ExamLab question???

[jayalakshmi charugundla]

Hi,

After Line 12 , how many objects are eligible for garbage collection?
The answer is one object.

It is better to explain with the diagram and its really very helpful to me . Actually they told that we have to draw a small picture everytime whenever we see this type questions in the real exam too..

    

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;         //line 12
System.gc();
}
}

[Ankit Garg]

Actually they told that we have to draw a small picture everytime whenever we see this type questions in the real exam too

Who they?? You are not obliged to draw a diagram for these type of questions. If you can figure out the answer without any diagram, then you don't need to draw a diagram. If I have to create a diagram for this question, it would be like this

 at line #11 (A a3=new A(a2);)

  a1=null
         a2                a3
         |                 |
         V                 V
  +--------------+   +--------------+
  |      a       |<---------a       |
  +------|-------+   +--------------+
         |
         V
  +--------------+
  |      a       |
  +--------------+
----------------------------------------------------------------------
at line #12 (a1=a3;)

         a2                a3
         |                 |
         V                 V
  +--------------+   +--------------+
  |      a       |<---------a       |<--a1
  +------|-------+   +--------------+
         |
         V
  +--------------+
  |      a       |
  +--------------+
----------------------------------------------------------------------
at line #13 (a1.a=new A(null);)

          a2                 a3
          |                  |
          V                  V
  +--------------+   +--------------+
  |      a       |   |      a       |<--a1
  +------|-------+   +------|-------+
         |                  |
         V                  V
  +--------------+   +--------------+
  |      a       |   |      a       |
  +--------------+   +--------------+
----------------------------------------------------------------------
at line #14 (a2.a=null;)

          a2                 a3
          |                  |
          V                  V
  +--------------+   +--------------+
  |      a       |   |      a       |<--a1
  +--------------+   +------|-------+
                            |
        GC                  V
  +--------------+   +--------------+
  |      a       |   |      a       |
  +--------------+   +--------------+

[jayalakshmi charugundla]

Ankit, I couldn't understand the program even with this diafram. If you have time, could you explain this program from line 9 (A a1 =null;) to end?

[Ankit Garg]

Lets see this one line at a time

9. A a1=null;    A reference of class A is created an initialized to null.
10. A a2=new A(new A(null));   Here a2 points to an Object of class A. The class A has a field named a and the instance of class A which we pass to the constructor will be set to the field a. We can split this line to understand it better

A a2=new A(null);
A temp = new A(null);
a2.a = temp;
temp = null;

11. A a3=new A(a2);    This one is also not that tough. a3 points to an instance of A and a3.a points to a2.

The rest of the statements should be easy to understand I think...

[jayalakshmi charugundla]

so because of a2.a=null , one class obejct is eligible for gc. And the other object is referanced by a1 and a3. Am I right?

Why you have given two boxes for one object ? You could have given only one that is enough. thats why i didn't understand.

what actually the second box says? tell me by words

[Ankit Garg]

so because of a2.a=null , one class obejct is eligible for gc. And the other object is referanced by a1 and a3. Am I right?

Yes that's right.

Why you have given two boxes for one object ?

Which two boxes??