Introducing Classes
T
he class is at the core of Java. It is the logical construct upon which the entire Java
language is built because it defines the shape and nature of an object. As such, the
class forms the basis for object-oriented programming in Java. Any concept you wish
to implement in a Java program must be encapsulated within a class.
Because the class is so fundamental to Java, this and the next few chapters will be devoted
to it. Here, you will be introduced to the basic elements of a class and learn how a class can be
used to create objects. You will also learn about methods, constructors, and the this keyword.
Class Fundamentals
Classes have been used since the beginning of this topic. However, until now, only the most
rudimentary form of a class has been used. The classes created in the preceding
chapters primarily exist simply to encapsulate the main( ) method, which has been used to
demonstrate the basics of the Java syntax. As you will see, classes are substantially more
powerful than the limited ones presented so far.
Perhaps the most important thing to understand about a class is that it defines a new data
type. Once defined, this new type can be used to create objects of that type. Thus, a class is
a template for an object, and an object is an instance of a class. Because an object is an instance
of a class, you will often see the two words object and instance used interchangeably.
The General Form of a Class
When you define a class, you declare its exact form and nature. You do this by specifying the
data that it contains and the code that operates on that data. While very simple classes may
contain only code or only data, most real-world classes contain both. As you will see, a class'
code defines the interface to its data.
A class is declared by use of the class keyword. The classes that have been used up to this
point are actually very limited examples of its complete form. Classes can (and usually do)
get much more complex. A simplified general form of a class definition is shown here:
class classname {
type instance-variable1;
type instance-variable2;
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