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  1. #1
    j2me64's Avatar
    j2me64 is offline Senior Member
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    Sep 2009
    Zurich, Switzerland
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    Default how many objects are eligible for gc?

    i'm just preparing for the scjp and i got some understanding problems with the following code

    Java Code:
     * @author j2me64
     * Question 11 p. 284 out of Book SCJP
    class Beta {}
    class Alpha {
    	static Beta b1;
    	Beta b2;
    public class Tester {
    	public static void main(String[] args) {
    		Beta b1 = new Beta();
    		Beta b2 = new Beta();
    		Alpha a1 = new Alpha();
    		Alpha a2 = new Alpha();
    		a1.b1 = b1;
    		a1.b2 = b1;
    		a2.b2 = b2;
    		a1 = null;
    		b1 = null;
    		b2 = null;
                    // do stuff

    the question is: When line 16 (// do stuff) is reached, how many objects will be eligible for garbage collection?

    the right answer is 1. but for me it's not very clear which object is eligible and why there are not more then 1 eligible objects. i think it has to do with the static Beta b1 in the class Alpha. it would be great if somebody can give an explanation.

  2. #2
    DarrylBurke's Avatar
    DarrylBurke is offline Forum Police
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    Madgaon, Goa, India
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    I'd say that the Alpha object previously referenced by the variable a1 is the only object eligible for GC.

    -- a2 isn't nulled
    -- b1 being static is referenced from the Alpha class and remains referenced as long as the class is loaded
    -- b2 is referenced from a2

    It's bad form to access a static member via an instance variable.
    Java Code:
    a1.b1 = b1;
    should be
    Java Code:
    Alpha.b1 = b1;
    I suppose it's allowed for trick questions ;-\



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