public class WalkablesTest 
{
    public static void main(String[] args) 
    {
        Person[] list = new Person[3];
        list[0] = new Person("Jack");
        list[1] = new Duck("Jeff");   // A compile-time error
        list[2] = new Person("John");
        
        // Let everyone walk
        Walkables.letThemWalk(list);
    }
}