Q.1. Introduction

Most of the examples provided in this book are relatively small. These examples do not require an extensive design process, because they use only a few classes and illustrate introductory programming concepts. However, some programs are more complex—they can require thousands of lines of code or even more, contain many interactions among objects and involve many user interactions. Larger systems, such as air-traffic control systems or the systems that control a major bank’s thousands of automated teller machines, could contain millions of lines of code. Effective design is crucial to the proper construction of such complex systems.

Over the past decade, the software-engineering industry has made significant progress in the field of design patterns—proven architectures for constructing flexible and maintainable object-oriented software. Using design patterns can substantially reduce the complexity of the design process. Designing an air-traffic control system will be a somewhat less formidable task if developers use design patterns. Design patterns benefit system developers by