2.34 Jb0270: java oop: packages  (Page 2/6)

I will also refer you to Java OOP: The Guzdial-Ericson Multimedia Class Library where I discuss the use of the classpath environment variable with a Java multimedia class library.

Some rules

There are some rules that you must follow when working with the classpath variable, and if you fail to do so, things simply won't work.

For example, if your class files are in a jar file, the classpath must end with the name of that jar file.

On the other hand, if the class files are not in a jar file, the classpath must end with the name of the folder that containsthe class files.

Your classpath must contain a fully-qualified path name for every folder that contains class files of interest, or for every jarfile of interest. The paths should begin with the letter specifying the drive and end either with the name of the jar file or the name ofthe folder that contains the class files. .

If you followed the default JDK installation procedure and are simply compiling and executing Java programs in the current directory youprobably won't need to set the classpath. By default, the system already knows (or can figure out) how to allow you to compile and execute programs in the current directory and how to use the JDK classes that come as part ofthe JDK.

However, if you are using class libraries other than the standard Java library, are saving your class files in one or more different folders,or are ready to start creating your own packages, you will need to set the classpath so that the system can find the class files in yourpackages.

Developing your own packages

One of the problems with storing all of your class files in one or two folders is that you will likely experience name conflicts between class files.

Every Java program can consist of a large number of separate classes. A class file is created for each class that is defined in your program, even if they areall combined into a single source file.

It doesn't take very many programs to create a lot of class files, and it isn't long before you find yourself using the same class names over again. Ifyou do, you will end up overwriting class files that were previously stored in the folder.

For me, it only takes two GUI programs to get in trouble because I tend to use the same class names in every program for certain standard operations suchas closing a Frame or processing an ActionEvent . For the case of the ActionEvent , the body of the class varies from one application to the next so it doesn't make sense to turn it into a library class.

So we need a better way to organize our class files, and the Java package provides that better way.

The Java package approach allows us to store our class files in a hierarchy of folders (or a jar file that represents that hierarchy) while only requiring that the classpath variable point to the top of thehierarchy. The remaining hierarchy structure is encoded into our programs using package directives and import directives.

Now here is a little jewel of information that cost me about seven hours of effort to discover when I needed it badly.