Hi Don,

Don C. wrote :
<<In OOP, you spend your time trying to find Is-A relationships, but they
don't exist. Unsurprisingly, hardly anyone is "really" doing OOP. It
seems to be unimplementable.>>

Oops, what is the matter? . A Is-relation between two classes is just inheritance. f.i. cB inherits cA, ergo cB is a special case of cA.


I think you talk about the problem to decide :  Do we have a
Has-Relationship or do we have a Uses-Relationship. ?!
I agree, it is often difficult to figure out the difference between Has- and
Uses- Relationship.
But when you keep in mind that a  Has-Relationship only exist when we can
say instead of :
cA has 2(n) cB objects ,
cA consists of 2 cB objects.
-------------------------
In other words : cA consists of 2(n) cB objects means  Has-Relationship.
Bjoern

BLS wrote:
> Hi Don,
> 
> Don C. wrote :
> <<In OOP, you spend your time trying to find Is-A relationships, but they
> don't exist. Unsurprisingly, hardly anyone is "really" doing OOP. It
> seems to be unimplementable.>>
> 
> Oops, what is the matter? . A Is-relation between two classes is just
> inheritance. f.i. cB inherits cA, ergo cB is a special case of cA.

Yes, that's exactly right. I'm saying that inheritance is rarely appropriate. If you read the Design Patterns GoF book, you'll see that the #1 rule of OO design is, "Prefer encapsulation to inheritance".
But the defining feature of OO design is inheritance. Without inheritance, OOP doesn't have polymorphism.

What I find extremely common in practice is "Can-be-treated-as-A" polymorphic relationships. Most of the time, "is-A" relationships are an approximation to "Can-be-treated-as-A" relationships. I think that polymorphism is very important, but inheritance-based polymorphism has many problems.

Hi Don,
okay, now I understand what you mean. Takes a while. You may have noticed
<g> that english is not my favourite language.
GoF.
Yes of course, I know about it. I read Bruce Eckel s Thinking in Pattern and
Prof. Moessenboek OOP in Oberon 2 instead because I have heard that GoF
Design Patterns is a bit too theoretical. Can you confirm ? Or will you say
it is a "You need it" book ?
Thanks,
Bjoern


"Don Clugston" <dac@nospam.com.au> schreef in bericht news:dveppu$1im5$1@digitaldaemon.com...
> BLS wrote:
> > Hi Don,
> >
> > Don C. wrote :
> > <<In OOP, you spend your time trying to find Is-A relationships, but
they
> > don't exist. Unsurprisingly, hardly anyone is "really" doing OOP. It seems to be unimplementable.>>
> >
> > Oops, what is the matter? . A Is-relation between two classes is just inheritance. f.i. cB inherits cA, ergo cB is a special case of cA.
>
> Yes, that's exactly right. I'm saying that inheritance is rarely appropriate. If you read the Design Patterns GoF book, you'll see that the #1 rule of OO design is, "Prefer encapsulation to inheritance". But the defining feature of OO design is inheritance. Without inheritance, OOP doesn't have polymorphism.
>
> What I find extremely common in practice is "Can-be-treated-as-A" polymorphic relationships. Most of the time, "is-A" relationships are an approximation to "Can-be-treated-as-A" relationships. I think that polymorphism is very important, but inheritance-based polymorphism has many problems.
>

BLS wrote:
> Hi Don,
> okay, now I understand what you mean. Takes a while. You may have noticed
> <g> that english is not my favourite language.

I know the feeling. I'm an expat myself, and don't speak the local language (German) as well as I'd like... Your English is about 100 times better than my German.

> GoF.
> Yes of course, I know about it. I read Bruce Eckel s Thinking in Pattern and
> Prof. Moessenboek OOP in Oberon 2 instead because I have heard that GoF
> Design Patterns is a bit too theoretical. Can you confirm ? Or will you say
> it is a "You need it" book ?

Well, GoF was the book that launched the Patterns movement, everyone quotes it all the term and uses terminology from it. A lot of the code in it isn't spectacularly good, and parts of the book seem a bit clumsy, but it was certainly groundbreaking. So, it's true that it's too theoretical, but I think it's still probably a 'you need it on your shelf' book. At least, you need a good idea of what's in it.

Thanks Don !