Should UML role names be reflected in class attributes?

Question

When class is associated with another, like Flight in the linked image, which has zero or one Plane associated with it, should this not be reflected in the class' attributes? That is, shouldn't class Flight in this example have an assignedPlane field alongside its flightNumber, departureTime and so on... otherwise how can Flight be said to 'be aware' of its associated Plane?

Answer 1

If you would have an attribute 'assignedPlane : Plane' in class Flight, then the role 'assignedPlane' on the association would be redundant. But merely having the role 'assignedPlane' on the association does not imply that class Flight is aware of it. If Flight would be implemented as a Java class or a class in any other OO programming language, it is not clear from your class diagram whether or not class Flight has any reference to (or knowledge of) the assignedPlane.

If you want to communicate to your audience that class Flight owns a reference called 'assignedPlane' to zero or one instance of Plane, then you could add a thick dot (or a small filled circle) at the end of the association. Here is an example from the UML 2.5 specification:

In this example, 'size' is an attribute of Window, although it is displayed as an association. If you would have a class diagram that shows all attributes of Window, then 'size : Area' would be one of them.

The dot notation is a relatively new notation and it is not widely used. Instead, many designers use the arrow notation, also shown in figure 9.12. According to UML, this so-called navigability arrowhead specifies that the associated instance of Area can be accessed efficiently from Window, irrespective of how and why this is the case. Most designers would assume that it means that Window has an attribute 'size : Area', but strictly speaking, this is not what the UML specification says.

Using both the arrow and the dot, as in figure 9.12, is valid, but redundant.

Answer 2

No.

Showing an association in diagram is equivalent to having an attribute of a specific class. If there is an association(with a role) then this role is an attribute of respective Class.

For reference please see the Figure 9.12 (with description) in section 9.5.5 of UML 2.5 specification.