Garbage Collection

[Henry Zhi Lin]

I got the following from exam lab,

  class A{
          A a;
          public A(A a){
               this.a=a;
          }
          public static void main(String args[]){
               A a1=null;
               A a2=new A(new A(null));
               A a3=new A(a2);
               a1=a3;
               a1.a=new A(null);
               a2.a=null;  //Line12
               System.gc();
          }
     }  

How many object can be garbage collected here?

My answer is 0, but the correct answer is 1.

I am so confused here and find there is no alternative way to solve problem like this. Can anyone tip me how to solve such gc problem?

Thanks.

[Henry Wong]

Let's name the object OA, OB, OC, OD, etc., as they are being created. This is to distinguish them from the reference that points to it. Furthermore, we will follow the instance variable that is part of these objects, to draw all the connections.

---------------------
After...

A a1=null;
A a2=new A(new A(null));
A a3=new A(a2);

You will get...

a1 --> null
a2 -------> 0A --> OB --> null
            ^
            |
a3 --> OC --| 
   

---------------------
After ...

a1=a3;

You will get...

a2 -------> 0A --> OB --> null
            ^
            |
a3 --> OC --|
       ^
       |
a1 ----| 
   

---------------------
After ...

a1.a=new A(null);

You will get...

a2 -------> 0A --> OB --> null
a3 --> OC --> OD --> null
       ^
       |
a1 ----| 
   

----------------------
After ...
a2.a=null;

You will get ...

a2 -------> 0A --> null
a3 --> OC --> OD --> null
       ^
       |
a1 ----| 
   

And as you can see, the B object is now unreachable.

Henry

[Henry Zhi Lin]

Thanks Henry,

You method are really cool. It is very clear what really happens.