garbage collection

[Raju Champaklal]

is system.gc() and finalize in the exam?

 How many objects are eligible for garbage collection when control reaches (1)?
public class Eligible {
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
Eligible obj = new Eligible();
new Eligible();
}
System.gc(); // (1);
}
}
Select the one correct answer.
(a) 0.
(b) 5.
(c) 10.
(d) Hard to say.   

this is from mughals book....

[atluri sandeep]

the answer to the code is c

[Minhaj Mehmood]

d ??

[Jacob Sonia]

The answer to the code should be c i believe, coz all the 10 objects formed are eligible for GC

[Raju Champaklal]

the book says the answer is d......maybe because of the system.gc()..but i dont think system.gc is in the exam

[Kamil Wojcik]

In SCJP 6 the topic of using System.gc() has been removed from the exam!

[Ankit Garg]

System.gc has been removed from the exam, but questions about how many objects are eligible for GC will be on the exam. So the question without considering the System.gc call is valid. The answer should be c according to me...

[Leonardo Carreira]

Sorry, i'm wondering about, is there any Java function that could be executed to check how many objects in JVM?..
Perhaps, if there's, it's easier to know the correct answer..

Yes, i think the answer to the code is C, because in the second new Eligible();, refer to null reference..
Although in the code there's looping and it would execute new Eligible(); 10 times, but only 5 objects has the same reference, (so only one object would be created from this looping), and the 5 other objects has no reference..
these 10 objects is eligible for GC..

    
public class Eligible {
  public static void main(String[] args) {
     for (int i = 0; i < 5; i++) {
       Eligible obj = new Eligible(); //this code will create one object which has reference
       new Eligible(); //this code will create one object which has no reference
     }
   System.gc(); // (1);
  }
}

[Ankit Garg]

Sorry, i'm wondering about, is there any Java function that could be executed to check how many objects in JVM?..
Perhaps, if there's, it's easier to know the correct answer.

The question is not about how many objects are garbage collected, its about how many are eligible for GC. This can be figured out from the code easily. If you just want to check how many objects are garbage collected, add a finalize method to the class like this

public class Eligible {
	public static void main(String[] args) {
		for (int i = 0; i < 5; i++) {
			Eligible obj = new Eligible();
			new Eligible();
		}
		System.gc(); // (1);
	}
	protected void finalize() {
		System.out.println("released");
	}
}

Adding a finalize method is a way of seeing how many objects were garbage collected. But this doesn't prove how many objects are eligible for GC, actual number of objects eligible for GC might be more than the number of objects garbage collected...

[Leonardo Carreira]

Thanks Ankit Garg for your reply..

as what Raju Raju Champaklal said, the book says the answer is D //10 objects are eligible for GC..
in this case, the number of objects were garbage collected same with the number of objects are eligible for GC..
so.., what's the best way to know the number of objects are eligible for GC?..
is it only through our logic?..

Thanks..

[Ankit Garg]

Leonardo Carreira wrote:in this case, the number of objects were garbage collected same with the number of objects are eligible for GC..

How can you say that?? If you run my program, most of the times you'll get the "released" text 10 times, but its not necessary. Not all objects will be garbage collected every time. 10 objects are eligible for garbage collection, but how many of them will actually be garbage collected before the program ends (i.e. JVM shuts down) cannot be predicted.

what's the best way to know the number of objects are eligible for GC?..
is it only through our logic?..

As far as I know, that is the only way of finding the number of objects eligible for GC i.e. using logic...

[Leonardo Carreira]

Ankit Garg wrote:
How can you say that?? If you run my program, most of the times you'll get the "released" text 10 times, but its not necessary. Not all objects will be garbage collected every time. 10 objects are eligible for garbage collection, but how many of them will actually be garbage collected before the program ends (i.e. JVM shuts down) cannot be predicted.

Okay, Thanks Ankit Garg..