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Old August 6th, 2007, 09:02 AM
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Default Need help with Polymorphism

Hi friends ! This is my first post in P2P Wrox ! I know VB6 but very new to Java (and OOP).

In the following code, I've created a super class (MySuper) that has a subclass (MySub1).
objSuper is reference of MySuper
objSub1 is a object of MySub1

Both the classes has a method x(). Naturally the method is over ridden in MySub. MySub also has another method y().

Now if I comment the objSuper.y(); line, the code compiles perfectly. But with that line I'm gettin a compile error "cannot find symbol method y()".

I'm reading Just Java 2 (6th Ed) by Pere van der Linden. In the summary of Chapter 8 he wrote "superclass = subclass // always valid"
But then why this code is not working ? I'm really confused.
Please help me. Thanks in advance ! :)
public class Test
    public static void main(String [] args)
        MySuper objSuper;
        MySub1 objSub1 = new MySub1();
        objSuper = objSub1;
        // x() is overridden, y() is NOT overridden 
        objSuper.x(); // OK
        objSuper.y(); // Compile ERROR - "cannot find symbol method y()"
class MySuper
    public void x()
        System.out.println("In MySuper's x()");
class MySub1 extends MySuper
    public void x()
        System.out.println("In MySub1's x()");
    public void y()
        System.out.println("In MySub1's y()");
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Old August 8th, 2007, 06:24 AM
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What do you actially need it's just type cast.

Use ((MySub1) objSuper).y(); for calling your method.

This problem occures because after objSuper = objSub1; type of objSub1 becomes MySuper for Java not MySub1 (as it is indeed).
There isn't such a method .y() for MySuper type. That's why Java complains.
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