Suppose I have an interface IModel which some one has to use it (the one who implement the controller (MVC)) and assume it contains funcA(). in the other side a programmer need to implement specific class for the IModel. I'm the one who design the interface and I want to force the implement of funcB.

my wish is to direct the programmer who implements funcA by implementation of funcB. other case is forcing transparent functionality of the specific "IModel". for example the interface contain openFile() but the specific class must contain the transparent functionality of encription (the file)

what is better?

1. define two interfaces: one I1 for those who use it and other one for those who implement the specific class.
   when I1 <-- I2 mean I2 extends I1.
2. own implemnetation of abstract calss like AbsModel which implements Imodel and contains abstract function.

thanks ofek