Overriding

[Athira Vineeth]

Hi

I have two class A and B, where B extends A.

public class ClassA {
public void printMe(){
	System.out.println("Inside A");
}
}

public class ClassB extends ClassA{
	public void printMe(){
		System.out.println("Inside B:Print Me");
	}
	public void printMy(){
		System.out.println("Inside B :Print My");
	}
}

Main class

public class ClassMain {
public static void main(String args[]){
	ClassA a=new ClassB();
	a.printMe();
}
}

In this case it will print" Inside B:Print Me"
That means Object a of reference ClassA has access to the method defined inside ClassB.

But if i call a.printMy();then it is showing compilation error.
Why so?

Thanks in Advance
Athira

[Walter Ho]

You're missing a basic concept here.Suggest you read more on inheritance and
casting.

[Micheal Quai]

"ClassA a=new ClassB(); "

1.a refer to Class A . there is a printME in ClassA.
2.then you "new " ClassB() to a .and ClassB() override the printME
3.a can not see the classB()

my english is so poor.
i hope that can help you .

[Campbell Ritchie]

I think your third line, Micheal Quai, is mistaken. It is not that you can’t see ClassB, but more that you can see it. The object is an instance of ClassB, so the ClassB version of the method is used.

[Micheal Quai]

Campbell Ritchie wrote:I think your third line, Micheal Quai, is mistaken. It is not that you can’t see ClassB, but more that you can see it. The object is an instance of ClassB, so the ClassB version of the method is used.

i still don't understand why the printMY() method can't not be use.by "a"?

[Manuel Petermann]

Lets make it a little more clear.

class Animal{
    public void move(){
        System.out.println("Animal is moving");
    }
}

class Tiger extends Animal{
    @Override
    public void move(){
        System.out.println("Tiger is moving");
    }
    public void roar(){
        System.out.println("Tiger roars.");
    }
}

class ClassMain {  
public static void main(String args[]){  
    Animal a=new Tiger();  
    a.roar();  //Error here
}  
} 

In this case you are assigning a new Tiger to an Animal variable.
This is possible because your Tiger is an Animal.

Problem now is, that an Animal cannot roar. A Tiger can.
The variable a just knows about the things defined in the Animal class.
If the above example would work, what do you suggest java should do when you type:

class ClassMain {  
public static void main(String args[]){  
    Animal a=new Animal();  
    a.roar();  //Error here again
}  
} 

[Winston Gutkowski]

Athira Vineeth wrote:But if i call a.printMy();then it is showing compilation error.
Why so?

And just to add to what Manuel said: Java uses the compile-time type to determine what methods can be 'seen', but the runtime type to determine which version of a method to run. This is what allows polymorphic behavior (check the tutorials).

HIH

Winston