Steve Spicklemire wrote:
> 3) (Last question I promise!.. for now) Is there any real difference
> between the plain lines, open bubbles, and closed bubbles other than
> the number of (master) objects with relations to the (slave?). If both
> objects refer back to eachother, should there be two bubbles? (That
> doesn't seem to be an option with ObjectDomain.) Is there a good
> source on the net that talks about this stuff? I've been reading some
> of the specs at omg.org... and I'm guessing there's something out
> there more friendly ... but I haven't found it yet...
First off, a disclaimer: I don't think UML is all that helpful fr
modeling Zope applications, or ZPatterns applications. I'll explain why
some other time.
The "bubbles2" you're talking about are to indicate aggregation and
These are different from the cardinality of the relationship.
See http://www.cat-box.net/steve/umltalk/page11.html for an example of a
plain association, an association that says "a person may keep any
number of cats, but a cat only has one owner", and an association with
the roles "pet" and "owner".
On http://www.cat-box.net/steve/umltalk/page13.html, the diagram shows
aggregation and composition. The examples are of a car, and a tooth. The
enamel and fillings in a tooth are tightly bound together, and not
really separable without pain (!). However, you can remove the engine
and wheels from a car, and consider them separately, without too much
trouble. So, the car example is aggregation, the tooth example is
In UML, objects linked by composition are semantically equivalent to
attributes. So, in the tooth example, I could have had the enamel and
collection of fillings as attributes of the tooth.
Aggregation indicates a responsibility in the relationship. So, in the
Car example, if I sell my car, or scrap my car, the engine and wheels go
as well. However, if I break my car's engine, that has no implications
for the rest of the car or its wheels, except that I need a new engine.
When you're expressing object-oriented designs, an aggregate
relationship implies that the controlling object (the "master", with the
diamond next to it) is responsible for creating, destroying, and
answering queries about its aggregate objects.
If you want to learn UML, I recommend "Practical Object-Oriented Design
with UML", Mark Priestley, pub. McGraw Hill, ISBN 0-07-079599-5.
I think Coad's notation is better and clearer, as it allows you to
communicate information about objects and their classes all in one
place. Coad's notation is more about showing interactions of objects,
whereas UML makes that awkward, and is mostly used for expressing static
information about relationships between classes.
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