How to learn Designing in Java

Hi

I have been a java programmer for few years now. I have learnt java progamming concepts and apis and few design patterns. Now i would like to learn how to be very good at designing java applications on my own. Even thoough i know few design patterns, i am unable to start designing applications. For example, what are the things that i should learn to decide when should the class should be abstract or by looking at a code i should be able to tell that this class should actually be an interface and should have sub classes etc.

Could you please guide me to learn things like these.