Unfortunately, you can't easily debug it, because the JID class is in another library (Tinder).
If you really have the passion, you could download the Tinder sources (not the released 1.2.2 version, but the unrelease 1.3.0-SNAPSHOT), include them in the project's classpath and use the sources instead of the Tinder.jar.
Here's the JID class:
http://svn.igniterealtime.org/svn/repos/tinder/trunk/src/main/java/org/xmpp/pack et/JID.java
