Right after I completed it, the tutorial was used in a workshop at the NetBeans Platform Certified Training in Warsaw. So, a group of between 20 and 30 students used it and managed to finish the tutorial, In other words, the code works. Go here if you want to use it too:
Some interesting details about this tutorial—it doesn't use the Matisse GUI Builder, so you could use the code in any IDE and it doesn't use the NetBeans Platform, so that it is applicable to any desktop Java application. It is quite simple and short, after which you can follow other more complex tutorials on the NetBeans Platform Learning Trail.
Many thanks to Marek Kliś, in whose great blog I found the code used above! Dziekuje bardzo, Marek.