Rigorous Object-Oriented System Design

One of the least systematically explored stages in object-oriented design is the process whereby groups of classes are organised into progressively larger units. The identification and evaluation of subsystems is still largely an intuitive, or expert activity. An effective design process should be capable of layering systems, yielding subsystem units which are cohesive internally and loosely coupled externally. This paper evaluates and compares responsibilitydriven and event-driven design techniques for their ability to layer systems. Each of these approaches elevates a different modularising principle: contract minimisation and existence dependency. The expertise is distilled as sets of semi-formal rules, which may be applied to effect system transformations. Important design patterns, such as Mediator, Chain of Responsibility, Template Method, Command and Composite are shown to be derived automatically during the transformation process. Finally, a complete formalisation of one design method is presented.