Confusion in value passing

[Soumya Manjunath]

Ranchers,

Below is the code snippet.

 
package com.whizlab.trial;

interface I{
	int i = 0;
}

class A implements I{
	int i = I.i + 1;
}

class B extends A{
	int i = I.i + 2;
	static void printAll(A obj){
		System.out.println(obj.i);
	}
	public static void main(String args[]){
		A a = new B();
		B b = new B();
		I i = new B();
		printAll(a);
		printAll(b);
		printAll((A)i);
		}
}

When I ran this program, I got the output 1 1 1. But I was expecting 2 2 2 as the actual object that we pass to the printAll method is B.
When we invoke the method printAll, the parameter obj of the printAll method will have the bit pattern to get to the actual object, that is B.
So when we try to print obj.i, shouldn't it be class B's i value..?

Can someone please help me understand this..?

[Abhay Agarwal]

Hi

It is because instance variable are decided at compile time.

Since method take a reference (obj) of "A" type , it will print 1 everything we invoke printall method

foe eg -

Change method to

static void printAll(I obj){  
	         System.out.println(obj.i);  
	     }

This will print 0 0 0

So , underlying concept to understand from this question is that Instance variables are decided at compile time.
Polymorphic behaviour is not there for instance variables

[Soumya Manjunath]

Thanks for your reply.

I modified the code a little bit.

package com.whizlab.trial;

interface I{
	int i = 0;
	void sayHello();
}

class A implements I{
	int i = I.i + 1;
	public void sayHello(){
		System.out.println(" Class A ");
	}
}

class B extends A{
	int i = I.i + 2;
	public void sayHello(){
		System.out.println(" Class B ");
	}
	static void printAll(A obj){
		System.out.println(obj.i);
		obj.sayHello();
	}
	public static void main(String args[]){
		A a = new B();
		B b = new B();
		I i = new B();
		printAll(a);
		printAll(b);
		printAll((A)i);
		}
}

The output is:

1
Class B
1
Class B
1
Class B

If the printAll method can print Class B, it is because of the polymorphic behaviour, right..? Then why not 2 as the value of obj.i ?

Please explain.

[Ankur kothari]

dude because in the all the cases it is the reference A whose instance variable i is being referred. Because in the printAll method, the parameter is of type A but at runtime the reference A refers to the object B therefore "Hello B" is being printed......


Man i am on fire....

[Siva Masilamani]

You know the variable always depends on reference type.

If you look at the reference type in printAll method it is of type A whose i value is 0

hence it prints 1 in all the cases.

[Ankur kothari]

@ siva: that was simple but much clear