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.
A class is a user defined data type. It is a blueprint or prototype from which objects are created. It represents the set of properties or methods that are common to all objects of one type.
A class is declared by use of the class keyword. A simplified general form of a class definition is shown here:
class classname {
Type instanceVariable1;
Type instanceVariable2;
// ...
Type instanceVariableN;
Type methodName1(parameterList) {
// body of method
}
Type methodName2(parameterList) {
// body of method
}
// ...
Type methodNameN(parameterList) {
// body of method
}
}Collectively, the methods and variables defined within a class are called members of the class.
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.
Here is a class called Box that defines three instance variables: width, height, and depth.
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.
To create an object of type Box you will use the new keyword.
Box myBox = new Box();This statement will create a new instance, myBox, of Box. Each time you create an instance of the class it creates an object of it having its own copy of instance variables defined by the class.
To access the variables, you will use the dot (.) operator.
myBox.width = 100;This assigns the value 100 to the copy of width that is contained within the myBox object.
/* 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, which is the recommeded way.