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From: Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Partially overriding a method?
Date: Thu, 21 Apr 2011 21:10:07 -0400
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On 4/21/2011 11:31 AM, raphfrk@gmail.com wrote:
> I was wondering if it is possible to override a method but only for
> certain sub-classes of the method that the super-class supports.
>
> For example:
>
> class MainClass {
>
>      public static void main(String[] args) {
>
>          System.out.println("Started");
>
>          MainClass mc = new SubClass();
>
>          mc.check("Testing");
>          mc.check(7);
>
>      }
>
>      void check(Object x) {
>          System.out.println(x.toString());
>      }
>
> }
>
>
> class SubClass extends MainClass {
>
>      void check(String x) {
>          System.out.println("Sub class: " + x);
>      }
>
> }
>
> The call to mc.check() calls the main class's version of the method.
>
> However, if I change the sub-class to:
>
> class SubClass extends MainClass {
>
>      void check(Object x) {
>          System.out.println("Sub class: " + x);
>      }
>
> }
>
> then it uses the sub-class always.

     Right.  You've made the mistake (and you're not the first,
nor the last) of confusing overRIDING with overLOADING.  In your
first example, the `check' method of SubClass does not override
the `check' method of MainClass; is is an overload (of the `check'
identifier).  SubClass has two different methods named `check':

	void check(String) ... // inherited
	void check(Object) ... // defined locally

     In your second example things are quite different: SubClass
has only one `check' method:

	void check(Object) ... // overrides MainClass method

Note that this `check' has exactly the same signature as the
MainClass `check'; that's why it overrides.  In your first example
the two `check' methods have different signatures and have nothing
to do with each other, as you may see by experimenting with

	class HeroClass extends MainClass {
	    double check(short shrift, String along) ...
	}

-- 
Eric Sosman
esosman@ieee-dot-org.invalid