I was also looking for a good java editor. First I tried vi with java syntax highlighting, but somehow the vi for the Z (
vi@killefiz) doesn't work well for me.
So I tried jed (
jed@killefiz) and it works nicely, syntax higligting, indenting, etc. but I wasn't able to make the java 'tab completion' work, which I found as a jed s-lang plugin:
jdoc That's a pity as this would be really a killer feature

Anyway I find java development on the Z quite a pain in the a$$, as I'm usually using ant (works with the Z's J2ME jre when adding the jdk 1.4.x rt.jar to the classpath). But ant is quite slow on the Z and you will spend much time with watching it building, *sigh* But it works at least.
An interresting thing could be also the curses based (textmode) IDEs which exist. Like
RHIDEhappy hacking