cb;
	}

	public static void main(String args[]) {
		CardBoard c1 = new CardBoard();
		CardBoard c2 = new CardBoard();
		CardBoard c3 = c1.go(c2);
		c1 = null;
	}

}

how many object here are eligible for garbage collection? to this question answer is 2. can somebody please explain how it counts up to two.
I can understand that c1 is one what is the other one and how. please help

[Ganish Patil]

Vidya sagari wrote:how many object here are eligible for garbage collection?

after what line of code? please always Quote Your Sources

[Vidya sagari]

Vidya sagari wrote:

class CardBoard {
	Short Story = 200;

	CardBoard go(CardBoard cb) {
		cb = null;
		return cb;
	}

	public static void main(String args[]) {
		CardBoard c1 = new CardBoard();
		CardBoard c2 = new CardBoard();
		CardBoard c3 = c1.go(c2);
		c1 = null;
//do Stuff
	}

}

after doStuff is reached, how many object here are eligible for garbage collection? to this question answer is 2. can somebody please explain how it counts up to two.
I can understand that c1 is one what is the other one and how. please help
[it is a question from OCPJP practice questions]

[Ganish Patil]

At line code

c1 = null

object whose reference was stored in reference variable c1 will be eligible for GC.

If there is no code that uses reference of object of CardBoard stored in c2 reference variable then at line where comment //doStuff is written, an object whose reference is stored in reference variable c2 will also be eligible for GC.

So conclusion is at line //doStuff two objects will be eligible for GC.

[Ganish Patil]

See this diagram

[Dave Tolls]

Only two objects are created and, at the comment line, only one is eligible, surely.
Where does the reference 'c2' stop pointing at its original object?
At '//do stuff' it's still active.

ETA:
Ah...missed the Short attribute.

[Ganish Patil]

Dave Tolls wrote:Where does the reference 'c2' stop pointing at its original object?
At '//do stuff' it's still active.

I think I should have said at line 15 where the main method ends

[Joe Bishara]

Ganish Patil wrote:At line code

c1 = null

object whose reference was stored in reference variable c1 will be eligible for GC.

I agree with this

Ganish Patil wrote:...at line where comment //doStuff is written, an object whose reference is stored in reference variable c2 will also be eligible for GC

I don't agree with this.

The program creates 4 objects:

the object referenced by c1 (a CardBoard object)

the object referenced by c1.Story (a Short object)

the object referenced by c2 (a CardBoard object)

the object referenced by c2.Story (a Short object)

At line //doStuff, the 2 objects that are eligible for GC are:

the object referenced by c1 (a CardBoard object) - because c1 is null

the object referenced by c1.Story (a Short object) - due to island of isolation

Ganish Patil wrote:I think I should have said at line 15 where the main method ends

At line 15 where the main method ends, 4 objects are eligible for GC.

[Ganish Patil]

Joe Bishara wrote:

Ganish Patil wrote:At line code

c1 = null

object whose reference was stored in reference variable c1 will be eligible for GC.

I agree with this

Ganish Patil wrote:...at line where comment //doStuff is written, an object whose reference is stored in reference variable c2 will also be eligible for GC

I don't agree with this.

The program creates 4 objects:

the object referenced by c1 (a CardBoard object)

the object referenced by c1.Story (a Short object)

the object referenced by c2 (a CardBoard object)

the object referenced by c2.Story (a Short object)

At line //doStuff, the 2 objects that are eligible for GC are:

the object referenced by c1 (a CardBoard object) - because c1 is null

the object referenced by c1.Story (a Short object) - due to island of isolation

Ganish Patil wrote:I think I should have said at line 15 where the main method ends

At line 15 where the main method ends, 4 objects are eligible for GC.

I was wrong, completely agreed with you,  

[Ganish Patil]

Mistakes I made,

Completely, I missed It is Short not short so total 4 objects were created.

At //doStuff line second object of CardBoard was still referenced by c2.

Thank you so much Joe Bishara, for correcting me

[Joe Bishara]

You’re welcome  

[Vidya sagari]

SO Do you mean that the answer is actually 4? I am new to java and I don't understand where and how a Short object is created. can you please elaborate on that?

[Joe Bishara]

Vidya sagari wrote:SO Do you mean that the answer is actually 4?

No. The answer to the original question is 2. At the line containing the //doStuff comment, the 2 objects that are eligible for GC are:

the object referenced by c1 (a CardBoard object)

the object referenced by c1.Story (a Short object)

Vidya sagari wrote:I don't understand where and how a Short object is created. can you please elaborate on that?

The source code shows that every CardBoard object has-a Short object.

class CardBoard {
    Short Story = 200;
}

[Vidya sagari]

WOW!!! thank you so much. I finally got the thing.

[Ganish Patil]

Vidya sagari wrote:SO Do you mean that the answer is actually 4? I am new to java and I don't understand where and how a Short object is created. can you please elaborate on that?

See here Short is a wrapper class.

CardBoard has a field named Story whose type is Short so when you create an object of CardBoard then one object of Short class is also gets created.

You can see we created two objects of CardBoard class i.e. c1 and c2 where each of them have one object of Short referred by Story variable, so total 4 objects are created. i.e. Two objects of CardBoard and two objects of Short.

Your question was how many objects are eligible for GC on line where you have comment //do Stuff  so till that line one object of CardBoard is eligible for GC i.e. object of CardBoard referred by c1 and you know that object also contains one object of Short referred by Story variable so this Short object also becomes eligible for GV means total 2 objects are eligible for GC i.e. one object of CardBoard and one object of Short.

Object of CardBoard referred by c2 will not be eligible at line where comment  //do Stuff is, because that second object of CardBoard is still refered by c2 varable. so your answer is 2 objects will be eligible on line where comment //do Stuff is.

[Vidya sagari]

Thank you for explaining