Re: Objects and Relations

On Feb 21, 3:06 am, "David BL" <davi..._at_iinet.net.au> wrote:
> On Feb 21, 12:03 am, "Walt" <wami..._at_verizon.net> wrote:
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> > 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.

David. RM uses a more elementary, antecedent and efficient terminology into naming entities, objects, classes, or other OO related concepts. Once you understand that in an RM perspective a class is the representation of some relation and that in RM a relation systematically defines a data type. To understand thoroughly such difference, you need to understand better RM concepts.

Up to you either to educate yourself. Received on Wed Feb 21 2007 - 10:05:57 CET