迭代器模式：提供一种方法顺序访问一个聚合对象中的各个元素，而又不暴露其内部的表示。

　　UML类图:

　　

　　煎饼屋和餐厅合并了！但是有个小问题，虽然两家都同意实现相同的菜单项MenuItem，但是煎饼屋想使用ArrayList储存菜单项，而餐厅则使用数组，为了使女招待能同时访问两家的菜单，我们需要为菜单提供一个统一的访问接口。

　　先来看菜单项MenuItem，两家店的实现相同

class MenuItem    {        string name;//名称        string description;//描述        bool vegetarian;//是否是素食        double price;//价格        public MenuItem(string name, string description, bool vegetarian, double price)        {            this.name = name;            this.description = description;            this.vegetarian = vegetarian;            this.price = price;        }        public string getName()        {            return name;        }        public string getDescription()        {            return description;        }        public double getPrice()        {            return price;        }        public bool isVegetarian()        {            return vegetarian;        }          }

　　接口Menu，它只定义了一个创建迭代器的方法，实现这个接口的类将提供各自不同的迭代器

interface Menu    {         Iterator createIterator();    }

　　煎饼屋菜单，实现了Menu接口

class PancakeHouseMenu : Menu    {        ArrayList menuItems;        public PancakeHouseMenu()        {            menuItems = new ArrayList();            addItem("K&B's Pancake Breakfast", "Pancakes with scrambled eggs,and toast", true, 2.99);            addItem("Regular Pancake Breakfast", "Pancakes with fired eggs,and sausage", false, 2.99);            addItem("Blueberry Pancake", "Pancakes with fresh blueberries", true, 3.49);            addItem("Waffles", "Waffles,with your choice of blueberries or strawberries", true, 3.59);        }        public void addItem(string name, string description, bool vegetarian, double price)        {            MenuItem menuItem = new MenuItem(name, description, vegetarian, price);            menuItems.Add(menuItem);        }               public Iterator createIterator()        {            return new PancakeHouseIterator(menuItems);        }        //菜单的其他方法    }

　　餐厅菜单同样如此

class DinerMenu : Menu    {        static int Max_ITEMS = 6;        int numberOfItems = 0;        MenuItem[] menuItems;        public DinerMenu()        {            menuItems = new MenuItem[Max_ITEMS];            addItem("Vegetarian BLT", "(Fakin') Bacon with lettuce & tomato on whole wheat", true, 2.99);            addItem("BLT", "Bacon with lettuce & tomato on whole wheat", false, 2.99);            addItem("Soup of the day", "Soup of the day,whit a side of potato salad", false, 3.29);            addItem("Hot dog", "A hot dog, with saurkraut, relish, onions, topped with cheese", false, 3.05);        }        public void addItem(string name, string description, bool vegetarian, double price)        {            MenuItem menuItem = new MenuItem(name, description, vegetarian, price);            if (numberOfItems >= Max_ITEMS)            {                Console.WriteLine("Sorry,menu is full! Can't add item to menu!");            }            else            {                menuItems[numberOfItems] = menuItem;                numberOfItems++;            }        }        public MenuItem[] getMenuItems()        {            return menuItems;        }        public Iterator createIterator()        {            return new DinerMenuIterator(menuItems);        }        //菜单的其他方法    }　　

　　接下来看看迭代器，迭代器有一个接口，定义了一些操作

interface Iterator    {        bool hasNext();        object next();        //更多的方法，例如remove...    }

　　不同的菜单有不同的迭代器，这是煎饼屋的

class PancakeHouseIterator:Iterator    {        ArrayList items;        int position = 0;        public PancakeHouseIterator(ArrayList items)        {            this.items = items;        }        public object next()        {            MenuItem menuItem = (MenuItem)items[position];            position++;            return menuItem;        }        public bool hasNext()        {            if (position>=items.Count)            {                return false;            }            else            {                return true;            }        }    }

　　这是餐厅的

class DinerMenuIterator:Iterator    {        MenuItem[] items;        int position = 0;        public DinerMenuIterator(MenuItem[] items)        {            this.items = items;        }        public object next()        {            MenuItem menuItem = items[position];            position++;            return menuItem;        }        public bool hasNext()        {            if (position>=items.Length||items[position]==null)            {                return false;            }            else            {                return true;            }        }    }

　　选择女招待可以方便的用统一的方法操作两家的菜单了

class Waitress    {        Menu pancakeHouseMenu;        Menu dinerMenu;        public Waitress(Menu pancake, Menu diner)        {            this.pancakeHouseMenu = pancake;            this.dinerMenu = diner;        }        public void printMenu()        {            Iterator pancakeIterator = pancakeHouseMenu.createIterator();            Iterator dinerIterator = dinerMenu.createIterator();            Console.WriteLine("MENU\n----\nBREAKFAT");            printMenu(pancakeIterator);            Console.WriteLine("\nLUNCH");            printMenu(dinerIterator);        }        private void printMenu(Iterator iterator)        {            while (iterator.hasNext())            {                MenuItem menuItem = (MenuItem)iterator.next();                Console.Write(menuItem.getName() + ",");                Console.WriteLine(menuItem.getPrice() + "--  ");                Console.WriteLine("     "+menuItem.getDescription());            }        }    }

　　迭代器的好处是将来如果有新的菜单加入，而它使用了不同的存储方法（例如hashtable），只需要让它实现迭代器接口，而不必修改女招待的操作方式。