Two layers of constraints for an extended object model in Eiffel

Static constraints represent the contractual responsibilities and obligations of methods in Eiffel classes. Dynamic constraints are considered in the analysis and design level of system development. This paper attempts to show that some classes of problems would benefit from the explicit use of dynamic constraints in the programming phase as well. It shows how the separation and layering of static and dynamic constraints help in object-oriented software evolution. 1 Introduction One of the great benefits of object-oriented systems is the open-closed principle advocated by (Meyer 1989). It allows designers to build systems which can evolve by opening and reusing the closed quality components and extending or refining their behaviour. The semantics of a system can evolve incrementally by including new methods for various classes (types) (van Biema 1990) in Eiffel. This work introduces the notion of dynamic constraints as an additional mechanism for the evolution of object-oriented softw..