15. Design Patterns

One of the interesting things about software development is that when you create a software system, you are actually modeling a real-world system. For example, in the Information Technology industry, it is safe to say that IT is the business—or at least IT implements the business. To write the business software systems, the developers must thoroughly understand the business models. As a result, the developers often have the most intimate knowledge of a company’s business processes.

We have seen this concept throughout this book as it relates to our educational discussions. For example, when we discussed using inheritance to abstract out the behaviors and attributes of mammals, the model was based on the true real-life model, not a contrived model that we created for our own purposes.

Thus, when we create a mammal class, we can use it to build countless other classes, such as dogs and cats and so on, because all mammals share certain behaviors and attributes. This works when we study dogs, cats, squirrels, and other mammals because we can see patterns. These patterns allow us to inspect an animal and make the determination that it is indeed a mammal, or perhaps a reptile, which would have other patterns of behaviors and attributes.

Throughout history, humans have used patterns in many aspects of life, including engineering. These patterns go hand-in-hand with the holy grail of software development: software reuse. In this chapter, we consider design patterns, a relatively new area of software development (the seminal book on design patterns was published in 1995).

Design patterns are perhaps one of the most influential developments that have come out of the object-oriented movement in the past several years. Patterns lend themselves perfectly to the concept of reusable software development. Because object-oriented development is all about reuse, patterns and object-oriented development go hand-in-hand.

The basic concept of design patterns revolves around the principle of best practices. By best practices, we mean that when good and efficient solutions are created, these solutions are documented in a way that others can benefit from previous successes—as well as learn from the failures.

One of the most important books on object-oriented software development is Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. This book was an important milestone for the software industry and has become so entrenched in the computer science lexicon that the book’s authors have become known as the Gang of Four. In writings on object-oriented topics, you will often see the Gang of Four referred to as the GoF.

The intent of this chapter is to explain what design patterns are. (Explaining each design pattern is far beyond the scope of this book and would take more than one volume.) To accomplish this, we explore each of the three categories (creational, structural, and behavioral) of design patterns as defined by the Gang of Four and provide a concrete example of one pattern in each category.