need help!!on interfaces and abstarct classes

hello,
I am little frustarted since I am not getting an idea on -" when and how to use certain features of java"?. For example: i donot have any clue on interfaces and abstarct class uses and advantages. Also I am interested to know even if for simple on line forms can these features can be used. if es then how to go about it. Can any one help me on this.

Thanks in advance
Hema

Your question is very general and thus difficult to answer briefly. The usages of interfaces and abstract classes is a very broad topic.

From a 50,000 foot perspective, they enable polymorphism, which is one of the fundamental principles of object oriented programming. If you aren't comfortable with this topic, you may be looking for a tutorial on object-oriented programming principles.

Polymorphism enables abstraction and indirection, which allows for "separation of concerns" in tiered applications, which are important concepts for even simple forms.

You will probably get much better answers if you can make your question a little more specific and concrete. Where and/or how are you having difficulty with using interfaces and abstract classes?

Hope the following link is helpful for you:
http://www.javaworld.com/javaworld/javaqa/2001-04/03-qa-0420-abstract.html