The file MosaicDrawLauncherApplet.java contains the source code for the applet. One interesting point in the applet is that the text of the button changes depending on whether a window is open or not. If there is no window, the text reads “Launch MosaicDraw”. When the window is open, it changes to “Close MosaicDraw”, and clicking the button will close the window. The change is implemented by attaching a WindowListener to the window. The listener responds to WindowEvents that are generated when the window opens and closes. Although I will not discuss window events further here, you can look at the source code for an example of how they can be used.

6.8.4Creating Jar Files

As the final topic for this chapter, we look again at jar files. Recall that a jar file is a “java archive” that can contain a number of class files. When creating a program that uses more than one class, it’s usually a good idea to place all the classes that are required by the program into a jar file, since then a user will only need that one file to run the program. Subsection 6.2.4 discusses how a jar file can be used for an applet. Jar files can also be used for stand-alone applications. In fact, it is possible to make a so-called executable jar file. A user can run an executable jar file in much the same way as any other application, usually by double-clicking the icon of the jar file. (The user’s computer must have a correct version of Java installed, and the computer must be configured correctly for this to work. The configuration is usually done automatically when Java is installed, at least on Windows and Mac OS.)

The question, then, is how to create a jar file. The answer depends on what programming environment you are using. The two basic types of programming environment—command line and IDE—were discussed in Section 2.6. Any IDE (Integrated Programming Environment) for Java should have a command for creating jar files. In the Eclipse IDE, for example, it’s done as follows: In the Package Explorer pane, select the programming project (or just all the individual source code files that you need). Right-click on the selection, and choose “Export” from the menu that pops up. In the window that appears, select “JAR file” and click “Next”. In the window that appears next, enter a name for the jar file in the box labeled “JAR file”. (Click the “Browse” button next to this box to select the file name using a file dialog box.) The name of the file should end with “.jar”. If you are creating a regular jar file, not an executable one, you can hit “Finish” at this point, and the jar file will be created. You could do this, for example, if the jar file contains an applet but no main program. To create an executable file, hit the “Next” button twice to get to the “Jar Manifest Specification” screen. At the bottom of this screen is an input box labeled “Main class”. You have to enter the name of the class that contains the main() routine that will be run when the jar file is executed. If you hit the “Browse” button next to the “Main class” box, you can select the class from a list of classes that contain main() routines. Once you’ve selected the main class, you can click the “Finish” button to create the executable jar file.

It is also possible to create jar files on the command line. The Java Development Kit includes a command-line program named jar that can be used to create jar files. If all your classes are in the default package (like the examples in this book), then the jar command is easy to use. To create a non-executable jar file on the command line, change to the directory that contains the class files that you want to include in the jar. Then give the command

jar cf JarFileName.jar *.class

where JarFileName can be any name that you want to use for the jar file. The “*” in “*.class” is a wildcard that makes *.class match every class file in the current directory. This means