设计模式——迭代器与组合模式（一）

于 2017-09-18 08:57:23 发布
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分类专栏： 设计模式
版权声明：本文为博主原创文章，遵循 CC 4.0 BY-SA 版权协议，转载请附上原文出处链接和本声明。
本文链接：https://blog.youkuaiyun.com/qq_33786266/article/details/78015397
本文介绍如何使用迭代器模式解决不同存储结构菜单的统一访问问题，通过实现迭代器接口，使得不同菜单可以被统一打印。

11（一）.

OO原则：类应该只有一个改变的理由。

OO模式：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。



例如：要合并两个存储方式不同的餐厅菜单，且需要一个“女招待”来打印用户定制的菜单，甚至告诉你某个菜单项是否为素食的。

煎饼屋用的是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
}
而另一家餐厅用的是数组，有很多其他代码依赖这两个菜单项，更改会带来不便。那么该如何解决？

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循环打印这两个菜单，那如果后面还要合并其他餐厅，我们就要继续加第三个循环，很明显这很不方便。


所以我们可以使用迭代器。


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()，删去方法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
}


对另一份菜单采取同样的做法。


于是“女招待”可以这样写：

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
}








此时，我们只需写一个printMenu中的循环就可以了，不需要改其他地方。“女招待”不需要知道菜单是怎么存储的。



又或者，我们可以采用java自己的Iterator接口。


对于PancakeHouseMenu类的处理：


1.删除之前写的PancakeHouseMenuIterator类；


2.然后在菜单的代码前import java.util.Iterator，并加入下列代码：




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


 


对于DinerMenu类：




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;
		}
	}
}


添加一个公共接口：






public interface Menu {
	public Iterator createIterator();
}


更新“女招待”代码：






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
}







这是目前的设计：























































                

        

    

    
  



    



            

  



          




        



    





    



      

      

        
      

      





	

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