In Chapter 3 we have seen that a class is a reference data type. Just like we have primitive data types in Java, which are predefined by the Java language, classes are also data types, but these are defined by a programmer. Just like we can use the primitive data in different applications, the classes defined by a programmer can also be reused in a number of applications.
The methods in a class are like the operators for the data type (class). In Exercise 2.1, we defined the class
HelloWorld as a place holder for the Java application. A Java application is defined in a method called
main(), so let us call those classes that contain the application (method main) as an application class. All classes need not contain ...