garbage collection

[vini singh]

At what point only a single object is eligible for garbage collection?

class Eco
{
public static void main(String []args)
{
Eco e1=new Eco();
Eco e2=new Eco();
Eco e3=new Eco();
e3.e=e2;
e1.e=e3;
e2=null;
e3=null;
e1=null;
}

Eco e;

}


output is:Never in this program as until last refernce is null none of the objects is eligible and when last refernce is nulled all three are eleigible.

what is meant by last refernce.n xplain the output

[Campbell Ritchie]

That exercise looks familiar; please quote Your Source. And please use the code button to preserve indentation and legibility.

When you get to the end of the method, work out which object the e1 reference points to, then work out what all its fields are, and whether they have any references. Write it on a sheet of paper and that ought to explain your problem.

[Harvinder Thakur]

Vini,

IMHO, you will get prompt answers for such questions in SCJP forum

Anyways, let me try and explain it with a diagram:

After:- Eco e1=new Eco();
Only one object is created on the heap which is referenced by reference variable e1.
        +-----+
e1 ---> | Eco | 
        +-----+
 
After:- Eco e2=new Eco();
Two objects are on the heap which are referenced by reference variables e1 & e2.
        +-----+
e1 ---> | Eco | 
        +-----+
 
        +-----+
e2 ---> | Eco | 
        +-----+
 
After:- Eco e3=new Eco();
Three objects are on the heap which are referenced by reference variables e1 & e2 & e3.
        +-----+
e1 ---> | Eco | 
        +-----+
 
        +-----+
e2 ---> | Eco | 
        +-----+
 
        +-----+
e3 ---> | Eco | 
        +-----+
 
After:- e3.e=e2;
Reference variable e in object e3 is made to refer to object e2. That means now we have two references pointing to object e2.
        +-----+
e1 ---> | Eco | 
        +-----+
 
        +-----+
e2 ---> | Eco | <--- e3.e
        +-----+
 
        +-----+
e3 ---> | Eco | 
        +-----+
 
After:-e1.e=e3;
Reference variable e in object e1 is made to refer to object e3. That means now we have two references pointing to object e3.
 
        +-----+
e1 ---> | Eco | 
        +-----+
 
        +-----+
e2 ---> | Eco | <--- e3.e
        +-----+
 
        +-----+
e3 ---> | Eco | <--- e1.e 
        +-----+
 
After:- e2=null;
Here, e2 is made null but the object to which it was pointing is still accessible through *e3.e*
        +-----+
e1 ---> | Eco | 
        +-----+
 
        +-----+
        | Eco | <--- e3.e
        +-----+
 
        +-----+
e3 ---> | Eco | <--- e1.e 
        +-----+
 
After:- e3=null;
Here, e3 is made null but the object to which it was pointing is still accessible through *e1.e* and notice that the object in the middle can be accessed by *e1.e.e* which is the same as *e3.e*
 
        +-----+
e1 ---> | Eco | 
        +-----+
 
        +-----+
        | Eco | <--- e3.e
        +-----+
 
        +-----+
        | Eco | <--- e1.e 
        +-----+
 
After:- e1=null; 
All three objects are eligible for garbage collection as e1 reference which was allowing a link to all the three objects has been nulled.
        +-----+
        | Eco | 
        +-----+
 
        +-----+
        | Eco | <--- e3.e
        +-----+
 
        +-----+
        | Eco | <--- e1.e 
        +-----+

Hope this helps clear the picture