public class Walkables 
{
    public static void letPersonsWalk(Person[] list) 
    {
        for (Person person : list) 
        {
            person.walk();
        }
    }
    
    public static void letDucksWalk(Duck[] list) 
    {
        for (Duck duck : list) 
        {
            duck.walk();
        }
    }    
}