The Complete Reference
The 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 a d learn how a class can be used to create objects. You will also learn about methods, constructors, and the this keyword.
Classes have been used since the beginning of this book. However, until now, only the most rudimentary form of a class has been used, The classes created in the pre ceding 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
specifying the data-that it contains and the code that operates on that data. While very simple classes m. y contain only code or only detail, most real-world classes cont sin both. As you will see, a class’ code defines the interface to its data.A class is declared by u~e of the class keyword. The classes that have been used to this point arc actually very limited examples of its complete form, Classes can (and usually do) get much more complex. The general [arm of a class ‘definition is shown here:
The data, or variables, defined within a class are called instance variables. The code
is contained within methods. Collectively, the methods and variables defined within a class are called members of the class. In most classes, the instance variables are acted
upon and accessed by the methods defined for that class. Thus, it is the methods that determine how a class’ data can be used. Variables defined within a class are called instance variables because each instance of the class (that is, each object of the class) contains its own copy of these variables. Thus, the data for one object is separate and unique from the data for another We will come back to this point shortly, but it is an important concept to learn early. All methods have the same general form as-main which we have been using the use. However, most methods will not be specified as static or public. Notice that the general form of a class does not specify a main method. Java classes do not need to have a main method: You only specify one if that class is the starting point for your program. Further, don’t require a main method at all.