Managing Conceptual And Physical Complexity

Introduction

Rarely is an application comprised of just one class. In reality, applications are constructed from many classes, each providing a unique service. This chapter introduces you to the concepts and terminology associated with building complex application behavior from a collection of classes. This is referred to as compositional design or design by composition.

The study of compositional design is the study of aggregation and containment. In this chapter you will learn the two primary aggregation associations: simple and composite. You will also learn how to use a UML class diagram to illustrate the static relationship between classes in a complex application. To do this you will need to know how to express simple and composite aggregation visually.

The study of aggregation also entails learning how a whole class accesses the services of its part classes. The concepts of message passing and sequencing will be demonstrated by introducing you to a new type of UML diagram known as the sequence diagram.