Is an Object eligible for GC even if its instance variable is references to another variable?

Question

I'm studing for the Java Certification 1Z0-803 and I hava a doubt about garbage collection:

import java.util.*;

class X {
    List<String> list = new ArrayList<>();
}

public class TestGC {
    // Is an Object eligible for GC even if its instance variable is references to another variable
    public static void main(String[] args){
        X x = new X(); // 1
        List<String> list = x.list;
        x = null; // 2, Is X object reference eligible for garbage collection here?

        list.add("a");
        list.add("b");
        list.add("c");

        for(String item : list) {
            System.out.println(item);
        }
        list = null;// 3, Or X object reference eligible for garbage collection here, after list is set to null
    }
}

x is referencing the object X created at the position 1.
This class X has a instance variable of the type List.
If I referenced the instance variable list on x of the type X in a local variable list and then set x to null, the object referenced for x will be eligible for GC in this line (position 2) or because I'm referencing an instance variable of this object, this object only be eligible for GC when its instance variable does not have anything reference to it (position 3)?

Just a side note: at point 3, the `ArrayList` is eligible for garbage collection, even if you don’t set `list` to `null`, as the variable’s scope ends there. But even if the variable had a larger scope, the fact that it is never read afterwards, makes the object unreachable. If you are interested in this subtle aspect, read [“finalize() called on strongly reachable object…”](http://stackoverflow.com/q/26642153/2711488). Since your code actually does nothing, an optimizing execution environment could collect the objects even earlier or not create them in the first place. — Holger, Sep 14 '16 at 14:49

score 7 · Accepted Answer · answered Sep 13 '16 at 14:29

7

Initially, you have

stack --> x --> list

x is thus reachable from the stack.

Then you have

stack --> x --> list
     \          /
      \--------/

x is still reachable from the stack, and list is reachable, too, through x, and through the local variable on the stack

Then you set x to null, so you have

 stack      x --> list
     \          /
      \--------/

And you thus see that there is now way to reach x anymore from the stack. The fact that there is a path from x to the list is irrelevant. So the VM is allowed to collect x:

 stack           list
     \          /
      \--------/

answered Sep 13 '16 at 14:29

JB Nizet

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Great answer. It took me more time to think this through than it took you to create those nice little ascii arts. If you dont mind asking: did you "draw" them manually? – GhostCat Sep 13 '16 at 14:31
@GhostCat yes. Hand-made craftamanship :-) – JB Nizet Sep 13 '16 at 14:46
Thank you so much. Your explanation is very clear. – Erick Alves Sep 13 '16 at 19:38

score 2 · Answer 2 · answered Sep 13 '16 at 14:26

It will be at 2. You aren't referencing the variable of x, you are referencing the List. The fact that x also has a reference to the List does not affect its eligibility for garbage collection. The List is not "inside" x, only a reference to the List is.

The List itself, of course, is not eligible for garbage collection because it is still referenced in the main, but the List and x are independent objects.

score 1 · Answer 3 · answered Sep 13 '16 at 14:24

1

Position 2, because at this point the X object is no longer reachable.

answered Sep 13 '16 at 14:24

Welbog

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Is an Object eligible for GC even if its instance variable is references to another variable?

3 Answers3