consider this sample code:
1. public class GC {
2. private Object o;
3. private void doSomethingElse(Object obj) { o = obj; }
4. public void doSomething() {
5. Object o = new Object();
6. doSomethingElse(o);
7. o = new Object();
8. doSomethingElse(null);
9. o = null;
10. }
11. }
When the doSomething method is called, after which line does the Object created in line 5 become available for garbage collection?
A. Line 5
B. Line 6
C. Line 7
D. Line 8
E. Line 9
F. Line 10
Answer: D
why D? it's true that when Line 6 is executed the object created in Line 5 is now referenced by the instance var o and the local var o and when Line 8 is executed the object now is referenced by only the local ref var o, so why the answer is D and what happens after Line 9 is executed?? thanks.
The main reason this question is confusing IMO is that there are 2 variables named o. One is the instance variable o and the other is the local variable o inside method doSomething().
Time instance var o local var o
Before Line 5: null
Line 5: null Object#1
Line 6: Object#1 Object#1
Line 7: Object#1 Object#2
Line 8: null Object#2 <- No more references to Object#1
So on (or after executing) line 8, Object#1 is eligible for collection.
The question is a bit confusing because it is after line 3 is called the second time that the object is eligible because it clears the second reference o.
来源：https://stackoverflow.com/questions/12182383/when-an-object-is-eligible-for-a-garbage-collector