Java debugger: jdb.

jdb manpage:

http://download.oracle.com/javase/6/docs/technotes/tools/solaris/jdb.html

A general debugging start:

jdb -classpath /home/potato/6/class -sourcepath /home/potato/6/src

A very good tutorial:

http://www.packtpub.com/article/debugging-java-programs-using-jdb