[Note: This chapter depends on the terminology and concepts discussed in Section 1.2, Object Technology Concepts.]
In Chapter 2, you worked with existing classes, objects and methods. You used the predefined standard output object System.out, invoking its methods print, println and printf to display information on the screen. You used the existing Scanner class to create an object that reads into memory integer data typed by the user at the keyboard. Throughout the book, you’ll use many more preexisting classes and objects.
In this chapter, you’ll create your own classes and methods. Each new class you create becomes a new type that can be used to declare variables and create objects. You can declare new classes as needed; this is one reason why Java is known as an extensible language.
We present a case study on creating and using a simple, real-world bank account class—Account. Such a class should maintain as instance variables attributes such as its name and balance, and provide methods for tasks such as querying the balance (getBalance), making deposits that increase the balance (deposit) and making withdrawals that decrease the balance (withdraw). We’ll build the getBalance and deposit methods into the class in the chapter’s examples.
In Chapter 2 we used the data type int to represent integers. In this chapter, we introduce data type double to represent an account balance as a number that can contain a decimal point—such numbers are called floating-point numbers. [In Chapter 8, when we get a bit deeper into object technology, we’ll begin representing monetary amounts precisely with class BigDecimal (package java.math) as you should do when writing industrial-strength monetary applications.]