If we don't know anything about the implementation and only have requirement specifications, then we should go for an interface . e.g.: Servlet If we are talking about an implementation, but not a complete one (partial implementation), then we should go for an abstract class . e.g.: GenericServlet, HttpServlet If we are talking about a complete implementation that is ready to provide service, then we should go for a concrete class . e.g.: MyOwnServlet Difference between Interface and Abstract Class Interface Abstract Class If we don't know anything about the implementation and only have requirement specifications, then we should go for an interface. If we are talking about an implementation, but not a complete one (partial implementation), then we should go for an abstract class. Inside an interface, every method is always public and abstract, whether we declare it or not. Hence, an int...