The Mysterious Case of the Missing Reusable Class Libraries

Where are the reusable class libraries as promised by the literature on Object-Oriented (OO)methods? We argue that within corporate data processing groups, resource pressures prevent the development of reusable class libraries based on an optional, additional, generalisation of some specific OO application. We observe that the development of such general libraries need not wait for the completion of the specific application, if the application is regarded as an example of the type of processing required for the domain. The products of any particular application development could be (i) the application itself; and (ii) more importantly, a set of general tools, specifically class libraries, for building similar applications. In terms of a class library, we believe that reusable class libraries are not simply specialised/ generalised application classes. That is to say generalised reusable classes do not simply evolve from vertical refinement. More typically, they involve horizontal expansion of the application's domain via client-server/association relationships. Examples of class libraries created by horizontal extension of the initial development process are given. Seven factors are described that promote the construction of class libraries during development. Object-oriented analysis (OOA) and design (OOD) methodologies are assessed for their ability to facilitate the development of the horizontal classes. Most of the current methods do not support the development of horizontal class structures and hence a contributing factor to the lack of reusable components may be our current generation of OOA and OOD methodologies.