up cast of object

[guo mark]

look at the scope.

class Foo{
public void print(){
System.out.println("this is Foo");
}
}
class Fooer extends Test{
public void print(){
System.out.println("this is Fooer");
}
}
public class RunFoo{
public static void main(String [] args){
SubTest subtest = new SubTest();
Test test = subtest;
test.print();
}
}

I thought it would print "Foo" but in fact it printed "Fooer".It seemed that the Fooer is not up cast to Foo,but why?

[Matthew Phillips]

It is called dynamic binding. When you assign subtest to a variable of type test it does not change the type of the actual object. When you run the print method the JVM runs the print method of the actual object.

[Lionel Siau]

I'm confused. What is SubTest ? A subclass of Test? Your above code seems to be missing something.

[Rajinder Yadav]

You're confused :^)
class Fooer extendes Test not Foo!!!
NICE EXAMPLE CODE

Originally posted by Lionel Siau:
I'm confused. What is SubTest ? A subclass of Test? Your above code seems to be missing something.

[Rajinder Yadav]

Here is something that shows polymorphic functions.
The base reference 's' of class Shape contains a reference to an object of type 'Circle', this is why the Circle.draw() is called.
Dynamic binding has to do with the 'type referenced', not the reference's type!

public class Test {
   public static void main(String[] args) {
      Circle c = new Circle();
      Shape s = c;
      s.draw();
   }
}
class Shape {
   void draw() {
      System.out.println("Shape");
   }   
}
class Circle extends Shape {
   void draw() {
      System.out.println("Circle");
   }   
}

[ February 27, 2002: Message edited by: Rajinder Yadav ]

[guo mark]

thanks all first.Its ture that there are someting wrong with my code,i forget to change Test and SubTest to Foo and Fooer.But i have got the idear,so thanks to all again.