[Uml-devel] UML question

[Andrew Sutton]

On Sunday 25 May 2003 10:06 pm, P. Fleury wrote:
> Hi Anthony,
>
> I also think that an aggregation give an information about the relations
> of the life-time of the related objects. Hence the filled and unfilled
> diamonds in some relations (aggregation is filled). The 1..* relation
> does not indicate any life-time issue, and my understanding of UML is
> that you can use it for things which can be dynamically associated. This
> also matches your car+wheel+people example.

we're getting closer :)

> >What I mean is, there is a definate distinction between relationships and
> >aggregations. Relationships have straight lines and aggregations are the
> > same but with diamonds on the end. So given that, what's the difference
> > between a 1 to many relationship and a 1 to many aggregation. I think the
> > answer is that a class exists in it's own right when a 1 to many
> > relationship is used, but when an aggregation is used, it means the class
> > is _comprised_ of that aggregate too.
> >
> >I think, a car would have a 1...*  aggregation of wheels, but not a 1...*
> >aggregation of persons.

first, the "relationship" you're talking about is an "association". an 
association has different forms of aggregation: none, shared, and composite. 
each defines different lifecycle semantics for related or associated objects. 

an association with no diamond means that there is non-aggregate association. 
one object uses 1..* of another object, but is neither responsible for 
creating or deleting them.

an association with a hollow diamond is shared aggregration. in otherwords, 
something else is the actual owner, but this uses them too. i'm not sure that 
this is really so different than non-aggregate associations.

the filled in diamond represents a composite aggregation. that's strong 
ownership. any composites are created when the owning class is created and 
destroyed when the owning class is destroyed.

at least that's how i think it works...

andy