Subj : Re: Polymorphism sucks [Was: Paradigms which way to go?]
To   : comp.programming,comp.object
From : Robert C. Martin
Date : Sat Jul 09 2005 06:58 pm

On 9 Jul 2005 11:43:39 -0700, "topmind" <topmind@technologist.com>
wrote:


>Here is an example borrowed from c2.com. We taxonomize drinks by making
>each orthogonal factor a level:
>
>DRINKS
>
>
>	Coke
>	--Diet
>	----Caffeinated
>	----No Caffeine
>	--Regular
>	----Caffeinated
>	----No Caffeine
>	Lemon-Lime soda
>	--Diet
>	----Caffeinated
>	----No Caffeine
>	--Regular
>	----Caffeinated
>	----No Caffeine
>	Iced Tea
>	--Diet
>	----Caffeinated
>	----No Caffeine
>	--Regular
>	----Caffeinated
>	----No Caffeine
>
>Any one of the three factors (flavor, dietness, caffeiness) could be
>put at the top level, but we cannot put them all there. Thus, we must
>make one the "king" arbitrarily and demoting one arbitrarily to the
>leaves.
>
>Also note what happens if you add a 4th factor to this tree.

Using inheritance to represent degrees of freedom is not always a good
idea.  In this particular case we are probably better off using
composition:

        |Stimulant|<---------|Coke|-------->|Sweetener|
             A                                   A
             |                                   |
       +-----+---------+                +--------+---------+
       |               |                |                  |
  |Caffeine|     |NullStimulant|      |Sugar|         |Nutrisweet| 

This is the GOF "Bridge" pattern (or strongly related to it).  

Trees aren't always the right design solution.
OO is not primarily about making trees.
You *can* make trees with OO, but you don't have to; and you often
shouldn't.

-----
Robert C. Martin (Uncle Bob)  | email: unclebob@objectmentor.com
Object Mentor Inc.            | blog:  www.butunclebob.com
The Agile Transition Experts  | web:   www.objectmentor.com
800-338-6716   


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 but to set a limit to infinite error."
    -- Bertolt Brecht, Life of Galileo

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