Introducing Classes
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 Posts 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 book. However, until now, only the most rudimentary form of a class has been used. The classes created in the preceding Posts 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.
The general form of a class definition is shown here:
class classname {
type instance-variable1;
type instance-variable2;
// ...
type instance-variableN;
type methodname1(parameter-list) {
// body of method
}
type methodname2(parameter-list) {
// body of method
}
// ...
type methodnameN(parameter-list) {
// body of method
}
}
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 thus far. 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, applets don't require a main( ) method at all.
C++ programmers will notice that the class declaration and the implementation of the methods are stored in the same place and not defined separately. This sometimes makes for very large .java files, since any class must be entirely defined in a single source file. This design feature was built into Java because it was felt that in the long run, having specification, declaration, and implementation all in one place makes for code that is easier to maintain.
A Simple Class
Let's begin our study of the class with a simple example. Here is a class called Box that defines three instance variables: width, height, and depth. Currently, Box does not contain any methods (but some will be added soon).
class Box {
double width;
double height;
double depth;
}
As stated, a class defines a new type of data. In this case, the new data type is called Box. You will use this name to declare objects of type Box. It is important to remember that a class declaration only creates a template; it does not create an actual object. Thus, the preceding code does not cause any objects of type Box to come into existence.
To actually create a Box object, you will use a statement like the following:
Box mybox = new Box(); // create a Box object called mybox
After this statement executes, mybox will be an instance of Box. Thus, it will have
"physical" reality. For the moment, don't worry about the details of this statement.
Again, each time you create an instance of a class, you are creating an object that
contains its own copy of each instance variable defined by the class. Thus, every Box object will contain its own copies of the instance variables width, height, and depth. To access these variables, you will use the dot (.) operator. The dot operator links the name of the object with the name of an instance variable. For example, to assign the width variable of mybox the value 100, you would use the following statement:
mybox.width = 100;
This statement tells the compiler to assign the copy of width that is contained within the mybox object the value of 100. In general, you use the dot operator to access both the instance variables and the methods within an object.
Here is a complete program that uses the Box class:
/* A program that uses the Box class.
Call this file BoxDemo.java
*/
class Box {
double width;
double height;
double depth;
}
// This class declares an object of type Box.
class BoxDemo {
public static void main(String args[]) {
Box mybox = new Box();
double vol;
// assign values to mybox's instance variables
mybox.width = 10;
mybox.height = 20;
mybox.depth = 15;
// compute volume of box
vol = mybox.width * mybox.height * mybox.depth;
System.out.println("Volume is " + vol);
}
}
You should call the file that contains this program BoxDemo.java, because the main( ) method is in the class called BoxDemo, not the class called Box. When you compile this program, you will find that two .class files have been created, one for Box and one for BoxDemo. The Java compiler automatically puts each class into its own .class file. It is not necessary for both the Box and the BoxDemo class to actually be in the same source file. You could put each class in its own file, called Box.java and BoxDemo.java, respectively.
To run this program, you must execute BoxDemo.class. When you do, you will see the following output:
Volume is 3000.0
As stated earlier, each object has its own copies of the instance variables. This means that if you have two Box objects, each has its own copy of depth, width, and height. It is important to understand that changes to the instance variables of one object have no effect on the instance variables of another. For example, the following program declares two Box objects:
// This program declares two Box objects.
class Box {
double width;
double height;
double depth;
}
class BoxDemo2 {
public static void main(String args[]) {
double vol;
// assign values to mybox1's instance variables
mybox1.width = 10;
mybox1.height = 20;
mybox1.depth = 15;
/* assign different values to mybox2's
instance variables */
mybox2.width = 3;
mybox2.height = 6;
mybox2.depth = 9;
// compute volume of first box
vol = mybox1.width * mybox1.height * mybox1.depth;
System.out.println("Volume is " + vol);
// compute volume of second box
vol = mybox2.width * mybox2.height * mybox2.depth;
System.out.println("Volume is " + vol);
}
}
The output produced by this program is shown here:
Volume is 3000.0
Volume is 162.0
As you can see, mybox1's data is completely separate from the data contained in mybox2.
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