To install java on the N900, download it first from:

http://java.sun.com/javase/downloads/embedded.jsp

use the:

"ARMv6 Linux - Headful (Early Access)" "EABI, glibc 2.5, Hard Float (VFP), Little Endian"

edition.

put it on the N900, and untar it with 'tar xvfz'.