Big Java, 4th Edition

Chapter 3. Implementing Classes

CHAPTER GOALS

• To become familiar with the process of implementing classes

• To be able to implement simple methods

• To understand the purpose and use of constructors

• To understand how to access instance variables and local variables

• To be able to write javadoc comments

  G To implement classes for drawing graphical shapes

In this chapter, you will learn how to implement your own classes. You will start with a given design that specifies the public interface of the class—that is, the methods through which programmers can manipulate the objects of the class. Then you will learn the steps to completing the class. You need to implement the methods, which requires that you find a data representation for the objects, and supply the instructions for each method. You need to document your efforts so that other programmers can understand and use your creation. And you need to provide a tester to validate that your class works correctly.

Instance Variables

In Chapter 2, you learned how to use objects from existing classes. In this chapter, you will start implementing your own classes. We begin with a very simple example that shows you how objects store their data, and how methods access the data of an object. You will then learn a systematic process for implementing classes.

Our first example is a class that models a tally counter, a mechanical device that is used to count people—for example, to find out how many people attend a concert or board a bus (see Figure 1).

Whenever ...