Re: Using overload to implement the equivalent of a switch

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 13 Aug 2008 12:20:24 -0700

Message-ID:

<48a33e09$0$17029$7836cce5@newsrazor.net>

Everyone wrote:

Hello!
I've been trying to use polymorphism -via- overloading+overriding to
simulate a dynamic switch

class AbstractBase{
public abstract void doSomething(Object anObject);
public void activate(Object anObject){
   doSomething(anObject);
}
}

class ResponsibleChild{
public void doSomething(Object anObject){
   System.out.println("Type not supported");
}
public void doSomething(Integer anInt){
System.out.println("Integered");
}
public void doSomething(String aString){
System.out.println("Strung");
}

}

class Main{
public static void main(String args[]){
   AbstractBase reference = new ResponsibleChild();
   reference.activate(Integer.valueOf(10));
   reference.activate("Strung!!");
}
}

The output I expected from this was ;

Integered
Strung

However the actual output is ;
Type not supported
Type not supported

Shouldn't the call
a. be made in the context of the object, and
b. narrow to the best possible match for the method signature

?

Regards,
Abhishek

The compiler has to be able to statically tell which method signature
its trying to invoke, doSomething(Object) in your example. At runtime,
the VM will see that doSomething(Object) has been overridden by your
derived class, and invoke the derived instance.

What you are trying to do is something called multi-dispatch. Java does
not support this concept at the language level.
--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>