Here is my advice and this will surely boost your JAVA concepts and knowledge within minimum time:
Two books I always have with me are:
1.
JAVA 60 minutes a day - Rich Raposa
Link:
*ebooks.ebookmall.com/ebook/120029-ebook.htm
2.
Head First JAVA - Sierra and Bates
Link:
*oreilly.com/catalog/9780596004651/
First book will be like a teacher to you, it has those small doubts answered which makes a solid foundation. A really good assortment on graphics programming too, I learned it from here only. Its has all from simple console apps to database programming. And that too in lucid manner.
Second book is like a treat on the move. It will entertain you and enhance further your knowledge. Take it as just an intro to JAVA. You wont find good chapters on graphics programming in it but some examples which gives you a general idea.
Third book that you might want is an online behemoth of examples, called JAVA Almanac, it will help you in getting those piece of codes which you will not be able to find out in other books.
Here is the Link to Online Examples, it's free and the best for getting nifty code snippets:
3.
The Java Developers Almanac 1.4
Link:
*www.exampledepot.com/
I hope it helps