Originally posted by Andrew Tibets: I understand that there can be only an instance of the association class for any given pair of objects
Make that "for any given link between two objects". A given pair of objects might well be linked more than once.
- Can be Association Class added for one-to-one association? Is it common?
Yes, of course it can - why not?
Is it common? I don't know...
For example, there is association Flight-Airplane 1:1, and association class Assign. Class Assign has two attributes: data, time.
- Is it possible by instance of association class Assign find corresponding pair of object Flight and Airplane?
I think that depends on how the association class is implemented. If it were possible, I probably would decide to not depict it as an association class, though.
- Although the multiplicity of association is 1:1, is it correct that in different time Airplane can be assigned to different flights, or multiplicity must be *:1 in this case?
The latter. The normal rules for associations still apply, it's just that the association itself has some attributes.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
At association class example I describe three style guidelines you may be interested in: 1. Model Association Classes On Analysis Diagrams. The figure shows that association classes are depicted as class attached via a dashed line to an association � the association line, the class, and the dashed line are considered one symbol in the UML. The reason why I only show them on analysis class diagrams is because you can't directly implement them in any OO language that I know of, that instead you need to resolve them. The implication is that the concept really isn't so useful.
2. Do Not Name Associations That Have Association Classes. The class has a name, so name it intelligently.
3. Center The Dashed Line of an Association Class. It makes it clearer.