Re: Good Design Strategies <Was comments on comments>

[jellinghaus-robert@CS.YALE.EDU (Rob Jellinghaus)]

In article <6127@medusa.cs.purdue.edu> rjh@cs.purdue.EDU (Bob Hathaway) writes:
>In article <4343@enea.se>, sommar@enea.se (Erland Sommarskog) writes:
>>As for bottom-up vs. top-down, Meyer explains this very well in his
>>book, and it's certainly applicable to the recent discussion.
>
>Could you elaborate on this, does he advocate top-down, bottom-up, ???
>I haven't read Meyer's book.  

Meyer advocates neither, although his strategy (which he terms simply
"object-oriented design") is closer to bottom-up than to top-down.  His
claim is that the best way to obtain reusability, maintainability, and
reliability (which should all result from good software engineering
technique), the best method is to focus design on the data objects that
the program is manipulating.  The resulting design can often be very
obvious, as it isn't too hard to find the objects in most applications.
Object-oriented languages then package the objects and their internal
representation together with the routines that operate on them, and
thus separate interface from implementation.  A good object-oriented
system quite often will consist largely of software components that
have been taken from older projects, which is not really a possibility
in either the top-down or the bottom-up model.  If the map of a top-
down system is a tree, with the principal function at the top, then
an object-oriented system is a net, with each object providing
services to other objects, with no clear hierarchical arrangement;
this enables the system to be locally modified without global effects.

That's a very brief summary of Meyer's arguments.  He goes into more
detail, with examples, in the book.

To respond briefly to Bill Wolfe's comments about Eiffel being flaky:
I can't say how good a language Eiffel is in the real world.  However,
Meyer uses it very effectively as an exemplar of a good object-oriented
programming language, and at every step throughout the book he discusses
the design decisions he made in creating Eiffel, and what the impli-
cations are for object-oriented programs.  He discusses the pros and 
cons of Ada, Smalltalk, etc., etc. at length, and he talks about the
essential features of any "true" (in his definition) object-oriented
language: multiple inheritance, genericity, polymorphism, dynamic
binding, etc.  Even if you don't agree with his decisions, his
exposition is very clear and (in my opinion) well-written.

(You can probably tell I like the book!)

>Bob Hathaway
>rjh@purdue.edu


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