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

Responsibility-driven design specifies object behavior before object structure and other implementation considerations are determined. We have found that it minimizes the rework required for major design changes.

Use cases, scenarios or scripts are roughly synonymous terms for important ways to focus our design activities. I prefer the term use case (although quickly saying it three times can leave your tongue tied) because it emphasizes usage. A use case is a textual description of a sequence of interactions between an actor (roughly corresponding to an external agent or class of users) and the system we are designing. Use cases were first described by Ivar Jacobson in his book “Object Oriented Software Engineering A Use Case Driven Approach.” Use cases have been around in various forms for quite some time. Jacobson, however, made the keen observation that use cases can be treated as refineable, extensible and even reusable specifications of system requirements. We’ve had these same goals for object designs. We know that it is harder to actually accomplish them than it is to talk about them.

The goal of is to improve encapsulation. It does so by viewing a program in terms of the client/server model.