hello,
if you feel that those topics are your only weakness, then I recommend to use the
Sun tutorial and catch up with networking (only a little), Serialization and RMI - in that order. For best practices regarding Serialization (and many other things) I recommend
Effective Java (one of the 2 books you ever need) additionally.
If you want to improve your general Java, programming and OO skills you can have a look at my very subjective suggested
learning path.
Kai