A subsystem is a set of classes (and possibly other subsystems) collaborating to fulfill a set of responsibilities. Although subsystems do not exist … - Rebecca Wirfs-Brock

Experienced object designers explore the design space from many different angles. They refine ideas of how their system should respond while they are in the middle of building and discarding ideas about how their design should work. Getting a design to gel involves making assumptions, seeing how they play out, changing one’s mind or perspective slightly and re-iterating. Design is a difficult, involved task. It inherently is a non-linear process. Yet, we are asked to trace our design results back to system requirements. And, if we uncover some implications during design, we’d like to tune our system requirements to reflect necessary design compromises.

Users can work with analysts and object designers to formulate and tune system requirements. People from business, analytical and object design disciplines can come together, learn from each other and generate meaningful descriptions of systems that are to be built. Each participant and each project has slightly different concerns and needs. Practical application of use cases can go a long way to improve our ability to deliver just what the customer ordered.

Object-oriented programming languages support encapsulation, thereby improving the ability of software to be reused, refined, tested, maintained, and extended. The full benefit of this support can only be realized if encapsulation is maximized during the design process. We argue that design practices which take a data-driven approach fail to maximize encapsulation because they focus too quickly on the implementation of objects. We propose an alternative object-oriented design method which takes a responsibility-driven approach. We show how such an approach can increase the encapsulation by deferring implementation issues until a later stage.