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Thinking in Java
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Testing applets

You can perform a simple test without any network connection by starting up your Web browser and opening the HTML file containing the applet tag. As the HTML file is loaded, the browser will discover the applet tag and go hunt for the .class file specified by the code value. Of course, it looks at the CLASSPATH to find out where to hunt, and if your .class file isn’t in the CLASSPATH, then it will give an error message on the status line of the browser to the effect that it couldn’t find that .class file.

When you want to try this out on your Web site, things are a little more complicated. First of all, you must have a Web site, which for most people means a third-party Internet Service Provider (ISP) at a remote location. Since the applet is just a file or set of files, the ISP does not have to provide any special support for Java. You must also have a way to move the HTML files and the .class files from your site to the correct directory on the ISP machine. This is typically done with a File Transfer Protocol (FTP) program, of which there are many different types available for free or as shareware. So it would seem that all you need to do is move the files to the ISP machine with FTP, then connect to the site and HTML file using your browser; if the applet comes up and works, then everything checks out, right?

Here’s where you can get fooled. If the browser on the client machine cannot locate the .class file on the server, it will hunt through the CLASSPATH on your local machine. Thus, the applet might not be loading properly from the server, but to you it looks fine during your testing process because the browser finds it on your machine. When someone else connects, however, his or her browser can’t find it. So when you’re testing, make sure you erase the relevant .class files (or .jar file) on your local machine to verify that they exist in the proper location on the server.

One of the most insidious places where this happened to me is when I innocently placed an applet inside a package. After uploading the HTML file and applet, it turned out that the server path to the applet was confused because of the package name. However, my browser found it in the local CLASSPATH. So I was the only one who could properly load the applet. It’s important to specify the full class name including the package in the CODE parameter of your applet tag. In many published applet examples, the applet is not put inside a package, but it’s generally best to use packages in production code.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire