Maintaining the consistency of class libraries during their evolution

Two important problems of object-oriented reuse are the propagation of design and implementation specifics of the base software to the inheritors, and the protection of the inheritors against changes in the base software. In this paper, we argue that the simple inheritance rules of existing object-oriented languages are not sufficient for properly dealing with these problems. In the proposal presented in this paper, programmers are enabled to make metalevel declarations of the internal protocols and dependencies of their classes. Additionally, changes of the base module are automatically monitored to filter out information about the alterations that may invalidate already existing inheritors. Based on these informations, the subclassing semantics is adjusted such that the maintenance of the base module properties and the protection of the inheritor is ensured during their integration. In this way, language support is provided for keeping the behavior of reusable software consistent during its evolution.