Structural and Behavioural Views on OMT-Classes

Object-oriented specification languages provide means to specify the static structure, as well as the allowed dynamic behaviour of objects. Here, the dynamic behaviour is usually described by giving the methods and a state transition diagram which defines the allowed sequences of methods on objects of a certain class. Specialized classes are defined using the inheritance relationship. In order to enable reusability while guaranteeing type substitutability, subclass specifications have to be compatible with respect to static and dynamic aspects with their corresponding superclass specifications. Classes have to provide a large shopping list of operations to satisfy the needs of all possible users. The interests of specific users are often restricted to a subset of operations and thus to a restricted behaviour. This paper describes a formalization of class descriptions given by attributes, operations, as well as state transition diagrams. It defines compatibility between sub- and superclasses and introduces the notion of views in the sense of hiding parts of a class description. It turns out that a view has the same properties as a (virtual) superclass.