ACT THREE, Scene 1: In Which We Do Design

Your goal is to invent and arrange objects in a pleasing fashion. Your application will be divided into neighborhoods where clusters of objects work toward a common goal. Your design will be shaped by the number and quality of abstractions and by how well they complement one another. Composition, form, and focus are everything.

—Rebecca Wirfs-Brock and Alan McKean⁸

Identifying objects (or object classes) is a difficult part of object-oriented design. There is no ‘magic formula’ for object identification. It relies on the skill, experience and domain knowledge of system designers (that would be you). Object identification is an iterative process. You are not likely to get it right the first time.

You begin finding objects by looking for real-world analogues in your requirements. That gets you started, but it's only the first step. Other objects hide in the abstraction layers of your domain. Where to find these hidden objects? You can look to your own knowledge of the application domain, you can look for operations that crop up in your requirements and in your architectural concepts of the system. You can even look to your own past experience designing other systems.