Frameworks are skeletal structures of programs that must be fleshed out to build a complete application. For example, a windowing system or a simulat… - Rebecca Wirfs-Brock

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.

A subsystem is a set of classes (and possibly other subsystems) collaborating to fulfill a set of responsibilities. Although subsystems do not exist as the software executes, they are useful conceptual entities.

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.