Strengthening the Semantics of UML Collaboration Diagrams

A serious weakness of the UML is the lack of appropriate means for specifying the semantics of operations in a class diagram and the interpretation of actions on a statechart diagram. As required in the corresponding request for proposals [11], such specification should describe the preand postconditions of operations and actions, their effect on the current state, as well as the calls or signals that are sent during their execution. The semantics of actions, like call or send actions, has to be the same in all models, that is, it should be specified once and for all on the meta level. As operations and their interpretation differ in every model, they have to be specified on the level of individual models. In both cases, it is desirable to use UML to specify the semantics: In the case of operations, it has to be given by application developers, i.e., users of the UML, who should not be forced to learn yet another notation. And by specifying the semantics of actions by UML diagrams on the meta level, people without a strong background in formal methods, like tool developers, teachers, and advanced users, could benefit from the semantics specification. With this motivation, in [8] a meta-modeling approach to the semantics of call actions in statechart diagrams is proposed which is based on collaboration diagrams. In a collaboration diagram, a collaboration specifying a graph of objects and links provides the context of an interaction describing the flow of messages and signals. The integration of structure and communication makes these diagrams suitable for specifying the semantics of operations and actions. However, the semantic weakness of collaboration diagrams, in particular concerning the collaboration part, limits their expressiveness and prevents their use for precise specifications. In fact, according to the UML specification [12], a collaboration is just the context of an interaction. It does not entail any structural requirements beside the obvious one that objects and links have to be present as soon as they are involved in an interaction. In this note, we sketch how, without changing the syntax, the semantics of collaboration diagrams can be strengthened (and formalized) in order to allow for the specification of preand postconditions and structural effects thus providing a powerful means for action semantics on both the model level and the meta model level.