UML22 — 7.3.4 Association Class

The specification should state when navigation between an association class and the endpoints of that association class are allowed. When there is an association class between two classes, OCL allows navigation between these classes and the association class itself (see Sections 7.5.4 and 7.5.5 of the OCL 2.0 specification ptc/2005-06-06). However, navigability in UML2 is defined with respect to the metaclass Property and the semantics describe navigability in terms of navigating across an association via a property, i.e., from one endpoint to the other (see the definition of isNavigable() in Section 7.3.44, subsection Additional Operations, and descriptions of navigability in sections 7.3.3 - Association, 7.3.7 - Class, and 7.3.44 – Property.) No mention of navigability to and from association classes is found in section 7.3.4 – Association Class, nor any place else in the specification. One simple possibility would be for navigation involving association classes to respect navigation between its endpoints. For example, if classes C1 and C2 are connected by an association class A, then if one can navigate from C1 to C2 (C1->C2), then one can navigate from C1 to A (C1->A) and from A to C2 (A->C2). Another simple possibility would be to always allow navigation from an association class to its endpoints while requiring navigation from an endpoint to the association class to respect navigability. For example, if the association is one-way navigable from C1 to C2 (C1->C2) then one could navigate from C1 to A (C1->A) and from A to C2 (A->C2) as above and, in addition, from A to C1 (A->C1). Anything more complex than these two simple alternatives requires deeper investigation into the semantics of the UML metamodel or even changing the metamodel. For example, the association ownedEnd: Property for Association in section 7.3.3 could subset Classifier::attribute instead of Classifier::feature. Or in the definition of Association Class in 7.3.4 one could allow the inherited associations ownedAttribute: Property and ownedEnd: Property to overlap, i.e., be non-disjoint, although this may have technical difficulties because these two associations are compositions.

No Data Available