I think you mean JAVA editor and not javascript editor, as javascript is written in the html format.
Now there are editors that USE java, but they dont necessarily EDIT java, usually php driven like tinymce which is commonly used in storebuilders etc
Try places like sourceforge.net or sun java site or google open source editors etc...
OOPS.. heres the tnyMCE- and it does say edits javascript..just never thought of it that way
http://tinymce.moxiecode.com/
Karen