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Message:

general

Posted by santhosh on October 10, 2000 at 11:26 AM

> > Here's a little problem I've been trying to solve.

> > I have an abstract base class with two concrete subclasses.
> > For the sake of simplicity, let's go back to the same old
> > examples:

> > abstract class Animal ...
> > class Dog extends Animal ...
> > class Cat extends Animal ...

> > Right, I want to write a method which does something with
> > Animals, but does a different thing with Dogs than Cats.
> > Trying to avoid using instanceof and downcasting, I wanted
> > to use overloaded methods, one method for each specific
> > type of animal that I need to handle:

> > void buy(Cat c) ...
> > void buy(Dog d) ...

> > So far so good. But in my client code, I store references
> > to Dogs and Cats as Animals because I don't know exactly
> > what type of Animal might be coming.

> > Animal animal = new Dog();

> > Now, when I try to call: buy(animal);
> > the compiler complains that there
> > is no method "buy" which accepts an Animal object.

> > So, my questions are:
> > 1. Is there any way to do this without instanceof and downcasting?
> > 2. If so, how?
> > 3. If not, why not? The JVM has enough runtime information to
> > know exactly what type the object is. I thought that's
> > what polymorphism is all about.
> > 4. Am I doing something obviously stupid here? I've thought
> > about it so much I'm just not sure now.

> > Any help greatly appreciated.

> > Cheers,

> > Chris

>
> ALl you need to do is implement dynamic polymorphism in your code
> Steps involved
> a. Create the buy function to be a virtual function ( in java abstract function )
> b. in the derived class Dog and Cat implement the Buy functions accordingly . ( need not declare them virtual)
> c. Now create the instance of dog as u described
> Animal a = new DOG()
> a.Buy()
> this function call will call the dog's bug functions.

> What u are trying to imeplement in you code is dynamic polymorphism and you are using overloading which is wrong.
> Overloading is to implement static polymorphism !.
> Hope this helped.