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From: Loryn Jenkins <loryn@s054.aone.net.au>
Subject: Re: Which wastes more time? (Was Re: Software landmines (loops))
Date: 1998/09/10
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Date: 1998-09-09T18:12:02+00:00
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Patrick Doyle wrote:
> 
> In article <35F56290.AB5491D9@s054.aone.net.au>,
> Loryn Jenkins  <loryn@acm.org> wrote:
> >Patrick Doyle wrote:
> >>
> >> I gave an example earlier of a reporting program which generates
> >> a report based on a moment in time.  If a person sells his car,
> >> he will either do so before or after this program is run;
> >> in either case, if he is a CAR_OWNER at the start, he will
> >> be a CAR_OWNER for the duration of the program.
> >
> >That's what I thought. And my response to that is: Why do you need class
> >PERSON at all, in this situation?
> 
> Sorry for the duplication.  It seems I didn't understand your
> point, so I did the only rational thing and proceeded to
> ignore it.  :-)
> 
> >You're not intending to use it
> >polymorphically with CAR_OWNER. In fact, the only reason for having it
> >is that it existed before, and now you simply want to extend it to cover
> >this CAR_OWNERship situation.
> 
> How do you know that?
> 
> class PERSON
> feature
>         name: STRING
>         age: INTEGER
> end
> 
> class CAR_OWNER inherit PERSON
> feature
>         cars: ARRAY [CAR]
> end
> 
> CAR_OWNER is a subtype of PERSON.  In the portion of the report
> that lists everyone's age, this polymorphism is important.

You're starting to hint at it, but haven't said it yet: What would you
imagine these two abstractions being used for in this imagined
application?

Be a little more specific: give me an example.

Loryn Jenkins