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We propose a precise definition of UML active classes through associated labelled transition systems using the algebraic specification language Casl. We are convinced that the first step to make UML precise is to find an underlying formal model for the systems modelled by UML, and we argue that labelled transition systems are a sensible choice. This modelization will help understanding the UML constructs and will improve their use in practice. One of our aims is, in the future, to use the powerful animation and verification tools available for algebraic specifications with UML specifications. We simplify the problem of the applicability of our semantics by restricting the state machine constructs considered. This restriction does not, however, narrow the UML subset in study because the restricted constructs can be replaced by equivalent combinations of other constructs. Because of some ambiguities in the UML official semantics, we discuss the several options at hand and choose, for each ambiguous case, the semantics that either makes more sense or that allows to simplify the problem the most.

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We discuss the nature of the semantics of the UML. Contrary to the case of most languages, this task is far from trivial. Indeed, not only the UML notation is complex and its informal description is incomplete and ambiguous, but we also have the UML multiview aspect to take into account. We propose a general schema of the semantics of the UML, where the different kinds of diagrams within a UML model are given individual semantics and then such semantics are composed to get the semantics on the overall model. Moreover, we fill part of such a schema, by using the algebraic language Casl as a metalanguage to describe the semantics of class diagrams, state machines and the complete UML formal systems.

This paper presents an attempt, perhaps unorthodox, at bridging the gap between the use of formal techniques and the current software engineering practices. After years of full immersion in the development and use of formal techniques, we have been led to suggest a Virtuous Cycle philosophy, better marrying the rigour of formalities to the needs and, why not, the wisdom of current practices. What we have called Well-Founded Software Development Methods is a strategy compliant with that philosophy, that essentially aims at proposing methods where the formalities provide the foundational rigour, and perhaps may inspire new techniques, but are kept hidden from the user.In a stream of papers we have outlined an approach -a possible instantiation of a particular well-founded method -which is Model-Driven and adopts a UML notation. Here, after introducing our basic philosophy and the Well-Founded methods strategy, we outline in summary our sample approach and, as a new contribution, we show in some detail how to handle the Model-Driven Design (or Platform Independent Design) phase.