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From: cory@three.mv.com (Cory Kempf)
Newsgroups: comp.object
Subject: Re: object-oriented design is *hard*!
Keywords: object-oriented design OOD
Message-ID: <469@three.mv.com>
Date: 10 Sep 90 01:36:04 GMT
References: <1264@mtunq.ATT.COM>
Organization: EnigamI,  Nashua, NH.
Lines: 64

psrc@mtunq.att.com (Paul S. R. Chisholm) writes:

>But I'm *really* struggling with object-oriented design!
>I've *finally* become very comfortable with the top-down approach; 
>			  Managing the intellectual process is very
>straightforward.

The top down approach is nice for proceedural programming, but it is 
probably not the best for OOP.  I have found that a bottom up method
works better...

Start out by identifying the objects.  Figure out what properties they
have, what messages they should receive, and how they should act.  For
example, a window might want idle time messages, mouse down messages,
update messages, keydown messages, etc.

Now look at all of those objects.  Find objects that have things in 
common -- similar properties, similar messages, or similar actions.

Often, you will notice that a certain object will need to be broken
up into two or three subobjects, and others can be combined into a 
superobject.  Keep working at this, and you will have an inheritance 
tree (several, usually).

At the same time, you should be doing your containment tree.

>OOD is different.  I seem to need to keep everything in my head, design
>everything, and only then start coding.  

This is quite true.  Only I would add that after you start coding, you
will find that you want to twiddle the design some more.  This is, in
general, good.

>					The only way I could get far
>enough to come up with an object-oriented design is do the bulk of a
>top-down design, and then notice the common elements!  Even so, I don't
>really know where to start, or what the natural next steps are when
>I've finished one step.

If you start with a top down proceedural approach, you will only be 
making more work for yourself, and will most likely not have a truly
object oriented design.  OOD is a different methodology.  In general, 
you first need to identify the objects, then find commonality.

Often I find that a CAD system helps.  In fact, the wall of my office
has two great big pictures (6 pages (reduced 50%) and 20 pages) of the
design of my current project... I don't think that I could have seen 
the inheritance with out the CAD program.  (I still need to do another
week or two worth of design: I stopped to test key parts for feasability
now I need to redesign a bit -- much better than re-coding)

>Anybody else experiencing this, or should I apply for a job flipping
>hamburgers at McDonalds?

If you can handle looking at the world from a different direction, you
shouldn't have too much trouble.  Otherwise, I would suggest avoiding
closing -- it is a real pain.  :-)

+C

-- 
Cory Kempf				I do speak for the company (sometimes).
The EnigamI Co.							603 883 2474
email: cory@three.mv.com, harvard!zinn!three!cory