How many object is eligible for garbage collection?

[Morteza Manavi-Parast]

This is from Devaka's diagnostic exam. I think it should be only one , but the correct answer is two object is eligible for garbage collection. I wonder where is the second object?

   class A{
     A aob;
     public static void main(String args[]){
          A a=new A();
          A b=new A();
          A c=new A();
          a.aob=b;
          b.aob=a;
          c.aob=a.aob;
          A d=new A().aob=new A();
          c=b;
          c.aob=null;
          System.gc();
     }
} 

[Devaka Cooray]

Hello Morteza,

This question is asked many times in this forum. Please read this topic.

Devaka.

[Ruben Soto]

Yeah, this is a tricky question if you are not thinking about operator precedence, and the fact that assignment has right associativity.

A d=new A().aob=new A();
is equivalent to
A d = ((new A().aob) = new A());

[Shin Kudo]

the trick here is after the line:

A d=new A().aob=new A();

the annonymous inner class object becomes eligible for GC.

[Morteza Manavi-Parast]

Thank you!

And just to clear any doubt, there is not any "annonymous inner class" invloved. The pitfall of this question, like already is said, is the right associativity of assignment operator. That's all!

[Ruben Soto]

Morteza Manavi-Parast wrote:Thank you!

And just to clear any doubt, there is not any "annonymous inner class" invloved. The pitfall of this question, like already is said, is the right associativity of assignment operator. That's all!

You are correct, Morteza.

I hope your studies are going well. I (and I'm sure many other people) have missed your insightful posts (the one about inner classes was a classic.)

[Ankit Garg]

Shin Kudo wrote:the trick here is after the line:

A d=new A().aob=new A();

the annonymous inner class object becomes eligible for GC.

Where is the anonymous inner class in this?? There is just a temporary intermediate object with no reference which becomes eligible for GC (I've marked it in bold). I hope this doesn't goes over everyone's heads

A d=new A().aob=new A();

[yen hoang]

Ankit Garg wrote:

Shin Kudo wrote:the trick here is after the line:

A d=new A().aob=new A();

the annonymous inner class object becomes eligible for GC.

Where is the anonymous inner class in this?? There is just a temporary intermediate object with no reference which becomes eligible for GC (I've marked it in bold). I hope this doesn't goes over everyone's heads

A d=new A().aob=new A();

You can say that again! There is nothing called anonymous inner class. Exactly, it's temporary object. And after all, there are two objects legal for GC. Right?

[Kieren Dixon]

yen hoang wrote:there are two objects legal for GC. Right?

True

[Ankit Garg]

yen hoang wrote:You can say that again! There is nothing called anonymous inner class.

Well actually I didn't see that people have already pointed it out that there is no anonymous inner class in this code, if that's what you mean. If you want to say that there is no anonymous inner class in this code, then it's fine. If you want to say that there are no anonymous inner classes in java, then I'm afraid you are wrong...

[yen hoang]

Ankit Garg wrote:

yen hoang wrote:You can say that again! There is nothing called anonymous inner class.

Well actually I didn't see that people have already pointed it out that there is no anonymous inner class in this code, if that's what you mean. If you want to say that there is no anonymous inner class in this code, then it's fine. If you want to say that there are no anonymous inner classes in java, then I'm afraid you are wrong...

Yes, I made mix up. I just want to say that in that code, there is not anonymous inner class. Thanks and regards!

[Morteza Manavi-Parast]

Thanks Ruben, I'm happy to hear that you found it useful!
And once again thank you for clearing my doubt on this question.

[NagarajGoud uppala]

Hi,
first of all thanks ruben.clearing the doubt with out any complications.
from the above discussion my understanding is, when we call the gc() it will remove the
objects which are not reffered by any reference variable and the objects
having the null value.
am i right???

[Ankit Garg]

NagarajGoud uppala wrote:
from the above discussion my understanding is, when we call the gc() it will remove the
objects which are not reffered by any reference variable and the objects
having the null value.
am i right???

Well it is not true. A call to gc() is just a request to the JVM to run the garbage collector. It is not sure if any objects will be garbage collected. This is why questions containing explicit call to gc() have been removed from SCJP 6.

The only thing that is sure about garbage collector is that it will run once before throwing an OutOfMemoryException. Other than that, you can't say when the garbage collector will run. In the exam, you'll only get questions asking how many objects are ELIGIBLE for garbage collection. No one will ask you how many objects are garbage collected as it cannot be determined...

[NagarajGoud uppala]

Hi Ankit,
thanks for your awesome explanation
you mean even gc() run explicitly or implicitly we can't make sure that all eligible objects are removed??
am i right? or correct me