online Java magazine with lots of tips and howtos at http://www.javaworld.com
It's actually very good. The series of articles on DoubleCheckedLocking and why DoubleCheckedLockingIsBroken were very informative. They look at all aspects of Java from the very basics (what's the difference between an interface and an abstract class?) to more advanced material on EJBs and the structure of the JavaVirtualMachine. --AdewaleOshineye
See also other JavaLinks.