duck-typing:

class A:
   f() { .... }
   g() { .... }

class X:
   m(value) {
      value.f()
      value.g()
      ...
   }

=======

class Y:
   m2() {
      ...
      x = X()
      x.m(B())
      
class B:
   f() { .... }

-------

interface I
   f()
   g()

---

class A { m1() }
class B : A { m2() }
class C : A {}
class D : B {}

        A
       / \
      B   C
     /
    D

A a = new A();
A a = new B();
A a = new C();
A a = new D();

a.m1()
XXX a.m2()

if (a is B) {
   B b = (B) a;
   b.m2();
}

A a = new D();

a is D -> true
a is B -> true
a is A -> true
a is C -> false

if (a is B b) {

}