Getting Class Correctness and System Correctness Equivalent - How to Get Covariance Right

Covariant typing policies, as they are available in Eiffel or Sather, result in a type system in which the correct typing of a system is not implied by the correct typing of each class. This hinders a secure, incremental development of class libraries in these object-oriented languages. Classes which are added to a library may invalidate others that are already in the library. The paper shows how a covariant typing policy may be omitted without losing flexibility. As a consequence we regain the equivalence of class correctness and system correctness (and hence class validity and system validity). The approach taken in this paper is based on the observation that inheritance is used either for the integration of subsystems or for the adaptation of general problem solutions to specific situations. It turns out that covariant typing policies are used for the latter situation. We show how adaptation may be solved by instantiation of schemes similar to generics. We give rules how such classe..