7.5 Applications

For a Java program to run as an application, it must have at least one public class that contains a public static method called main() that takes exactly one parameter, an array of String objects.

Here is a very simple sample application that outputs "Hello World!" and then exits:

class DoIt {
    public static void main  (String argv[]){
         System.out.println ("Hello World!");
    }
}

The main() method must be public so that the Java virtual machine can find it. If the method is not public, its name is not included in the compiler's output. The system does not create any objects prior to the start of the application's main() method, so the main() method must be static because it cannot be associated with an object.

If an application has a graphical user interface, then it typically creates a java.awt.Frame object in main(). The Frame object acts as the top-level window for the application.

In Sun's implementation of Java, you run a Java application by running the Java interpreter with a command-line argument that specifies the name of the class that contains the main(). The name of the Java interpreter is java. Here's the command-line for our sample application:

C:\> java DoIt

The capitalization of the class name on the command line must match the capitalization of the class name within the program. If the class is part of a named package, the name of the class must be qualified with the package name. For example, if you have a package called COM.geomaker and it contains the class called DoIt, you would use the following command to run the application:

C:\> java COM.geomaker.DoIt

Any additional information that you provide on the command line is passed to the application as command line arguments. These arguments are passed to the application using the String array passed to main(). The number of elements in the array is equal to the number of arguments passed to the application. If there are no arguments to the application, the length of the array passed to main() is zero.

References Methods; Packages