Interfaces

The fields in an interface are implicitly static and final. The fields, of course, are not part of the interface. The values are stored in the static storage area for that interface.

Factory Method design patter: instead of calling a constructor directly, you call a creation method on a factory object which produces an implementation of the interface -- this way, in theory, your code is completely isolated from the implementation of the interface, thus making it possible to transparently swap one implementation for another.

An appropriate guideline is to prefer classes to interfaces. Start with classes, and if it becomes clear that interfaces are necessary, then refactor. Interfaces are a great tool, but they can easily be overused.