> yes iam waiting for preferences only how?
There is no preference for any of them. It depends on the way the problem is modelled. One might decide to have abstract classes, another might want interfaces, some way just get going without any of them.
In real-time or virtual time, the considerations should remain the same as mentioned in:
http://www.javaworld.com/javaworld/javaqa/2001-04/03-qa-0420-abstract.html
http://www.javaworld.com/javaworld/javaqa/2001-08/03-qa-0831-interface.html