Re: Separation of concerns

Responding to Kowalski...

Hi everyone,
I am currently quite confused about the separation of concerns things
and search here for enlightenment.
I have some classes. lets say A and B that shall implement a certain
interface (e.g. Drawable).
Now it would not be very nice if A and B would implement then directly
or?
Should I subclass like A_Draw and B_Draw or what is the current
pattern to solve this?

I'm afraid I'm not sure I understand what you are asking. It sounds like you are describing:

[Drawable]
A
|
+-----+-----+
| |
[A] [B]

where the [Drawable] is an abstract superclass that defines a common interface that both [A] and [B] must implement in some acceptable fashion.

That is a matter of generalization rather than separation of concerns.

However, the notion of -able (drawable, persistable, etc.) is often symptomatic of an abuse of OO generalization that is related to separation of concerns. This is a variation on the sort of composition one finds in functional programming. One composes objects by providing disparate responsibilities through inheritance (often multiple inheritance). The resulting object is a hybrid of logically unrelated groups of properties.

That sort of composition violates OO separation of concerns because the object is not cohesive (i.e., focused on a single set of logically related concerns). The acid test is looking for a corresponding entity in the customer problem space. One can generalize various flavors of Vehicle and a domain expert like a car dealer will have no difficulty with the notion of a Honda Accord. But try asking that same car dealer to test drive a Drawable Honda Accord.


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