Question

I need help with this Java Program example. This example is from the book
Java: Seventh Edition by Herbert Schildt.

I have few doubts about this program and also doubts about the text(explanation of this topic) written in this book. This program is written under Inheritance –> Method Overriding. Here A is the super class of B and B is the super class of C. In this program callme() is a method written in the three classes where each callme() overides another callme() method.

1) In the program below, what does obtain a reference of type A means? (This concept is implemented in the line A r; in the Main method)
2) What is name space convention?
3) In this program, what does “r referes to an A object mean”? (This concept is implemented in the lines r.callme(); written in the main method.)

class A
{
void callme()
{
System.out.println("Im in the class A");
}
}

class B extends A
{
void callme()
{
System.out.println("Im in the class B");
}
}

class C extends B
{
void callme()
{
System.out.println("Im in the class C");
}
}

public class Dispatch 
{
public static void main(String args[])
{
A a = new A();
B b = new B();
C c = new C();
A r;

r = a;
r.callme();
r = b;
r.callme();
r = c;
r.callme();
}
}

Answer 1

This is a way to overcome the problem of method over-riding.
If you want to get rid of method over-ridden at some time during your development then you can use this way of DMD.

Referring your example with comments:

class A //Super class A
{
    void callme()   // method callme() that'll be overwritten next in subclasses
    {
        System.out.println("Im in the class A"); 
    }
}  
class B extends A   //Subclass B  inherited from A
{            
    void callme()   //method callme() of Super class A is over-hided here
    {
        System.out.println("Im in the class B"); 
    } 
}  
class C extends B            //Subclass C, inherited from B
{
    void callme()    // this time B subclass method callme() is over-hided
    {
        System.out.println("Im in the class C"); 
    }
}  

//Now suppose, during you development phase at some time, you don't want to use over-ridden methods, here is DMD to help you out at run time.

public class Dispatch
{ 
    public static void main(String args[])
    { 
        A a = new A(); 
        B b = new B(); 
        C c = new C(); 
        A r;
        // r is a reference to class A
        // this reference should be assigned to each type of object and called at
        // run time without compiling.
        r = a; 
        r.callme(); 
        r = b; r.callme(); 
        r = c; r.callme(); 
    } 
}