From: Robert Klemme <shortcutter@...>
Date: 2008-05-25T20:29:47+09:00
Subject: Re: OOP in Ruby?

On 23.05.2008 00:15, James Britt wrote:
> Robert Klemme wrote:
>> Without wanting to start an indepth discussion about OO, but aren't 
>> these just two sides of the same medal?  The message metaphor seems to 
>> lean a bit more on the client side while the ADT view is a bit more 
>> about how messages transform internal state (although ADT is mainly 
>> about observable state through sequences of method invocations).  In the 
>> end, in both cases an instance holds state which is changed via method 
>> invocation / method sending and the implementation determines how the 
>> state may be manipulated.
> 
> Here's the key difference to me:  Taking a message-orient POV, you do 
> not assume that messages == methods.  You think in terms of how an 
> object does things internally, and then how it will handle messages.

I believe there is usage for both approaches - even within the same 
project: usually I create classes from the outside, i.e. I decide about 
their responsibilities, their interface and then I decide how they 
should be implemented.  In other cases, e.g. when implementing a 
particular algorithm I probably will first think about the internals and 
then decide which input or interface a class needs.

In both cases though clarifying responsibilities of a class (-> CRC 
cards) is the first and most important step IMHO.

> In Java the assumption is that any message you send has to map to a 
> particular method.  It does not encourage the same degree of 
> encapsulation and emphasis on behavior being distinct from implementation.

I am not sure I get your point here.  Looking at e.g. interfaces in 
java.util what else is this than a separation of behavior (e.g. map 
lookup) from the implementation (HashMap, TreeMap)?

> In some ways it is similar to the difference in behavior-driven 
> development and test-driven development.
> 
> Point of view matters.

At least as a means for reasoning about how one does software 
development and which alternative approaches are around.

Thank you for your input!

Kind regards

	robert