Creating an Applet User Interface

Displaying Diagnostics to the Standard Output and Error Streams

Displaying diagnostics to the standard output can be an invaluable tool when you're debugging an applet. Another time you'll see messages at the standard output is when an uncaught exception occurs in an applet. Applets also have the option of using the standard error stream.

Where exactly the standard output and error are displayed varies, depending on how the applet's viewer is implemented, what platform it's running on, and (sometimes) how you launch the browser or applet viewer. When you launch the Applet Viewer from a UNIX shell window, for example, strings displayed to the standard output and error appear in that shell window, unless you redirect the output. When you launch the Applet Viewer from an X Windows menu, the standard output and error go to the console window. Netscape Navigator 2.0, on the other hand, always displays applet standard output and error to the Java Console, which is available from the Options menu.

Applets display to the standard output stream using System.out.print(String) and System.out.println(String). Displaying to the standard error stream is similar; just specify System.err instead of System.out. Here's an example of displaying to the standard output:

//Where instance variables are declared:
boolean DEBUG = true;
. . .
//Later, when we want to print some status:
if (DEBUG) {
    System.out.println("Called someMethod(" + x + "," + y + ")");
}

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Note: Displaying to the standard output and error streams is relatively slow. If you have a timing-related problem, printing messages to either of these streams might not be helpful.

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You should be sure to disable all debugging output before you release your applet.

Creating an Applet User Interface