If you are talking about the EJS version. You should be able to download EJS source (click link) just below the applet. If you want to find the .java file. Run EJS and load xml file, click [b]run[/b] to generate .java file.
If you are talking about the JDK 1.0.2 version. it is included in the ZIP file you would received if you clicked the "get file for offline use" button. I do not think you have read the posted message carefully, that is the reason why you did not find out how to get source code for traffic light system.