The above statement is taken from www.uml-diagrams.org section Assocation ends from the rather beginning.

• Is there a real-world example for this?
• What exactly means owned by association itself?
• What is association class or how is it implement in code?

CodePudding user response：

Here's my attempt. I also was unsure about the nature of an association in the past and until not so long ago where I sort of grasped it. For the full pain you have to go through pp. 197 of UML 2.5 (11.5 Associations)

What is an association?

When you have a number of classes (we may call them A, B, etc. here for brevity. They are designed to be somewhat self-consistent. So you have properties and operations which work with the latter. If our A is some mathematical class you may have a well working black box. Fine.

Now, when any class is in need of another (e.g. the B class needs objects of type A to have that math toolbox handy) you create some reference. In a programming language that will be some variable holding a reference to an instance of the needed class. That's the moment where you have an association.

In UML you can denote such an association as a property typed with the other class. Or (!) you draw a line for an association between both class boxes and place the role name towards the referenced class (telling the name of the variable the programmer shall use). This being quite simple got me some headaches since I saw notations with both property and role notation. I have not fosteres the UML spec as tho what this would mean, but I established a rule I was also teaching that this is not allowed. Tools like EA do not support roles in a correct way which forces you to manually maintain your diagrams. That is, if you have an association role from B to A and the diagram does not contain A then you should show the role as property (see my note about dot notation below). But once you place A on that diagram you should see the assocation line with the role name, but not the property (pretty confusing I admit).

Note: Dot notation

Since UML 2.5 (or was it 2.1?; but see p. 200 of UML 2.5) the OMG guys introduced the so called dot notation. That is, you have a little dot at the association line end along where the role name is. This dot indicates that the role name is actually the property of the referencing class. Pretty much what I was talking about above. And what I thought was the intention. So, I have no idea what a not-owned property would be in any case. May be a good question to be asked.

Associaton class

Assume you have class C which reflects some kind of relation between A and B (and eventually D, E, etc.). Doing that makes it an association class. It's doing that by exactly the same technique as above, just for multiple classes. (If a class has multiple relations to other classes it's not necessarily an association class, but only if it's main purpose is to set these into some relation. YMMV)

So here you have the case where an association owns the role since you have a real object. The association explained above exists only as a reference in the programming world, not a complex storage structure like an association class.