Untitled

Java Interface question regarding polymorphism
interface C {
    void shutUp();
}

class A {
    public void speak() {
       System.out.println("Class a");
    }
}

class B extends A implements C {
    public void shutUp() {
        System.out.println("Saying nothing...");
    }

    public void speak() {
        System.out.println("Class B");
    }
}

A obj = new B();
obj.speak()

A obj = new A();
obj.shutUp();

B obj = new B();
obj.shutUp();
obj.speak();

((C) obj).shutUp() // cast to C type
((B) obj).shutUp() // cast to B type

B obj = new B();
obj.shutUp();

C obj = new B(); // (!) but: you can't (without casting) call speak method
obj.shutUp();

A obj = new B();
try {
    Method shutUp = obj.getClass().getMethod("shutUp", null);
    shutUp.invoke(obj, null);
}
catch (Exception ignored) {}

A obj = new B();

public void speak() {}

B obj2 = new B();

public void shutUp() {}

B o = new B();
A aO = o;
C cO = o;

class D extends A {
    public void speak(){
        System.out.println("Class D");
    }
}

List<A> as = new ArrayList<A>();
as.add(new A());
as.add(new B());
as.add(new D());

A a0 = as.get(0);
A a1 = as.get(1);
A a2 = as.get(2);

a0.speak(); // >> Class A
a1.speak(); // >> Class B
a2.speak(); // >> Class D