Duck example strategy pattern - Head first design pattern

Question

I want to ask something about duck example on this book that made me confused and I feel contradictions.

1. Problem

Answer 1

You're correct. The solution offered by the book suffers from one huge problem: "FlyNoWay" is not a sub case of "FlyBehaviour". To have any meaning, FlyBehaviour must require the ability to fly. Classes that inherit from it will specify the behaviour (fly using wings etc.). A sub class cannot contain less than the class it inherits from.

Taking a closer look, "FlyNoWay" is just a pseudo class that was introduced to solve the problem of polymorphism in an inappropriate way.

The right method is to use interfaces.

class Duck
{
    swim();
}

class MallardDuck : IFlyable
{
    fly();
}

class RedheadDuck : IFlyable, IQuackable
{
    fly();
    quack();
}

What about code reuse? Well you have to make the interface as strict as possible, assuring that most changes in the interface will cause the program to not compile.