Re: Objects and Relations

"David BL" <davidbl_at_iinet.net.au> wrote in message news:1172023595.106358.33710_at_q2g2000cwa.googlegroups.com...
> On Feb 21, 12:03 am, "Walt" <wami..._at_verizon.net> wrote:
> >
> > In casual conversation, ER modelers will tend to use the simple word
> > "entity" in place of either "entity set" or "entity instance", leaving
the
> > listener to disambiguate by means of the context in whuch the word
appears.
> >
> > By analogy, in casual conversation, object oriented programmers will use
the
> > simple word "object" in place of either "object class" or "object
instance",
> > similarly leaving disambiguation up to the listener.
> >
> > The problem comes when the listener is not familiar with the underlying
mode
> > of thinking. In that case, the listener will sometimes disambiguate
> > incorrectly. That is why introductory tutorials on object oriented
> > programming tend to spell out "object class" or "object instance", at
least
> > until the reader can be presumed to have gotten accustomed to the mode
of
> > thinking. I'm sure you will have noticed this, if you've gone back and
read
> > some introductory material after gaining proficiency.
> >
> > Similarly, introductory material on ER modeling should spell out when
we
> > are talking about the "set of all vehicles" and when we are talking
about "a
> > particular vehicle". Some such material does this.
> >
> > Discourse in this newgroup tends to be a little more formal than casual
> > conversation, but far less formal than introductory tutorials.
> >
> > Hope this helps.
>
> I prefer to only use "object" to mean instance. Why would one say
> object when one means class? ie given that we have different words
> for these different concepts, let's use them!
>
> A word like "book" can mean an instance as well as a type, depending
> on the context. It could be argued that it really means a type, and
> its use for naming an instance is a curious feature of how we
> communicate because we don't generally want to go to the trouble to
> give things around us explicit names.
>
> I agree that "entity" can have both meanings as well. However, I was
> taking the statement "entities are illusionary" to mean that all
> instances of entities are illusionary, rather than only entity types.
>
> It is interesting that in UML we have class diagrams and object
> diagrams. An ERD is analogous to a class diagram. That could
> explain Jim's tendency to regard "entity" as being a type.
>
>

Also interesting is that in OOA (Object Oriented Analysis) Peter Coad lists an ERD as one of the items that might be input to the OOA process. What I'm going to suggest is that ERD is OOA projected onto the data centric perspective. Received on Wed Feb 21 2007 - 16:06:24 CET