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Designing with Runtime Class Information



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Answer to your question

Posted by Sharadha Eswar on August 28, 2000 at 5:05 PM

> 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()
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.


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