Java does not support extending more than one class (at the same level), and that's a good thing. You don't need this when writing software. If you think you do need this, then you should reconsider the design of your software.
Multiple inheritance introduces a number of subtle issues, like the diamond problem that the others are referring to.
The designers of the Java language decided that multiple inheritance is more trouble than it's worth and left it out of Java.
(C++ does support multiple inheritance and the diamond problem is solved there by making it even more complicated - by adding virtual inheritance). [ June 25, 2007: Message edited by: Jesper Young ]