Design Patterns discussion

[Wayne Cannon <wcannon AT sonic.net>]

I'm glad it was helpful to you.  It was the most confusing bunch of 
responses I've seen in a long time.

Frequently (almost every problem) you will find that a combination of 
more than one pattern can be used to implement a solution.  If you 
really read the descriptions, I think you will find that the GoF 
patterns themselves are pretty distinct from one another (maybe not in 
their structure, but in their use, intent, and the way the function -- 
as Paul Adolf pointed out).

The Bridge and Strategy patterns are very distinct.  The Bridge pattern 
deals with HOW you deal with multiple implementations, while the 
Strategy pattern deals with USING different implementations to implement 
different algorithms.  One way to implement the Strategy pattern is to 
use the Bridge pattern approach, but the Strategy pattern can also be 
implemented a number of other ways (e.g., via a switch statement).  You 
can use the Bridge pattern approach to implement the Strategy pattern, 
but not vice versa (the Strategy pattern is not a HOW and the Bridge 
pattern care nothing about how it is USED)..

The State and Strategy patterns are distinct.  This is going to sound 
familiar.  The State pattern deals with HOW you switch between different 
states/implementations, while the Strategy pattern deals with USING 
different implementations to implement different algorithms.  One way to 
implement the Strategy pattern is to use the State pattern approach, but 
the Strategy pattern can also be implemented a number of other ways.  
You can use the State pattern approach to implement the Strategy 
pattern, but not vice versa (the Strategy pattern is not a HOW and the 
State pattern cares nothing about how it is USED).

To carry this line of thought one step further, a Bridge pattern 
approach can be used to implement the State pattern, but the State 
pattern can also be implemented by other mechanisms.  The State pattern 
includes a little bit of WHAT the switching implementations are used for 
-- to switch states -- while the Bridge pattern says less about how it 
is used and what it is used for.

The important point is that you may use multiple patterns to solve one 
problem -- indeed, you may choose to use one pattern to implement 
another pattern.  For example, you may use a Bridge pattern to implement 
a State pattern, and the State pattern to implement a Strategy pattern.  
This does not make these patterns equal, or even similar, to one 
another.  If anything, some patterns are lower-level concepts than other 
patterns.  You frequently have multiple choices available to implement 
the higher-level patterns, only one of which might be a particular 
lower-level pattern.

Regards,
Wayne


Chris Finlayson wrote:

> Hi all:
>
> Thanks for all the prompt, thorough responses.  It's been quite helpful.
>
> Everyone's response has been terriffic.
>
>