Re: Clean Object Class Design -- Circle/Ellipse
Date: Thu, 11 Oct 2001 12:43:10 GMT
Message-ID: <3BC59460.3090400_at_mail.com>
Bob Badour wrote:
> "James A. Robertson" <jarober_at_mail.com> wrote in message news:<3BC199E9.8020103_at_mail.com>...
>
>>Use one I'm actually likely to stumble on in a real application I might
>>write.
>>
>>Bob Badour wrote:
>>
>>
>>>Using the real example of circle and ellipse:
>>>
>>>How does Smalltalk:
>>>* allow me to declare circle as a subtype of ellipse?
>>>
>>
>>I subclass Circle from Ellipse
>>
>>
>>
>>>* allow me to send the setFoci message to ellipse variables (that
>>>incidentally contain circle values) with two different focal points?
>>>
>>
>>I override the methods in the subclass so that each gets appropriate
>>behavior. If the ellipse variable contain circle values, ,my
>>application code is wrong>
>>
> > Are you saying that Smalltalk does not allow polymorphism? > >
>
>>>* prevent me from sending the setFoci message to circle variables?
>>>
>>See above.
>>
> > Overriding setFoci does not prevent me from sending the message, does > it?
No, but I can override it in such a way as to make it irrelevant - raising an exception, for instance. If the developer using the new class doesn't know how to use it, that's not really my problem, is it?
> > >
>>If I find that I'm overriding enough methods, subclassing was a bad
>>choice. You have created an example 'problem' that isn't much of a
>>problem.
>>
> > If Smalltalk made sufficient distinction between values and variables, > one would not need to override setFoci in the Circle class because it > would never inherit the method. Circle values would inherit the > operations on Ellipse values but Circle variables would not inherit > the operations on Ellipse variables.
If you understood Smalltalk, you wouldn't make such egregious errors in your postings either. Have you ever used Smalltalk? From what vast well of experience do you draw your silly statements from?
> Received on Thu Oct 11 2001 - 14:43:10 CEST