Introduction

In our discussions of object-oriented programs in the preceding chapters, we introduced many basic concepts and terminology that relate to Java object-oriented programming (OOP). We also discussed our program development methodology: We selected appropriate variables and methods for each program and specified the manner in which an object of our class collaborated with objects of Java API classes to accomplish the program's overall goals.

In this chapter, we take a deeper look at building classes, controlling access to members of a class and creating constructors. We discuss compositiona capability that allows a class to have references to objects of other classes as members. We reexamine the use of set and get methods and further explore the new J2SE 5.0 class type enum (introduced in Section 6.10) that enables programmers to declare and manipulate sets of unique identifiers that represent constant values. In Section 6.10, we introduced the basic enum type, which appeared within another class and simply declared a set of constants. In this chapter, we discuss the relationship between enum types and classes, demonstrating that an enum, like a class, can be declared in its own file with constructors, methods and fields. The chapter also discusses static class members and final instance variables in detail. We investigate issues such as software reusability, data abstraction and encapsulation. Finally, we explain how to organize classes in packages to help manage large applications and promote reuse, then show a special relationship between classes in the same package.

Chapter 9, Object-Oriented Programming: Inheritance, and Chapter 10, Object-Oriented Programming: Polymorphism, introduce inheritance and polymorphism, respectivelytwo additional key object-oriented programming technologies.