Interfaces are meant to define the public API of a type - and only that, not its implementation. So any method (or static member) you define in an interface is by definition public.
Since an interface can't contain any concrete implementation, there is no way to call any member methods from within. And declaring such methods but leaving the calls to them to subclasses or totally unrelated clients would mean your type definition is incomplete and brittle. That is why if you need to define protected or package access members, you can do so in an abstract class (which may also contain implementation).
