Self_Exam Chapter 3 SCJP Number 11

[Ted North]

Here is my impression of which objects are created and how the references relate to them:

class Beta { }
 class Alpha {
 static Beta b1;
 Beta b2;
 }
 public class Tester {
 public static void main(String[] args) {
 Beta b1 = new Beta(); Beta b2 = new Beta();
 Alpha a1 = new Alpha(); Alpha a2 = new Alpha();
 a1.b1 = b1;
 a1.b2 = b1;
 a2.b2 = b2;
 a1 = null; b1 = null; b2 = null
// do stuff
 }
 }

When line 16 is reached, how many objects will be eligible for garbage collection?
A. 0
B. 1
C. 2
D. 3
E. 4
F. 5

I selected D

The answer says that "B is correct. It should be clear that there is still a reference to the object referred to by a2, and that there is still a reference to the object referred to by a2.b2. What might be less clear is that you can still access the other Beta object through the static variable a2.b1 - because it's static" (K & B, 2000-something, p. 285).

What am I missing here? 0_o

*** Also which b1 and b2 are set to null? The class variable of Alpha and its instance variables or the variables declared in main and their instances?

[Henry Wong]

Can you explain the diagram? Some parts of it doesn't make sense, like how does an object variable point to a local variable?

Henry

[Praveen Kumar M K]

Ted,

I agree with the answer if the GC check was at line 14(I would suggest you take a crack at it again!), but line 16 looks like out of scope of main() method...so shouldn't all the 4 objects be available for GC at line 16?

Regards,
Praveen.

[Praveen Kumar M K]

You could try with this, it will give more clarity.

class Beta { }
 class Alpha {
 static Beta b1;
 Beta b2;
 }
 public class Tester {
 public static void main(String[] args) {
 Beta bee1 = new Beta(); Beta bee2 = new Beta();
 Alpha a1 = new Alpha(); Alpha a2 = new Alpha();
 a1.b1 = bee1;
 a1.b2 = bee1;
 a2.b2 = bee2;
 a1 = null; bee1 = null; bee2 = null
// do stuff
 }
 }