I did use it once, while at Uni, I think with the Java JDK, you need to replace the GCJ Java compiler, it's something like:
mv java java.gcj
mv javac javac.gcj
mv javadoc javadoc.gcj
ln -s java /path/sun/jdk/java
ln -s javac /path/sun/jdk/javac
ln -s javadoc /path/sun/jdk/javadoc
To use the Sun JDK instead of GCJ.
It has been a while, so don't try that unless you're happy un-doing it.