3.5 Summary

This chapter makes only one point: great software maintains focus on one task. To focus software, sharpen your ability to collect requirements and control your customers. If you're not careful, scope creep can confuse the basic theme of your software. When you've got a more complex problem, break each fundamental theme into a layer, or subsystem. In general, common layers are always evolving for Java technologies. Many of the accepted practices are sound, but others are suspect. Better layers share a common purpose and an effective interface.

Once you've designed effectively layered software and built clean software with a distilled purpose, maintain your clarity of purpose. To keep software focused on a central theme, you'll need to frequently refactor to loosen the coupling around tightly coupled components. Loose coupling is desirable at a lower level, and you can control it by testing and refactoring with techniques like interfaces. Also, pay attention to coupling at a higher level, so that each major subsystem is as isolated as possible. You'll improve reuse and isolate one subsystem from changes in others. In the next chapter, we'll discuss how to take the some extreme steps to reduce coupling between business domain models and services through increased transparency.