1.应用场景

(1)访问一个聚合对象的内容而无需暴露它的内部表示

(2)支持对聚合对象的多种遍历

(3)为遍历不同的聚合结构提供一个统一的接口

2.概念

提供一种顺序访问聚合对象元素的方法，并且不暴露聚合对象的内部表示。这个模式给使用者提供了一种方法，可以顺序访问一个聚集对象中的元素，而又不用知道内部是如何表示的。

3. Class Digram

• Aggregate 是聚合类，其中 createIterator() 方法可以产生一个 Iterator；
• Iterator 主要定义了 hasNext() 和 next() 方法。
• Client 组合了 Aggregate，为了迭代遍历 Aggregate，也需要组合 Iterator。

4. Implementation

public interface Menu {
	public Iterator createIterator();
}

public interface Iterator {
	boolean hasNext();
	MenuItem next();
}

public class MenuItem {
	String name;
	String description;
	boolean vegetarian;
	double price;
 
	public MenuItem(String name, 
	                String description, 
	                boolean 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 boolean isVegetarian() {
		return vegetarian;
	}
	public String toString() {
		return (name + ", $" + price + "\n   " + description);
	}
}

public class PancakeHouseMenu implements Menu {
	ArrayList<MenuItem> menuItems;
 
	public PancakeHouseMenu() {
		menuItems = new ArrayList<MenuItem>();
    
		addItem("K&B's Pancake Breakfast", 
			"Pancakes with scrambled eggs, and toast", 
			true,
			2.99);
 
		addItem("Regular Pancake Breakfast", 
			"Pancakes with fried eggs, sausage", 
			false,
			2.99);
 
		addItem("Blueberry Pancakes",
			"Pancakes made 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,
	                    boolean vegetarian, double price)
	{
		MenuItem menuItem = new MenuItem(name, description, vegetarian, price);
		menuItems.add(menuItem);
	}
 
	public ArrayList<MenuItem> getMenuItems() {
		return menuItems;
	}
  
	public Iterator createIterator() {
		return new PancakeHouseMenuIterator(menuItems);
	}
  
	public String toString() {
		return "Objectville Pancake House Menu";
	}

	// other menu methods here
}

public class PancakeHouseMenuIterator implements Iterator {
	ArrayList<MenuItem> items;
	int position = 0;
 
	public PancakeHouseMenuIterator(ArrayList<MenuItem> items) {
		this.items = items;
	}
 
	public MenuItem next() {
		MenuItem item = items.get(position);
		position = position + 1;
		return item;
	}
 
	public boolean hasNext() {
		if (position >= items.size()) {
			return false;
		} else {
			return true;
		}
	}
}

public class DinerMenu implements Menu {
	static final 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, with a side of potato salad", false, 3.29);
		addItem("Hotdog",
			"A hot dog, with saurkraut, relish, onions, topped with cheese",
			false, 3.05);
		addItem("Steamed Veggies and Brown Rice",
			"Steamed vegetables over brown rice", true, 3.99);
		addItem("Pasta",
			"Spaghetti with Marinara Sauce, and a slice of sourdough bread",
			true, 3.89);
	}
  
	public void addItem(String name, String description, 
	                     boolean vegetarian, double price) 
	{
		MenuItem menuItem = new MenuItem(name, description, vegetarian, price);
		if (numberOfItems >= MAX_ITEMS) {
			System.err.println("Sorry, menu is full!  Can't add item to menu");
		} else {
			menuItems[numberOfItems] = menuItem;
			numberOfItems = numberOfItems + 1;
		}
	}
 
	public MenuItem[] getMenuItems() {
		return menuItems;
	}
  
	public Iterator createIterator() {
		return new DinerMenuIterator(menuItems);
		// To test Alternating menu items, comment out above line,
		// and uncomment the line below.
		//return new AlternatingDinerMenuIterator(menuItems);
	}
 
	// other menu methods here
}

public class DinerMenuIterator implements Iterator {
    private MenuItem[] menuItem;

    int position=0;

    public DinerMenuIterator(MenuItem[] menuItem) {
        this.menuItem = menuItem;
    }

    @Override
    public boolean hasNext() {
        return position < menuItem.length && menuItem[position] != null;
    }

    @Override
    public MenuItem next() {
        MenuItem menuItem = this.menuItem[position];
        position++;
        return menuItem;
    }
}

public class Waiterss {
    PancakeHouseMenu pancakeHouseMenu;
    DinerMenu dinerMenu;

    public Waiterss(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) {
        this.pancakeHouseMenu = pancakeHouseMenu;
        this.dinerMenu = dinerMenu;
    }

    public void printMenu() {
        Iterator pancakeIterator = pancakeHouseMenu.createIterator();
        Iterator dinerIterator = dinerMenu.createIterator();

        System.out.println("MENU\n----\nBREAKFAST");
        printMenu(pancakeIterator);
        System.out.println("\nLUNCH");
        printMenu(dinerIterator);

    }

    private void printMenu(Iterator iterator) {
        while (iterator.hasNext()) {
            MenuItem menuItem = iterator.next();
            System.out.print(menuItem.getName() + ", ");
            System.out.print(menuItem.getPrice() + " -- ");
            System.out.println(menuItem.getDescription());
        }
    }
}

public class MenuTestDriver {
    public static void main(String[] args) {
        DinerMenu dinerMenu=new DinerMenu();
        PancakeHouseMenu pancakeHouseMenu=new PancakeHouseMenu();
        Waiterss waiterss=new Waiterss(pancakeHouseMenu,dinerMenu);
        waiterss.printMenu();
        Iterator iterator = pancakeHouseMenu.createIterator();
        while (iterator.hasNext()){
            MenuItem next = iterator.next();
            System.out.println(next);
        }

        System.out.println();

        Iterator dinerMenuIterator = dinerMenu.createIterator();
        while (dinerMenuIterator.hasNext()){
            MenuItem next = dinerMenuIterator.next();
            System.out.println(next);
        }
    }
}

5. JDK

• java.util.Iterator
• java.util.Enumeration