I got some feedback about the entry "Most Important Patterns" and especially the interface "pattern". I introduced the interface as pattern, but it is not a part of the standard GoF-family (I didn't meant the marker interface). The question remains, what is the intension of using interfaces? In Java there are some use cases for using it:
- Providing an API
- Encapsulating coarse grained functionality
- Making implementations interchangeable
- Hiding the technology
- Exposing services (methods)
Considering the items above there is one common intension - exposing coarse grained functionality and hiding low level implementation. The opposite to the "interface" is a domain object. It is an objectoriented entity - without an explicit interface. In Java you will use for the realization of an entity just a class. The real power of domain objects, is the inheritance of behavior and state and polymorphic capabilities.
Interfaces emphasize the "serviceorientation" and provide a procedural point of view of a system. Domain Objects on the other hands provide a fine grained, and object oriented access to the business logic. So using interfaces do emphasize the services, methods, and processes.