1.4 Running a Java Application

To run a Java application, you invoke the Java interpreter, java, with one or more arguments. The first argument is always the name of a Java class. Here is how to run the "Hello World" application:

C:\> java HelloWorld

The capitalization of the class name must match the name used in the class declaration in the source file. The interpreter loads the specified class and then calls its main() method.

A class can belong to a particular package. This allows the class to prevent classes in other packages from accessing its declared variables and methods. If a class is not specified as part of a package, it automatically becomes part of the default package. Because the HelloWorld class is part of the default package, you do not need to include the package name as part of the class name on the command line. If the HelloWorld class were part of a package called student.language, however, you would have to include the package name on the command line. For example, you would run the application as follows:

C:\> java student.language.HelloWorld

Any additional arguments specified on the command line are passed to the main() method in its String[] parameter. For the "Hello World" application, the String[] parameter is an empty array. If, however, there were command-line arguments, the first array element, String[0], would correspond to the first command-line argument specified after the class name, String[1] would correspond to the next command-line element, and so on. The name of the class does not appear as an element in the array of parameters passed to the main() method. This is different than in C/C++, where the first element in the array of command-line arguments identifies the program name and the second element is the first command-line argument.