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Thread: Question about garbage collection

  1. #1
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    Default Question about garbage collection

    Dear programmers

    While navigating in the internet I have seen the following Question:

    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?

    Answer: 2

    Maybe someone can tell me why the answer is 2?
    I seems to me that the answer should be 1
    (only c2 is eligible)


  2. #2
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    Default Re: Question about garbage collection

    I agree with you. It looks like here is error in problem statement or proposed answer. The same problem is several times mentioned on the web and it looks with the same considerations...

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    Default Re: Question about garbage collection

    Thanks a lot for the answer

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    Default Re: Question about garbage collection

    No, I think the answer is correct.
    We start off by initializing c1 to some C object (we'll call this: original_c1).
    We then initialize c2 by calling the m1() method. The m1 method takes c1 as the parameter and changes its reference to some new C object, then returns the new c1 value, which sets c2 to be equal to c1's new value (original_c2). Since c2 and c1 are now equal, there are no more references left to original_c1, so that can be garbage collected.
    Then we initialize c3 to some C object.
    Lastly, we set c2 to be equal to c3. So no more references to original_c2 exist, and that can also be garbage collected.

    So I agree with 2, and the two objects eligible for garbage collection are c1 and c2's original initialization values.

    EDIT: or maybe not if c1 is still referencing original_c2...
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    Default Re: Question about garbage collection

    Prove it to yourself - there is a method in the API which is "Called by the garbage collector on an object when garbage collection determines that there are no more references to the object." One could use this method, along with a gc call to support any claims as to how many objects of class C are eligible

    class C{
    	public static void main(String a[]) throws Exception{
    		C c1=new C();
    		C c2=m1(c1);
    		C c3=new C();
    		c2=c3; //6
    		System.gc();
    		Thread.sleep(1000);
    		anothermethod();
    	}
     
    	static C m1(C ob1){
    		ob1 =new C();
    		return ob1;
    	}
     
    	@Override
    	public void finalize(){
    		System.out.println(toString() + " Finalized");
    	}
    	static void anothermethod(){
     
    	}
    }

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    Default Re: Question about garbage collection

    I ran the code (with minor modifications). The object created in the m1 method is the only one that gets garbage collected. I was incorrect when I said the c1 variable's reference would change (that's really obvious to me to be wrong now, I don't know why I thought it would).
    If you take out line 6, nothing gets garbage collected.
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    Default Re: Question about garbage collection

    Aussiemcgr and Copeg thanks for helping me.
    I have also tested the code with finalize and saw that the garbage collection removed only one object.

    I have learnt something new.

    Thanks a lot

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