An Object-oriented Approach to Formal Specification

Formal methods for software development are becoming increasingly necessary as softwarebecomes an important part of everyday life. To handle the complexities inherent in large-scale software systems these methods need to be combined with a sound developmentmethodology which supports modularity and reusability. Object orientation, based on theconcept that systems are composed of collections of interacting objects whose behavioursare specified by classes, is such a methodology.This thesis presents the formal specification language Object-Z which is an extension ofthe formal specification language Z to facilitate specification in an object-oriented style.The major extension in Object-Z is the introduction of the class schema which capturesthe object-oriented notion of a class by encapsulating a single state schema with all theoperation schemas which may affect its variables. The class schema is not simply asyntactic extension but also defines a type whose instances are objects. Object-Z alsosupports single and multiple inheritance allowing classes to be reused in the definitionof other classes and polymorphism allowing a variable to be assigned to objects of morethan one class.The thesis also presents a set-theoretic model of classes in Object-Z which could form thebasis of a full formal semantics. The model, based on the histories of a class, i.e. thesequences of states and operations which an object of the class can undergo, facilitatesthe specification of liveness properties using a temporal logic notation. A fully-abstractmodel of classes in Object-Z, derived from the history model, is also presented. Thismodel is used to formally define a notion of behavioural compatibility in Object-Z whichcould form the basis of a theory of class refinement.