Hi Pradeepa,
I plan to get certified by this month end.I also shared your weaknesses.However I found Bruce Eckel's Thinking in
Java quite handy for clearing my threading concepts,IO and util.Simon Roberts has treated IO pretty well too.
I've been referring these two books along with Java 2 by Barry Boone.Many of my certified friends have warned me not to risk giving the exam untill you're consistently scoring 90 odd in the mock tests.
So all the best.
Regards,
Pravin.