ABSTRACTING OBJECT STATE

The term `state' has many different meanings, depending on the context in which it is used. In the implementation phase, it refers to the va lues of the instance variables of the object; during design, it refers to states used in finite state models or their derivatives. For both interpretations, problems have been identified: (1) the way clients of an object access its state is defined in an ad hoc manner, (2) the co nventional object-oriented model provides no means to change the inter face of an object dynamically on the basis its state or the type of cl ient accessing it, (3) finite state machine approaches are not uniform ly integrated in the object-oriented model nor do they provide suffici ent complexity reduction for modelling the dynamic behaviour, and (4) no support is provided for defining active states. In this paper we in troduce the concept of abstract object state as a solution to these pr oblems. This concept provides a systematic technique to explicitly def ine an abstraction of the object state which is placed at the interfac e of the object. The abstract object state consists of static states, but also of active states which can be seen as time derivatives of sta tic states. We show that the abstract object state provides a more uni form approach to object behaviour specification than do finite state m odels. The concepts discussed have been implemented in the layered obj ect model ((LOM)-O-AY), an extended object-oriented model which is sup ported by a translator to C++, an execution environment and an integra ted development environment.