設計模式——迭代器與組合模式（一）

11（一）.

OO原則：類應該只有一個改變的理由。

OO模式：迭代器模式——提供一種方法順序訪問一個聚合對象中的各個元素，而又不暴露其內部的表示。

例如：要合併兩個存儲方式不同的餐廳菜單，且需要一個“女招待”來打印用戶定製的菜單，甚至告訴你某個菜單項是否爲素食的。

煎餅屋用的是ArratList,

import java.util.ArrayList;

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
}

而另一家餐廳用的是數組，有很多其他代碼依賴這兩個菜單項，更改會帶來不便。那麼該如何解決？

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;
	}
  
	// other menu methods here
}

如果printMenu()上依次for循環打印這兩個菜單，那如果後面還要合併其他餐廳，我們就要繼續加第三個循環，很明顯這很不方便。

所以我們可以使用迭代器。

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

public class DinerMenuIterator implements Iterator {
	MenuItem[] items;
	int position = 0;
 
	public DinerMenuIterator(MenuItem[] items) {
		this.items = items;
	}
 
	public MenuItem next() {
		MenuItem menuItem = items[position];
		position = position + 1;
		return menuItem;
	}
 
	public boolean hasNext() {
		if (position >= items.length || items[position] == null) {
			return false;
		} else {
			return true;
		}
	}
}

DinerMenu中加入方法createIterator()，刪去方法getMenuItems()；

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
}

對另一份菜單採取同樣的做法。

於是“女招待”可以這樣寫：

public class Waitress {
	PancakeHouseMenu pancakeHouseMenu;
	DinerMenu dinerMenu;
	public Waitress(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());
		}
	}
	//其他方法
}

此時，我們只需寫一個printMenu中的循環就可以了，不需要改其他地方。“女招待”不需要知道菜單是怎麼存儲的。

又或者，我們可以採用java自己的Iterator接口。

對於PancakeHouseMenu類的處理：

1.刪除之前寫的PancakeHouseMenuIterator類；

2.然後在菜單的代碼前import java.util.Iterator，並加入下列代碼：

public Iterator creatIterator(){
	return menuItems.iterator();
}

 

對於DinerMenu類：

public class DinerMenuIterator implements Iterator {
	MenuItem[] items;
	int position = 0;
 
	public DinerMenuIterator(MenuItem[] items) {
		this.items = items;
	}
 
	public MenuItem next() {
		MenuItem menuItem = items[position];
		position = position + 1;
		return menuItem;
	}
 
	public boolean hasNext() {
		if (position >= items.length || items[position] == null) {
			return false;
		} else {
			return true;
		}
	}
	public void remove(){
		if(position<=0){
			throw new IllegalStateException("You can't remove an item until you've done at least one next()");
		}
		if(list[position-1!=null]){
			for(int i = position-1);i<(list.length-1;i++){
				list[i]=;ist[i+1];
			}
			list[list.length-1]=null;
		}
	}
}

添加一個公共接口：

public interface Menu {
	public Iterator createIterator();
}

更新“女招待”代碼：

public class Waitress {
	Menu pancakeHouseMenu;
	Menu dinerMenu;
 
	public Waitress(Menu pancakeHouseMenu, Menu 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());
		}
	}
 
	//其他方法
}

這是目前的設計：