Howdy, Kenny!
For the SCJD project, do you need to JavaDoc private fields and methods?
Well champ, I do think it is a good idea to write JavaDoc comments even for the private members, in order to ease maintenance. You don't have to include them in the documentation, but if you want to include them one day, they are already there. You can include, for instance, protected and public members.
It would seem using regular comments makes more sense, since these are private hence they are not to be exposed or known about to clients of the class.
You mean the // comments? Well, I myself don't like them. If they are used, then it means that you are having to explain your code, which is something the code should do on its own. The code should be as clean as possible, which means that the methods shouldn't be big (up to 15 excluding '{' and '}' should be enough) and variables should have a proper name (i.e. "serverReference" instead of "sRef"), as well as the methods.
I myyself included JavaDoc comments for all class members.