Questions on GC from Khalid A.Mughal, Chap 9

[Anuradha Prasanna]

On page 402 & 403 of this book, there are no answers for four GC questions.

1)How many objects are eligible for GC 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)
}
}

I think, 9 objects are eligible for GC. is that right?
but the answers are,
(A)0
(B)5
(c)10

can anyone tell the right answer with explanation?
--------------------------------------------------------------------------------------------------------------------------------------
2)How many objects are eligible for GC when control reaches (1)?

public class Link {
private Link next;
Link(Link next) {this.next = next; }
public void finialize() {System.out.println("X"); }

public static void main(String[] args) {
Link p = null;
for(int i=0; i<5; i++) {
p = new Link(p);
}
System.gc();        //(1)
}
}

I think, 0 objects are eligible for GC. is that correct?

--------------------------------------------------------------------------------------------------------------------------------------

3)How many objects are eligible for GC when control reaches (1)?

public class Elements {
public static void main(String[] args) {
int[] array = new int[4];
for(int i=0;i<4;i++) {
array[i]=i;
}
array[0]=array[1]=array[2]=array[3]=0;
System.gc();              //(1)
}
}

I think, 0 objects are eligible for GC here. Is that right?
--------------------------------------------------------------------------------------------------------------------------------------

4) How many objects are reachable when control reaches (1)?

public class Nullify {
private static void nullify(Integer[] array) { array = null; }

public static void main(String[] args) {
Integer[] array = new Integer[4];
for(int i=0;i<4;i++){
array[i]=i;
}
nullify(array);
System.gc();        //(1)
}
}

I think 5 objects are reachable.(Integer[] obj with reference array & 4 objects pointed to by the references array[0], array[1], array[2], array[3])
is it correct?

[rushikesh sawant]

EDIT:
1) 10 objects eligible for gc, variable "obj" is block scoped.
2) 0 objects eligible for gc.
3) 0 objects.
4) 0 object eligible for gc, wrappers are immutable.

i think these are the answers.

[Ankit Garg]

1) There are 10 objects eligible for GC. obj is declared inside the loop, so its scope is inside the loop, so after the loop is completed, it goes out of scope, thus the object referenced by it is also eligible for GC.
2) I also think 0 objects are eligible for GC.
3) Again I also think 0 objects are eligible for GC. There is actually only one object here which is the int array, and it doesn't goes out of scope or is not set to null.
4) I think 0 objects are eligible for GC. Read this for more info...

[Beaten by just 1 second ]

[rushikesh sawant]

hey ankit i think in last one, 1 object should be eligible for gc. is it right?
EDIT:
oh got it.

[Ankit Garg]

rushikesh sawant wrote:hey ankit i think in last one, 1 object should be eligible for gc. is it right?

Try this modified code

public class Nullify {
	private static void nullify(Integer[] array) { array = null; }

	public static void main(String[] args) {
		Integer[] array = new Integer[4];
		for(int i=0;i<4;i++){
			array[i]=i;
		}
		nullify(array);
		for(int i=0;i<4;i++){
			System.out.println(array[i]);  //prints 0,1,2,3
		}
		System.gc();        //(1)
	}
}

[rushikesh sawant]

yea.
um, what about second one? why 0? i think 3.

[Ankit Garg]

rushikesh sawant wrote:um, what about second one? why 0? i think 3.

Again try this modified code

public class Link {
	private Link next;
	String s;
	Link(Link next, String s) {this.next = next;this.s = s; }
	public void finialize() {System.out.println("X"); }

	public static void main(String[] args) {
		Link p = null;
		for(int i=0; i<5; i++) {  //4 Link objects created here
			p = new Link(p,String.valueOf(i));
		}
		Link temp = p;
		while(temp.next != null) {   //lets traverse the elements inside p (alias temp), this loop
                                     //will run 4 times proving that none of the elements was eligible for GC
			System.out.println(temp.s);
			temp = temp.next;
		}
		System.gc();        //(1)
	}
}

[rushikesh sawant]

Thanks for clarification.

[Abimaran Kugathasan]

Interesting......

[Anuradha Prasanna]

hi all,

thanks for the clarification!

but in question 4, the real question is how many objects are reachable? and NOT "HOW MANY OBJECTS ARE ELIGIBLE FOR GC?"
"I think 5 objects are reachable.(Integer[] obj with reference array & 4 objects pointed to by the references array[0], array[1], array[2], array[3]) " --> THAT MEANS NO OBJECTS ARE ELIGIBLE FOR GC, but 5 objects are reachable!

correct me if iam wrong!

[rushikesh sawant]

yea, 5 objects are reachable.

[Ankit Garg]

AFAIK, the real exam won't ask you how many objects are reachable. Knowing how many objects are reachable automatically tells us how many objects are eligible for GC, so the question is fair, but in the real exam you don't have to worry about whether question is asking for objects reachable or eligible...