Below is a quick visual guide to the code structure of the now-defunct Window System Example—I decided to release the source code since Applets do not generally work on today’s web. You can click on the code summaries to see the full versions of each file (there are a couple of other little clickable easter eggs in there, too).
If you try real hard, you might be able to get the Applet to run in your browser (see the link above; you might need to refresh the page or even re-launch your browser). After Oracle purchased Java™, they made some significant changes to the security policies, and newer operating systems and browser will not run Applets easily. Chrome in particular will just not show them, due to lack of support for a cross-platform plug-in architecture called NPAPI.
Safari can show and run Applets if the operating system allows it: see this explanation of how to trick out your browser and this explanation of how to set system security settings for Java Applet display.