objects eligible for gc

[Edward Lawrence]

Why isn't c3 eligible for garbage collection in the code below? Doesn't it equate to null after c1.go(c2) returns?

The question below was found in the book: SCJP Sun Certified Programmer for Java 6 Study Guide (Exam 310-065), the first self test question of Chapter 3.

QUESTION:- When // doStuff is reached, how many objects are eligible for GC?

class CardBoard {
  Short story = 200;
  CardBoard go(CardBoard cb) {
    cb = null;
    return cb;
  }
  public static void main(String[] args) {
    CardBoard c1 = new CardBoard();
    CardBoard c2 = new CardBoard();
    CardBoard c3 = c1.go(c2);
    c1 = null;
    // do Stuff
} }

[Kevin Workman]

What exactly would be GC'd if c3 *were* eligible?

[Edward Lawrence]

If c3 *were* eligible, I believe that nothing would be guaranteed for GC. However, c1 with its associated Short wrapper and c3 with its associated Short wrapper would be eligible. There would be 4 objects eligible for garbage collection. I am probably totally wrong. Can anyone provide a little insight?

[Paul Clapham]

Here's the insight you need: Garbage collection applies to objects, not to variables. So asking whether a variable can be garbage collected is meaningless. Look at the objects.

[Edward Lawrence]

I was thinking on the line of your insight until I read the following answer from the book:-

" Only one CardBoard object (c1) is eligible, but it has an associated Short wrapper object that is also eligible." The Short is a wrapper object.

[fred rosenberger]

c3 isn't an object, so it's eligibility for GC is always 'no', whether it points to null or something else.

[Edward Lawrence]

Thanks Fred. That is the fact/concept I have been missing.