Which kinds of associations do classes have in a system? How to best represent them in UML?

Question

This post is actually divided in two questions:

Which kinds of associations classes/interfaces usually have?

I'd say there are at least 3 kinds of dependencies:

• Inheritance : I think we all know this one)
• Dependency : with this I mean that if class A depends on class B, I can only instantiate class A if I pass it class B as constructor argument. Is this correct?
• Uses : class A uses class B if there is any kind of reference of class B in the code of class A that is neither inheritance of dependency.

Are these the only kind of associations between classes? Are my interpretations of what they mean correct?

Which kind of dependencies are of our interest to show on UML diagrams (mostly class/package diagrams)?

Up until now, I've mostly only put in class diagrams arrows depicting dependencies associations. But now that I think of it, most of the time I look at class diagrams, I'll find something along the lines of

where only Inherits and Uses associations are shown. But those Uses in the class diagram aren't really the same kind of Uses I've defined above. They are just a way of telling you there's a getter on a class of a type that's not a primitive.

I guess that if I had to look to some system's documentation I'd like to know how different classes in the system depend on each other. I'd definetely like to have some sort of Dependency Graph to look at.

Would it better to restrict class diagrams to Inherits/Uses associations and then have other diagram/graph that shows Depends/Uses associations?

How do you prefer it, and why?

Thanks

Answer 1

First, you should stop talking about inheritance, depends and uses relationships as "associations". The word "Association" has a particular meaning in UML; see below.

Of the three relationships you mentioned, it is conventional to only show the inheritance (technically Generalization) relationships in a class diagram. They are represented using solid lines with open triangle arrowheads.

If you wanted to show depends / uses relationships between classes, you should use the UML Dependency notation; i.e. a dashed line with a "vee" arrowhead.

The other kind of relationship that is commonly depicted in an class diagram is an Association; i.e. information-carrying relationships between instances of classes, that are typically represented using (for example) pointers, or database foreign keys / indexes.

I guess it is a matter of personal preference, but I wouldn't want to see depends/uses relationships on an object diagram. It is difficult enough to layout the Generalization and Association lines. Other lines are simply going to make the diagram too "busy".

The UML User Guide says:

"Remember that it is both undesirable and unnecessary to model all relationships among a set of abstractions in a single diagram or view. Rather, build up your system's relationships by considering different views on the system. Highlight interesting sets of relationships in individual diagrams."