If not exactly java, java-like JALIMO was there for the previous versions of maemo but not sure you can use it in maemo5. I also tried it in my n810 and was able to write programs and execute and this was about a year ago coz I stopped using it from the not-so-good feedback I got about it then. But I'm sure they have improved by now. Check this out: https://wiki.evolvis.org/jalimo/index.php/Maemo