As part of my work at the University of Auckland (which is drawing to a close soon) I have been looking at Java on Haiku. There is already some support for Java on Haiku. By running installoptionalpackage DevelopmentJava you can get the JamVM java virtual machine, GNU Classpath (including gjar, gjavah and maybe some other utilities), and ecj (the Eclipse Java compiler, which is written in Java). This is enough to run and compile basic Java apps. For instance, the popular Java buildtool Ant can run on Haiku.
This is a companion discussion topic for the original entry at https://www.haiku-os.org/blog/yourpalal/2012-03-01_haiku_java/