https://www.codecademy.com/ - a good start for learning the basics of Java. Will teach you the basics for free, need to buy Pro account to use any of the projects. (personally, I don't think the pro account is worth it)
www.javabat.com - A wide variety of quick coding exercises, may be used in class.
Https://repl.it/ - Online repository for programming problems (also has a working online IDE). Many past AP questions have been translated into repl.it as problems
http://www.khanacademy.org/ - online tutorials for just about anything