If you mean a file  on the local machine, you won't be able to consult
it from within an applet, which runs in a security-
protected "sandbox". If you mean a file on the server, you need to
set the codebase in the web  page, i.e.

<applet width="200" height="50" code="Applet.class"
codebase="."></applet>

and put the file in the same folder as the applet. Also, avoid J++
if you can help it!