Inheritance and subtyping methods, modifications and analytical quantification

Object-oriented languages provide support for reusability and modularity. Reusability is achieved by inheritance, whereas modularity is achieved by the encapsulation of methods. Inheritance can be considered to be an implementation technique that saves the effort associated with code duplication. Although inheritance results in a specialization or refinement of a superclass, it is possible that if one class inherits from another, the type of the objects generated by the derived class in not necessarily a subtype of the type of the objects generated by the original class. This paper reviews existing techniques for solving this problem and proposes an approach used in the design of a statically-typed object-oriented language called POOL, together with criteria for analyzing and comparing such techniques.