Roles in Object-oriented Systems

In many class-based object-oriented systems the association between an instance and a class is exclusive and permanent. Therefore these systems have serious difficulties in representing evolving objects which may take on different roles over time. Entities must be reclassified any time they evolve, e.g., if a person becomes a student and later an employee. Class hierarchies must be planned carefully and may grow exponentially if entities may take on several independent roles. The problem is even more severe for object-oriented database programming languages than for common object-oriented programming. Databases store objects over longer periods during which the represented entities evolve. This project shows how class-based object-oriented systems can be extended to handle evolving objects well. Class hierarchies are complemented by role hierarchies, whose nodes represent types an object classified in the root may evolve. At any point in time, an entity is represented by an instance of the root and an instance of every role type whose role it currently plays. The approach extends traditional object-oriented concepts, such as classification, object identity, specialization, inheritance and polymorphism in a natural way. The practicability of the approach was demonstrated first by an implementation in Smalltalk, which can be easily adapted for use in object-oriented database systems like GemStone. Roles can be made available in Smalltalk by adding a few classes. There is no need to modify the semantics of Smalltalk itself. Role hierarchies are mapped transparently to ordinary classes. In a further step, the Unified Modeling Language (UML) was extended by appropriate stereotypes in order to support roles and the implementation was transformed to Java.