Are you teaching someone Java? Do you know anyone who is taking an introductory training in Java? Do you want your 5 year old kid to start on Java? If yes, then you can help them further by introducing to free interactive IDEs made just for this purpose.
PMD comes with a set of handy rules, which more than often are sufficient for any project. However, in real life projects you also need some custom rules to be created. This article talks about writing custom PMD rules using XPath.
An interview with award winning scientist & proponent of the Hole in the Wall (HIW) experiment: Dr Sugata Mitra. He started the experiment at the slums at Kalkaji, Delhi in 1999 & proved that kids could learn computers without formal training and teachers.