Learning Java, 4th Edition

Running Java Applications

A standalone Java application must have at least one class containing a method called main(), which is the first code to be executed upon startup. To run the application, start the VM, specifying that class as an argument. You can also specify options to the interpreter as well as arguments to be passed to the application:

    % java [interpreter options] class_name [program arguments]

The class should be specified as a fully qualified class name, including the package name, if any. Note, however, that you don’t include the .class file extension. Here are a couple of examples:

    %java animals.birds.BigBird
    %java MyTest

The interpreter searches for the class in the classpath, a list of directories and archive files where classes are stored. We’ll discuss the classpath in detail in the next section. The classpath can be specified either by an environment variable or with the command-line option -classpath. If both are present, the command-line option is used.

Alternately, the java command can be used to launch an “executable” Java archive (JAR) file:

    % java -jar spaceblaster.jar

In this case, the JAR file includes metadata with the name of the startup class containing the main() method, and the classpath becomes the JAR file itself.

After loading the first class and executing its main() method, the application can reference other classes, start additional threads, and create its user interface or other structures, as shown in Figure 3-1.

Figure 3-1. Starting a Java ...