LeJOS is a
Java based
replacement firmware (OS) for
Lego Mindstorms, replaces the rather nasty
GUI drag & drop "programming language" that comes with the
Lego Robotics Invention System.
After installing the LeJOS firmware, can you create Java programs in your usual way. Then you'll compile the with the leJOS compiler. This compiler is really just the normal "
javac" compiler, but with a wrapper that specifies different base
classes (replacements for classes like System as well as additional classes like
Motor). You then
upload the compiled program to the RCX and run it there.