Contract-Java: Design by Contract in Java

Abstract

Design by Contract (DbC) is a programming methodology in which the meaning of program entities, such as methods and classes, is made explicit by the use of programming predicates named assertions. A non-incorrect program is simply one in which all assertions are true. This simple founding idea, which is the essence of DbC, give programmers a tool able to specify, test, debug, document programs, as well as a mechanism to construct fault tolerant systems. However, although well adapted to object-oriented programming (and other popular techniques such as unit testing), Design by Contract still has a very low practical acceptance and application. We believe that one of the main reasons for such is the lack of a proper support for it on most programming languages currently in use (such as Java). The existing tools that support Design by Contract in Java do not allow to take the full advantage of the methodology, either by relegating it to minor comment alike language constructs or by lacking a proper Disciplined Exception Mechanism. We propose a new Java extension, named “Contract-Java”, fully compatible with existing Java code, in which DbC is fully implemented and integrated in Java. Unlike current DbC approaches in Java, our language has a disciplined exception mechanism, simplifying and enhancing error handling (and enabling possible fine-tuned fault tolerance), which coexists peacefully with Java’s normal exception mechanisms. A disciplined exception mechanism, unlike try/catch based exceptions, ensures that a method either terminates successfully meeting its post-condition, or it fails propagating a DbC exception. Another feature in which our approach innovates is its enhanced debug information automatic generation in the presence of a contract failure (which may ease program development).