No abstract

No abstract

We present a denotational, strictly syntax-directed, semantics for Statecharts, a graphical, mixed specification/programming language for real-time, developed by Harel [H]. This requires first of all defining a proper syntax for the graphical language. Apart from more conventional syntactical operators and their semantic counterparts, we encounter unconventional ones, dealing with the typical graphical structure of the language. The synchronous nature of Statecharts makes special demands on the semantics, especially with respect to the causal relation between simultaneous events, and requires a refinement of our techniques for obtaining a denotational semantics for OCCAM [HGR]. We prove that the model is fully abstract with respect to some natural notion of observable behaviour. The model presented will serve as a basis for a further study of specification and proof systems within the ESPRIT-project DESCARTES.

We present a Hoare-style specification and verification approach for invariants in sequential OO programs. It allows invariants over non-hierarchical object structures, in which update patterns that span several objects and methods occur frequently. This gives rise to invalidating and subsequent re-establishing of invariants in a way that compromises standard data induction, which assumes invariants hold when a method is called. We provide specification constructs (inc and coop) that identify objects and methods involved in such patterns, allowing a refined form of data induction. The approach now handles practical designs, as illustrated by a specification of the Observer Pattern.

An algebraic specification is viewed as a black box that rewrites input to a ``most basic'' canonical form. We argue that a canonical form should be given for each specific specification, to prevent ``cheating'' in the implementation. Furthermore, we argue that the definition of the canonical form may sometimes require semantic rather than syntactic information.To relate an OO implementation to a specification requires opening the black box to some extent; we assess the choices to be made here.