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From "Stephen McConnell" <mcconn...@osm.net>
Subject RE: ExcaliburComponentManager
Date Fri, 07 Dec 2001 16:37:13 GMT

From: Leo Sutic wrote:
> > From: Stephen McConnell [mailto:mcconnell@osm.net]
> >
> > What is interesting here is that ExcaliburComponentManager
> > implements the
> > Initializable and Composable interfaces without declaring the required
> > exceptions and yet this compiles ok (which I confess I don't
> > understand).
>
> Stephen,
>
> it is a basic thing about OO programming - a subclass should be able to
> stand in for a superclass anywhere. That is, if SuperClass is a superclass
> to SubClass, then:
>
> void myMethod (SuperClass sc) { ... }
>
> ...
>
> myMethod (new SubClass ());
>
> should be valid, as the contract for SuperClass is inherited down to
> SubClass. That is, SubClass must deliver at least as much as SuperClass.
>
> Proceeding with the notion of contracts - each class requires
> something and delivers something. This contract is inherited down the
> hierarchy, and a subclass must not require more and must not deliver
> less than a superclass.
>
> Thus, since Initializable can be seen as a superclass of
> ExcaliburComponentManager, ECM must not require more and must not deliver
> less than Initializable.
>
> Initializable has a method initialize () throws Exception.
>
> Requires: No parameters, ability for caller to handle Exception.
> Delivers: Initializes the class.
>
> Now, when ECM does *not* throw Exception:
>
> Requires: No parameters.
> Delivers: Initializes the class.
>
> So ECM actually requires *less* than Initializable. This is perfectly
> valid - the contract is upheld by the subclass. You class, however, does
> throw an Exception and thus requires *more* than its superclass (ECM).

Leo:

Your quite right - the distinction here is that ECM does not need to throw
an exception relative to its own implementation characteristics.  However,
this limits the potential for supertypes because a supertype cannot
introduce anything that would potential raise an exception during initalize.
Conclusion - there isn't a fault in ECM in terms of its own internal
implementation - but elimination of the throws Exception delcaration
effectively eliminates the possibility for supertypes that need this (which
was my case).

Steve.


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