Modeling a Groupware Editing Tool with Cooperative Objects

The language described in this paper, called Cooperative Objects, fits in the category of strongly-typed OOL. Its type system, the semantics it retains for inheritance, and more generally all its specifically Object-Oriented features are strongly influenced by the Eiffel language [1]. In particular, an object system is organized by a dynamically evolving use relationship, describing how the objects communicate through services invocations, following a client / server style of interaction. In Eiffel, the contract that a server object ensures to fulfill towards its clients is expressed by a set of preconditions, postconditions and invariants. In Cooperative Object, the idea of programming by contract is retained, but the contract is expressed by a High-Level Petri Net (HLPN) expressing the precedence constraints, the concurrency and the synchronization between the provided services. This allows for an easy description of objects with an internally concurrent behavior. In Eiffel, the implementation of a class is given by providing the algorithms associated with each services, supposed to respect the class' contract. In Cooperative Objects, the implementation of a class is given by a HLPN, more detailed than that of its specification, making explicit how instances of the class will behave as clients of other objects. Petri Nets analysis results may ensure that the implementation PN is compatible with the specification PN. Finally, the semantics of a service invocation is also expressed by a Petri net. This allows to provide a formal semantics not only to the behavior of a single object, but also to a set of communicating objects. The paper presents the Cooperative Objects language. It first explains how the language may be used as a formalism for describing objects with concurrent behaviors, thus providing the specification of a concurrent class. It then shows how to provide an executable implementation of the class within the same formalism. The basic semantics of inter-objects communications is then detailed, in terms of Petri nets. Finally, some indications are given on how to use results from the PN theory to ensure the compatibility between a specification and its implementation, and between ancestor and descendants down an inheritance hierarchy.