Chapter 2. Designing Classes with a Single Responsibility

The foundation of an object-oriented system is the message, but the most visible organizational structure is the class. Messages are at the core of design, but because classes are so obvious this chapter starts small and concentrates on how to decide what belongs in a class. The design emphasis will gradually shift from classes to messages over the next several chapters.

What are your classes? How many should you have? What behavior will they implement? How much do they know about other classes? How much of themselves should they expose?