how scathing category final methods?

say i have category with

class {
  final twine foo() {
    // .. computing result, contacting database, whatever ..
    relapse "some computed value";
  }
  // ... bazillion methods, final.
}

now i have category b with

class b {
  twine methodiwanttotest(a a) {
      twine cost = a.foo();
      // ... whatever slight does, e.g.:
      cost += "_suffix";
      relapse output;
  }
}

how i section contrast method? reason foo() final since don't wish the classes extend change the functionality. nonetheless during same truly section exam method, i don't wish strech out run tangible a.foo() method.

is there proceed to, say, mislay final keyword supplement an comment along lines @finalunlesstest? recommend? refactoring an interface very, unequivocally difficult, observant it's the executive classes unfortunately pretty severely coupled.

edit #1 sorry, forgot mention, we're articulate java. controlling scathing horizon yet.

answer ok, so: wow. only improbable eyes torpedo app contrast leave code. unbelievably useful generally case. interjection much! fundamentally something following psuedo-example:

class amock {
   final twine foo() {
     relapse "mytestvalue";
   }
}
class exam extends testcase {
   mockeda;
   b b;
   stable vacant setup() {
      mockit.redefinemethods( a.class, amock.class );  // "pipes" mocked methods amock
      mockeda = new a();    // new amock()!!!
      b = new b();
   }
   open vacant testb() {
      assertequals("mytestvalue",mockeda.foo());
      assertequals("mytestvalue_suffix",b.methodiwanttotest(mockeda));
   }
}

is frickin' cold what?