marcus mock exam-- reference

[chao-long liao]

class ValHold{
        public int i = 10;
}
public class ObParm{
    public static void main(String argv[]){
        ObParm o = new ObParm();
        o.amethod();
    }
    public void amethod(){
         int i = 99;
         ValHold v = new ValHold();
         v.i=30;
         another(v,i);
         System.out.print( v.i );
     }//End of amethod
     public void another(ValHold v, int i){
          i=0;
          v.i = 20;
          ValHold vh = new ValHold();
          v =  vh;
          System.out.print(v.i);
         System.out.print(i);
      }//End of another
}

1) 10030
2) 20030
3) 209930
4) 10020
Ans is 4,in my opinion,I think the answer should be 10010,because v = vh, so the
System.out.print( v.i ) should be 10,so what's the problem ?
[ Jess added UBB [code] tags to preserve whitespace -- check 'em out! ]
[ July 13, 2002: Message edited by: Jessica Sant ]

[Amir Ghahrai]

Ok, the thing to remember here is that all variables are passed by value and not by reference. However, when the variable is a reference to an object, then any changes made to the object through that reference will be apparent to other methods. in the class another, you first change the variable i to 20 through the reference v. then you create a new instance of calss ValHold and store the reference to vh which has a new copy of the variable i which has a value of 10. Ofcourse when you change the variable i through the reference v, the change is apparent to the amethod method.
HTH
[ July 13, 2002: Message edited by: Amir Ghahrai ]

[Bindesh Vijayan]

Here is a clearer picture of what Amir said:

class ValHold {
  public int i = 10;
}
public class ObParm
{ 
  public static void main(String argv[])
  {        ObParm o = new ObParm();
       o.amethod();
  }
 
public void amethod() {
        int i = 99;
       ValHold v = new ValHold();       //0
       v.i=30;
       another(v,i);                       // 1 
       System.out.print( v.i );            //5
  }//End of amethod
   public void another(ValHold v, int i) //2
{          i=0;
        v.i = 20;                        //3
     ValHold vh = new ValHold();
         v =  vh;                       //4
         System.out.print(v.i);          
         System.out.print(i);
   }//End of another
}

At 0 you create a reference var and point it to an object created by new operator in memory
The variable v at 2 is a local member of the meth another(), when u call this method you are passing out object (at 1) of the ValHold that you created at 0.This means that v refers to an object that lies in memory created at 0.

Now at 3 when you set i=20, you are changing the value of object reference ,laying in memory, and the one referred (pointed) by v at 0
so at 3 there is a global change to the reference created by new operator at 0
At 4 you set the meth another()'s local variable v to refer to a new object created by new operator, which has i=10 .
When you come out of another() the local var v fails to exist and therefore the new reference that it had no is longer pointed by any reference( a perfect member for gc())
But inside amethod() v still holds the reference of object created at 0 and whoose member was altered at 3.
Semantics remains the same as mentioned by Amir..
Thanks
Bindesh Vijayan