Garbage Collection -K&B book

[neha verma]

Refer to Q12 K&B book Chapter 3-

Given:
1. class Eco {
2. public static void main(String[] args) {
3. Eco e1 = new Eco();
4. Eco e2 = new Eco();
5. Eco e3 = new Eco();
6. e3.e = e2;
7. e1.e = e3;
8. e2 = null;
9. e3 = null;
10. e2.e = e1;
11. e1 = null;
12. }
13. Eco e;
14. }
At what point is only a single object eligible for GC?    

A. After line 8 runs.
B. After line 9 runs.
C. After line 10 runs.
D. After line 11 runs.
E. Compilation fails.
F. Never in this program.
G. An exception is thrown at runtime.
Answer
I feel that the answer should be B. After line runs isn't e3 eligible for garbage collection. As e3 is set to null at line 9 and at line 6 e3.e =null, so e 3 should be eligible after line 9. Can somebody explain.
Thanks.

[Tony Smith]

In this type of question, the safest way to do it with a pencil and paper and draw out the whole thing instead doing it in the head.

I don't know if you mistype something, but you said "at line 6 e3.e =null" but I see line 6 as e3.e = e2;

The answer is G. An exception is thrown at runtime is because of line 8 e2 = null; Once you set any object to null, you can't no longer tough any of it's field. In this case e2.e. That causes the NullPointerException to be thrown.

so originally

e2====>object===>e
but once you do e2=null;

all you get is e2 object===>e You can't no loger get to that object.

[neha verma]

Given:
1. class Eco {
2. public static void main(String[] args) {
3. Eco e1 = new Eco();
4. Eco e2 = new Eco();
5. Eco e3 = new Eco();
6. e3.e = e2;
7. e1.e = e3;
8. e2.e = e1;
9. e2 = null;
10. e3 = null;
11. e1 = null;
12. }
13. Eco e;
14. }

At what point is only a single object eligible for GC?
Hi,I forgot to modify the code earlier. Please look at the modified code I have moved

 e2.e =null 

before we set e2 to null. In the K&B book answers sections it says that with the code above the answer would be

Never in this program

. But I think after line number 10, e3 should be eligible for garbage collection. Please clarify.

[Yogvinder Singh Ghotra]

Based on your last stated code

"e1.e = e3;"

If e3 is set to null..
the object can't be GCed after line 10 as e1.e refers to this Object??

and after 11th line, all three will be eligible for gc. Tony's approach is the right way to go for such questions