If you want to access files on the client computer via your applet, you will have to sign it. (Yes, theoretically you can put a java policy in the right place on the client but that isn't a very practical thing to do and almost nobody in real life ever does it.) However you'll find that in an IDE or applet viewer, that rule doesn't apply. It only applies in real life, namely in a browser.
If you are writing an applet that must be because you plan to have people download the applet from your web site and run it in their browsers. In which case you need to live within the applet security rules.
If that wasn't your plan then stop writing an applet and just write an ordinary application. (And if you have a Java book which teaches you about applets near the beginning then throw it away and get a newer one.)
Joined: Nov 23, 2010
so I went to create certificate (following instructions here:webpage)
this goes OK, asks me if I trust it, , I confirm and get:
What that means ?
Cacerts file is surely there.
I had 'cacerts.' in keystore file path (have no idea what dot stands for), in one of previous trials, same thing.