Introduction

This chapter explores object design for use-case realizations for the next iteration of the NextGen case study, which tackles support for external third-party services whose interfaces may vary, more complex product pricing rules, and pluggable business rules.

In the context of the design problems that will be discussed, new high-use UML notation will also be introduced.

The emphasis is to show how to apply the GoF and more basic GRASP patterns. It attempts to illustrate that object design and the assignment of responsibilities can be explained and learned based on the application of patterns—a vocabulary of principles and idioms that can be combined to design objects.

The Gang-of-Four Patterns

The additional patterns presented in this chapter are drawn from Design Patterns [GHJV95], a seminal and extremely popular work that presents 23 patterns useful during object design. Since the book was written by four authors, these patterns have become known as the “Gang-of-Four”—or “GoF”—patterns.^[1]

^[1] With a tangential reference to Chinese politics.

This chapter provides an introduction to some of the high-use GoF patterns; subsequent chapters present more.^[2] A thorough study of the Design Patterns book is recommended to grow as an object designer, although that book assumes the reader is already a designer with some experience; this book offers an introduction.

^[2] In practice, perhaps approximately 15 of these 23 patterns are frequently used.

A Shared Vocabulary

In addition to the visual vocabulary of UML notation, by the end of this chapter we will have a richer shared vocabulary of design, in terms of pattern names. Thus, it will be possible to increasingly communicate software design ideas primarily in UML diagrams, with some attached notes that indicate the patterns (Indirection, Strategy, ...) being applied.