For those unaware java is a high level programming language which courtesy of its tendency to be compiled to bytecode and then be interpreted is largely cross platform, it is of special note as it is a fairly high level language (something the DS is lacking in support for) and is now widely taught as part of computer programming courses.
Functions available at this stage are quite limited and it is suggested you aim for J2SE 1.4/version 1.4 of the java language as higher will not work.