1. a String is an object. There's a difference between a null reference to an object and an object that holds no information.
2. I suggest you go out and buy some books. Head First Java is great... if you're practicing for SCJP or whatever they call it now, Sierra and Bates have some good books out. There's also plenty of books you can borrow from the library or buy secondhand if budget is a concern.
It's okay to ask questions, but folks are volunteers here, and you're flooding the forums with your own questions, thus not sharing the volunteering. You should try to work some things out for yourself, then ask questions once you read a bit -- that would be more polite... and let's face it: the rule here is BeNice.
Search google for null vs empty string in java. Next time you post a question, search the internet first (or the forums.... see SearchFirst) -- don't expect people to do all the work for you.
I agree with Janeice. The best way to learn is to get a book, download a good IDE (netbean, etc) and hack up the codes to try. a good IDE will flag up any compilation error immediately. It better to learn how to fish, I believe that the spirit the guru are trying to cultivate here
I read Kathy Sierra book and couple of other books when i wrote scjp 4 years back. When i look back again those questions to strengthen my knowledge now, many of those concept are again new to me. That time i practiced on notepad with dcommand line execution and ide also. I was wondering how to remember so many concepts all the time to perform best in the project esp so many new technology. I feel overwhelmed and do not no start and end for this java. please advise