Basics on how Garbage collector works.

[Tabassum Momin]

Hello everyone,
I came across this question on net.

class C
{
public static void main(String a[])
{
C c1=new C();
C c2=m1(c1);
C c3=new C();
c2=c3; //6
anothermethod();
}
static C m1(C ob1){
ob1 =new C();
return ob1;
}
}
After line 6, how many objects are eligible for garbage collection?

--> I feel that answer of this question must be 1. I want to know whether I am correct or not.

[Ulf Dittmer]

Not a direct answer, but maybe this helps: https://coderanch.com/t/266329/java-programmer-SCJP/certification/garbage-Collector. It's certainly fun :-)

[Tabassum Momin]

Thank You for sharing that link. It was really helpful and interesting too...
As far, what I understood from that discussion is :
-->The object created within m1() method is the only object ready for garbage collection after the execution of line 6.

[Winston Gutkowski]

Tabassum Momin wrote:As far, what I understood from that discussion is :
-->The object created within m1() method is the only object ready for garbage collection after the execution of line 6.

Sounds right to me.

Basically, an object is available for garbage collection as soon as there is no reference in scope that points to it. And in order for a reference to be "in scope" (and I'm not sure if that's the correct term, but it's the way I always think of it) it has to either:
(a) have a name that can be used.
(b) be returned from a method as part of a compound statement, in which case it will be in scope for as long as it's needed in the statement.

For example:
BigInteger cube256 = BigInteger.valueOf(256L).pow(3);
returns an unnamed BigInteger object from valueOf(), which is then used to return its cube (a different object). The unnamed object will be eligible for gc as soon as the pow() method has finished, but the one assigned to cube256 will only be eligible when either:

cube256 is reassigned, or

no piece of code can "see" code256 any more.

Quite honestly, unless you need this information for passing the SCJP exam, it's rarely something you need to obsess about. If you follow good programming practises and avoid using lots of static fields, the chances of you holding on to objects for a lot longer than you need to is fairly slim.
One exception is Closeable objects (see java.io.Closeable), which you should remember to close() as soon as you're finished with them. And if you're using version 7, there is the new try-with-resources statement that helps you out with that stuff.

HIH

Winston

[Tabassum Momin]

Thank You. I am right now, preparing for SCJP exam, your suggestion will help me to focus on right path.
It is very well explained and had cleared all my doubts

[Winston Gutkowski]

Tabassum Momin wrote:Thank You. I am right now, preparing for SCJP exam, your suggestion will help me to focus on right path.
It is very well explained and had cleared all my doubts

Glad to help.

Winston