gistfile1.java

interface IMySuperInterface {
    void setNumPlats(int numero);
}

class MyAtivity extends Activity implements IMySuperInterface {
    //do the implentation and other stuff;
}

class MyFragment extends Fragment {
    IMySuperInterface defaultInstance = new IMySuperInterface {
        public setNumPlats(int numero) {
            Log.e("MyFragment", "IMySuperInterface.setNumPlats() was called without Activity attached!");
            //or throw ... depends on your needs
        }
    }
    IMySuperInterface theInterfaceInstance = defaultInstance;
    void onAttach(Activity activty) {
        if(activty instanceof  IMySuperInterface){
            theInterface = (IMySuperInterface)activty;
            return;
        }
        //you can also leave the defaultInstance but then setNumPlats will not do anything
        //better throw this error it will be easy to cach it in tests
        throw new RuntimeException("This fragment needs Activity that implements IMySuperInterface");
    }
    
    void onDettach() {
        //set it back to default instance to avoid NPE
        theInterface = defaultInstance;
    }
    
    void SomrMethodWhereIWanaUseIt(){
        theInterfaceInstance.setNumPlats(0);
    }
}