Programming by contract

"Why can\´t software be more like hardware?" has been the software engineer\´s lament for nearly as long as there have been large software systems. In particular, why isn\´t there a software components industry to rival the existing hardware components industry? Hardware components come with the following attributes: an interface that hides detail that would only confuse or at least distract me; an unambiguous interface specification written in a language I can understand (in the case of the integrated circuit, this may be a fairly complex language, but it\´s one I expect to learn if I\´m going to work with that hardware); a guarantee-the component has been tested and/or validated against its specification. All three items-especially the last one-are notably lacking for software components. Indeed, software tends to come with an antiguarantee, otherwise known as a disclaimer. All of the above points rely on a rigorous specification of the hardware component\´s interface. In a nutshell, programming by contract is about providing just such specifications for software components (that is, classes), and it provides the best hope of a basis for a true software component industry. The discussion focuses on object oriented software.