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From: Biju Thomas <bijuthom@ibm.net>
Subject: Re: Which wastes more time? (Was Re: Software landmines (loops))
Date: 1998/09/06
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Date: 1998-09-06T00:00:00+00:00
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Andrew Reilly wrote:
> 
> In article <35F1C0B9.8A50CEB0@s054.aone.net.au>,
>         Loryn Jenkins <loryn@s054.aone.net.au> writes:
> 
> > I find this test to be a very useful in finding whether a (sub-type)
> > inheritance relationship should hold:
> >
> > Is the child class a type of the parent class, and will it be for the
> > whole of its existence? And, is the parent class a supertype of the
> > child class, and will it be for the whole of its existence?
> >
> > That is, in this example:
> > (1) Is a CAR_OWNER a PERSON, and will it be for the whole of its
> > existence?
> > (A1) Yes.
> >
> > (2) Is a PERSON a CAR_OWNER, and will it be for the whole of its
> > existence?
> > (A2) No. Any PERSON can buy and sell cars, moving its status from car
> > owner to not multiple times during their life.
> 
> Supertyping is clearly /not/ an "is-a" relationship, though.  Cows
> are a (sub)type of mammal, and a cow "is-a" mammal (for the whole of
> its existence), but a mammal is not necessarily a cow.

How can you come to this last conclusion? A mammal is either a cow or
not a cow for its entire existence. It cannot be cow for some time and
human for the rest of its existence. Remember that we are talking about
a particular instance of mammal, not all mammals. The supertyping
described as an is-a relationship applies to a single instance, not all
instances. A *particular* Person can be a car owner one day and cease to
be a car owner next day. On the other hand, a *particular* mammal, which
is a cow, is a cow for its entire existence.

Biju Thomas