Overview

In previous chapters, I discussed three fundamental concepts of object-oriented design: objects, encapsulation, and abstract classes. How a designer views these concepts is important. The traditional ways are simply too limiting. In this chapter I step back and reflect on topics discussed earlier in the book. My intent is to describe a new way of seeing object-oriented design, which comes from the perspective that design patterns create.

In this chapter,

• I compare and contrast the traditional way of looking at objects—as a bundle of data and methods—with the new way—as things with responsibilities.

• I compare and contrast the traditional way of looking at encapsulation—as hiding data—with the new way—as the ability to hide anything. Especially important is to see that encapsulation can be used to contain variation in behavior.

• I compare and contrast the traditional way of using inheritance—for specialization and reuse—with the new way—as a method of classifying objects.

• The new viewpoints allow for containing variation of behaviors in objects.

• I show how the conceptual, specification, and implementation perspectives relate to an abstract class and its derived classes.

Perhaps this new perspective is not all that original. I believe that this perspective is one that many developers of the design patterns held when they developed what ended up being called a pattern. Certainly, it is a perspective that is consistent with the writings of Christopher Alexander, Jim Coplien, and the Gang of Four.

While it may not be original, it has also not been expressed in quite the way I do in this chapter and in this book. I have had to distill this way of looking at patterns from the way design patterns behave and how they have been described by others.

When I call it a new perspective, what I mean is that it is most likely a new way for most developers to view object orientation. It was certainly new to me when I was learning design patterns for the first time.