According to the code conventions from Sun Microsystems, we have two different types of comment.
(A) The first is an implementation comment which looks like this:
/*
* Comment here !
*/
(B) The other is a documentation comment that looks like this:
/**
* Comment here !
*/
All the private methods that need documenting are implementation dependent, so that would mean comments like (A).
All the public interface methods should go into your JavaDoc's, and those would be comments like (B).
However, in my opinion, one shouldn't be wasting time documenting all and every single private method.
If it's not going to add any value, and your private method is rather short,
the comments are just an added burden of maintenance.
If you want your code to be understandable, easy to maintain and re-factor,
then rather spend more time reading up on Uncle Bob's clean code book,
and less time refactoring your comments ...