Question from Practice exams

[Larsen Raja]

Hi,

public class Fiji {
    static Fiji base;
    Fiji f;
    public static void main(String[] args) {
        new Fiji().go();
        // GC point 1
    }
    void go() {
        Fiji f1 = new Fiji();
        base = f1;
        Fiji f2 = new Fiji();
        f1.f = f2;
        Fiji f3 = f1.f;
        f2.f = f1;
        base = null; f1 = null; f2 = null;
        // GC point 2
`  }
}

The answer is:
At GC point 1 -> 3 objects are eligible for GC.
At GC point 2 -> 0 objects are eligible for GC.

I understand, second anser. But I feel the first answer should be => 2 objects are eligible for GC.

Larsen.

[Stephan van Hulst]

Nope. The answer is 3. Why do you think it should be 2?

[Anayonkar Shivalkar]

Hi Larsen,

At GC point 2, there are two object eligible for GC (f1 and f2), so, option 2 is not the answer.

Now, GC point 1 comes after GC point 2, and at GC point 1, another object is eligible for GC(the one created by new Fiji()), so, at GC point 1, three objects are eligible for GC.

[Stephan van Hulst]

Well, at point 2 there are no object eligible, because the two objects created in the go() method are still reachable. As soon as the method returns though, they become unreachable.

You're correct about point 1 though.

[Anayonkar Shivalkar]

Hi Stephan,

At last statement of method go()

base = null; f1 = null; f2 = null;  

So, both objects created in method go() are now unreachable, and hence eligible for GC, right?

[Stephan van Hulst]

No, f3 still has a reference to the object that was referenced by f2, and that object's f field references the object that was initially assigned to f1.

[Anayonkar Shivalkar]

Yes Stephan, you are right. I missed it

f3 refers to f1.f, so f1 is not eligible for gc. Further, f1.f refers to f2, so f2 also is not eligible for gc. So... is it that both options are correct (i.e. no object is eligible for gc at GC point 2)?

Need to brush up gc fundamentals again

[Larsen Raja]

Got it. Just ignored the simple fact. Since nothing is returned from the method, all the objects created in the method should be available for GC. Additionally, nothing is persisted in the static field. Thank you guys.


Larsen.