Pass by value/ Pass by reference

[Rex Isaacs]

Hi,

I've a question regarding Pass by value/Pass by reference.

Please see the sample I tried.

class PassbyValue{
 
    int price,value;
    public PassbyValue(int i, int j){
      System.out.println("i :"+i+" j :"+j);
 
      price = i;
      value = j;
 
    }
    public int getPrice(){
        return price;
    }
    public int getValue(){
      return value;
    }
    public void setPrice(int pr){
      price = pr;
    }
    public void setValue(int va){
      value = va;
    }
}
 
public class PbvTest{
 
    void callme(PassbyValue ppbv){
 
        System.out.println(ppbv.getPrice()+ " "+ppbv.getValue());
        ppbv.setPrice(300);
        ppbv.setValue(400);
    }
 
    public static void main(String args[]){
 
      PassbyValue pbv = new PassbyValue(100,200);
      PbvTest pbt = new PbvTest();
      pbt.callme(pbv);
      System.out.println(pbv.getPrice() + "  "+pbv.getValue());
    }
}    

Here after calling the method callme() with pbv as the object, when I print the value of Price and Value, it prints 300 and 400 instead of 100,200.

My doubt is that if it is pass by value, the value should be 100,200 no? Can anybody explain why ?

Thanks,
Rex

[Chandra Bhatt]

Hi Rex,

Follow the link
Pass By Value or Pass By Reference

[Meena R. Krishnan]

You are passing the address and not the actual value. Any changes made to the object at that address is global.

[marc weber]

The value being passed is a copy of the reference.

Within main, the original reference is assigned to the local variable pbv. Within callme, a copy of that reference (i.e., a value) is assigned to the local variable ppbv. However, both of these point to the same object. So either reference can be used to change the object's state, and this change will be "visible" through the other reference.

On the other hand, if you reassigned ppbv to point to a different object...

void callme(PassbyValue ppbv) {
  System.out.println(ppbv.getPrice()+ " "+ppbv.getValue());
  ppbv = new PassbyValue(500, 600); //points to new object
  ppbv.setPrice(300);
  ppbv.setValue(400);
}

[Neo Phesus]

Hi Rex,

Could you please explain why did you think it should be 100 and 200?? and what difference does it make incase of a pass by value??

Regards,

[Neo Phesus]

OOps !! Sorry..!! I did not check the replies

[Meena R. Krishnan]

Also refer to the following thread for an explanation given by Fred Rosenberger w.r.to reference passing.

http://www.coderanch.com/t/262603/java-programmer-SCJP/certification/Garbage-collection-objects

[Zhao zhenhua]

Hello,Rex Isaacs:
Acturally,when you get "PassbyValue pbv = new PassbyValue(100,200);" that create an object and initialize the two variable and set Price=100 and Value=200;but when get "callme()" the reference is the "object",following

void callme(PassbyValue ppbv){
System.out.println(ppbv.getPrice()+ " "+ppbv.getValue()); ppbv.setPrice(300); ppbv.setValue(400); }

Here after calling the method callme() with pbv as the object, the value of Price and Value had been reset, so it prints 300 and 400 instead of 100,200.

If I got any wrong ,please tell me anyone about my answer include my caluse syntax.

[Rex Isaacs]

Thanks all for your valuable replies to make me understand the concept.

I got it now.

[ April 30, 2007: Message edited by: Rex Isaacs ]