Using Java Applets

David Wallace Croft
croft@alumni.caltech.edu

1997-10-15

What are Applets?

Applets are little programs that you can embed in your web pages.

Applets are "live".
Example: a multiplayer arcade game interface.

Applets are secure. They cannot read from or write to your hard drive while you are using them.

Adding an Applet

To add an applet to your web page, you can simply add an applet HTML tag such as the following.

<APPLET
  CODE="NCMECbanner.class"
  CODEBASE="http://www.missingkids.com/java/"
  WIDTH=342
  HEIGHT=128
  >
An applet would be here.
</APPLET>

It's Like the Image Tag

The HTML "applet" tag shares many attributes in common with the HTML "image" tag.

WIDTH (pixels, required)
HEIGHT (pixels, required)
VSPACE (pixels)
HSPACE (pixels)
ALT (alternative text)
ALIGN (left, right, top, texttop, middle, absmiddle, baseline, bottom, absbottom)

Notice that, like a graphics image, the applet can be loaded from a different server. In the above example, it is being loaded from www.missingkids.com.

Special Applet Attributes

The CODE applet tag attribute gives the name of the main applet class to be loaded. This class, once loaded, will call and load additional classes as needed.

The CODEBASE specifies the server and directory from which the classes are loaded. The codebase attribute is optional; if it is not specified, the browser will assume that the classes should be loaded from the same URL (server and directory) as the web page.

The optional ARCHIVE attribute provides a list of pre-compressed files including classes, multimedia, and data files that your applet may need. The archive files are downloaded and checked first before individual files are sought from the server.

Parameters

Some applets need additional parameters specified to customize operation. These parameters are akin to providing command-line arguments to a program. They are provided using the <PARAM> tag in the form of name/value pairs.

<APPLET
  CODE=SlideShow.class
  WIDTH=460
  HEIGHT=160
  >
  <PARAM NAME="music"     VALUE="tophit.au">
  <PARAM NAME="pausetime" VALUE="10"       >
  <PARAM NAME="slide0"    VALUE="me.gif"   >
  <PARAM NAME="slide1"    VALUE="bear.jpg" >
  <PARAM NAME="slide2"    VALUE="dog.gif"  >
</APPLET>

Troubleshooting

If your applet does not work, there could be a number of problems.

Your browser does not support applets (unlikely).
Your browser has the option to support applets turned off.
Your firewall filters out applet packets.
You have the wrong setting for the CODEBASE.
The applet you are calling has been hard-wired to be loaded only from a particular server or web page.
Your applet is loading then crashing.

To check if your applet is crashing in Netscape Navigator, click on "Communicator" and then "Java Console" to view the error output.

Sample Applet

Embedded in this web page is an applet which you put in your HTML pages right away. In Netscape Navigator, click on "View" and then "Page Source" to copy the applet tag and then paste it into your pages.

Back

http://www.alumni.caltech.edu/~croft/research/java/applet/
Copyleft 1997 David Wallace Croft
Posted 1997-10-15