UML as a Heterogeneous Multiview NotationStrategies for a Formal

On UML UML is a semi-formal notation, in the sense that it has a rather precise syntax (including well-formedness conditions) but an informal semantics (just a natural language description). The task of its formalization is not trivial and poses new problems, since UML has some relevant and novel features with respect to the existing speciication formalisms. Let us single out our personal view of some of those features. Multiview The UML model of a system consists of many diierent diagrams, each one describing a view either of the system or of some of its parts. For example, a class diagram describes the static structure of the system, while a state diagram describes the local reactive behaviour of a system component (an object of some class), a sequence or collaboration diagram describes some mutual interactions between some components of the system, and so on. It may happen that the functionality and some constraints on an operation of a class are in the class diagram, its local behaviour is in a state diagram, its interactions with the objects of another class are in a collaboration diagram, and those with the objects of a third diierent class are in a sequence diagram. Heterogeneous The various kinds of UML diagrams use very diierent notations, as a variant of entity relationships, state charts, message sequence charts, a kind of Petri nets (activity diagrams), et cetera. Extendable UML ooers some ways to the users to extend the kinds of the elements used in the diagrams, as stereotypes, tagged values and constraints. Moreover the language used for the constraints is not xed; a particular one, OCL, has been deened but its use is not mandatory and can be replaced by other languages, including also natural language text. Notation UML is only a notation and not a method (see in AR97] a detailed discussion on the diierence between notation/formalism and method); thus it can be used in diierent ways by diierent methods, and a method may consider only parts of UML or diierent parts in diierent phases of the development process (see, e.g., Wie98]).