Re: What databases have taught me
Date: Fri, 23 Jun 2006 15:34:42 GMT
David Cressey wrote:
> "Bruno Desthuilliers" <onurb_at_xiludom.gro> wrote in message
>>>multi-dimensional. OO textbooks like to use animals as an example. >> >>Alas... >> >>>They >>>like to build a polyphormic hierarchy like this: >>>Fish >>> - Shark >>> - Tunar >>>Bird >>> - Eagle >>> - Condor >>>Mammal >>> - Horse >>> - Dolphin >>> - Bat >>>This is the correct zooligical hierachy. >> >>Yes, and a very bad example of the use of subtyping in OO. Also, it >>somehow relies on the assumption that polymorphism is conditionned by >>class inheritance - which, while somehow the case with languages like >>Java, is not necessarily true for all OOPLs. >> >>>But what if there are features >>>(or behavior) that are common for all animals that can fly or that >>>lives in water?
> There is an even worse problem with the above example. Since most of us are
> not zoologists, we accept pretty much without question that a simple
> hierarchy is sufficient to classify all animals. However, our superficial
> understanding is conditioned on several assumptions that are way below the
> One such assumption is common ancestry. One of the reasons horses, dolphins
> and bats belong in the same category is that they have inheritaed some
> common characteristics from a presumed common ancestor. That common
> ancestor was presumable not the ancestor od all the eagles and condors.
> However, zoologists are toying with the hypothesis that DNA from multiple
> species can combine to give rise to a new species. If that turns out to be
> the case, then the traditional tree of zoological taxonomy may have to be
> replaced with some structure that accomodates multiple inheritance. This
> makes the example a whole lot messier, t osay the least.
Donkeys, mules, horses and burros already demonstrate that. Retroviruses incorporated into the germ lines of living organisms of differing species make things wierd, but even without such incorporation, cross-over diseases can cause different species to select for similar genotypes and/or phenotypes. Received on Fri Jun 23 2006 - 17:34:42 CEST