Re: Difference between compile time and runtime reference/objects

From:

"Matt Humphrey" <matth@iviz.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 10 Mar 2008 15:25:18 -0400

Message-ID:

<OrydnWk_yMu1FkjanZ2dnUVZ_vumnZ2d@comcast.com>

"lielar" <lielar@gmail.com> wrote in message
news:fef5cfcd-99a4-4897-b0d4-d66bc120517d@i12g2000prf.googlegroups.com...

Hi

I have the following code
-------------------------------------------------
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) {
          B b = new B();
          A a = new B();
          printAll(a);
          printAll(b);
     }

}

You have 2 instance variables both called i, which is almost always a
mistake. printAll() selects the instance variable based on the compile-time
type of the expression, so it always hits the one in A.

You can make printAll print '2' for B by allowing it dispatch through
object. You don't have to overload any methods to do this.
1) add getI () { return i; } to class A
2) replace the duplicate declaration of "i" in B with i = I.i + 2 in its
constructor
3) Make printAll use the method
System.out.println (obj.getI ())

Cheers,
Matt Humphrey http://www.iviz.com/