In the first, simple version, the whole of the program is implemented as a Java Applet. The following solution is more comprehensive. It forms the example that is used to illustrate UML in the talk The Four Faces of UML. You can try running it below:
The above Applet has been tested in Netscape version 4.05. As the previous version, this one has bugs in the Java virtual machine. So if your net browser does not run my applet, you can try to download JDK 1.1 PREVIEW RELEASE from http://developer.netscape.com/software/jdk/download.html . Its free, and it works for me.
You can download the Java code for this example. The tool part is in http://heim.ifi.uio.no/~trygver/javaclasses/editor1a, the server part is in http://heim.ifi.uio.no/~trygver/javaclasses/kernel1a.