Question

I am preparing for OCPJP, and I got stuck at the following mock exam question:

Given:

3. interface Animal { void makeNoise(); }

4. class Horse implements Animal {
5.     Long weight = 1200L;
6.     public void makeNoise() { System.out.println("whinny"); }
7. }

8. public class Icelandic extends Horse {
9.     public void makeNoise() { System.out.println("vinny"); }

10.    public static void main(String[] args) {
11.        Icelandic i1 = new Icelandic();
12.        Icelandic i2 = new Icelandic();
12.        Icelandic i3 = new Icelandic();
13.        i3 = i1; i1 = i2; i2 = null; i3 = i1;
14.    }
15. }

When line 14 is reached, how many objects are eligible for the garbage collector?

A. 0

B. 1

C. 2

D. 3

E. 4

F. 6

Their correct answer is E, i.e. four objects, but I’m not sure why. From my point of view, i2 and its weight will get eligible for garbage collection. Perhaps I’m missing something, please advise.

Answer 1

Lets call Icelandic() on line 11 IceA, line 12 IceB, and so forth.

After creation

i1 = IceA
i2 = IceB
i3 = IceC

After i3 = i1

i1 = IceA
i2 = IceB
i3 = IceA

After i1 = i2

i1 = IceB
i2 = IceB
i3 = IceA

After i2 = null

i1 = IceB
i2 = null
i3 = IceA

After i3 = i1

i1 = IceB
i2 = null
i3 = IceB

So only the Icelandic() created on line 12 remains. Now, each Icelandic() has a Long weight, so IceA and IceC are now unreferenced, meaning 4 objects (IceA, IceA.weight, IceC, IceC.weight) are available for GC.

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Other issues:

1. args is still args, they are not counting going out of scope in this question
2. Long weight is not declared statically, so each instance of the class has a weight object.