Guiding Specification and OO implementation of Data Types

Design by contract (DBC) is among the most popular techniques that are taught in introductory programming courses aiming at helping students to learn how to construct correct and robust software. Although we recognize the important role played by formal design as supported by DBC techniques, we have experienced for several years the frustration of not being able to guide students in writing contracts, both that fully specify all the relevant properties, and are monitorable. The fact that students are left with very poor specifications leads them to perceive contracts as unnecessary and even irrelevant, discouraging the further application of DBC. We addressed these problems through the adoption of property-driven algebraic specifications for the description of the observable behavior of programs. Our approach comprises a tool-assisted refinement process that supports the run-time checking of implementations against specifications. In this paper we present the approach and report on our experience of using it.