Difference between Interface, Abstract class and Concrete class

1)if we dont know anything about  implementation, just we have only requirement specification. then we go for interface. Ex:Servlet

If we know the implmentation, but not fully. We know the partial implementation then we go for Abstract class. Ex: Generic Servlet and HttpServlet

If we know fully implamentation and ready to provide service then we can go for concrete class. Ex: MyOwnServlet class