A reliable and flexible type system for object-oriented programming

The inheritance mechanism facilitates incremental modification and is the basis for the many advantages and popularity of the object-oriented paradigm. In particular, inheritance is overloaded to describe the different relationships that are possible between classes. Such overloading requires the development of a semantic model that facilitates 'safe' redefinitions between classes. Current approaches use type systems to ensure 'safe' redefinitions, and are not capable of handling all the different uses of inheritance naturally. This paper develops a uniform type system built on a hierarchy which distinguishes between abstract and concrete classes. It is shown how such a structure on the hierarchy can be exploited to describe all common uses of inheritance in a natural way. Finally, the necessary formalisms to define abstract and concrete classes and their relationships, the type system along with the static type checker, and the semantic model to interpret type safety are developed.