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- Writing testable code - an excellent collection of tips on how to structure and organize your code to make it easier to test (pdf )
- Joshua Bloch's book Effective Java is an excellent collection of tips that will help you become more fluent in Java.
- Bleeding at the keyboard
- Exception handling: http://www.onjava.com/pub/a/onjava/2003/11/19/exceptions.html - in general, make sure your error messages are concise and informative. Just like your code.
- http://www.artima.com is a good site for learning more about Java and programming in general.
- CACM article on good API design