设计模式 - 迭代器模式(iterator pattern) 具体解释

迭代器模式(iterator pattern) 详细解释

本文地址: http://blog.****.net/caroline_wendy

迭代器模式(iterator pattern) : 提供一种方法顺序訪问一个聚合对象中的各个元素, 而又不暴露其内部的表示;

建立迭代器接口(iterator interface), 包括hasNext()方法和next()方法;

不同聚合对象的详细的迭代器(concrete iterator), 继承(implements)迭代器接口(iterator interface), 实现hasNext()方法和next()方法;

详细聚合对象(concrete aggregate), 提供创建迭代器的方法(createIterator).

通过调用聚合对象的迭代器, 就可以使用迭代器, 统一输出接口.

面向对象设计原则:

一个类应该仅仅有一个引起变化的原因.

即一个类应该仅仅有一个责任, 即高内聚.

详细方法:

1. 菜单项, ArrayList和数组的基本元素.

/**
 * @time 2014年6月20日
 */
package iterator;

/**
 * @author C.L.Wang
 *
 */
public class MenuItem {

	String name;
	String description;
	boolean vegetarian; //是否是素食
	double price;

	/**
	 *
	 */
	public MenuItem(String name,
			String description,
			boolean vegetarian,
			double price)
	{
		// TODO Auto-generated constructor stub
		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;
	}

}

2. 详细的聚合对象(concrete aggregate), 包括创建迭代器的方法(createIterator).

/**
 * @time 2014年6月26日
 */
package iterator;

/**
 * @author C.L.Wang
 *
 */
public class DinerMenu {

	static final int MAX_ITEMS = 6;
	int numberOfItems = 0;
	MenuItem[] menuItems;

	/**
	 *
	 */
	public DinerMenu() {
		// TODO Auto-generated constructor stub

		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 the 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);

	}

	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;
		}
	}

	public Iterator createIterator() {
		return new DinerMenuIterator(menuItems);
	}

}

/**
 * @time 2014年6月20日
 */
package iterator;

import java.util.ArrayList;

/**
 * @author C.L.Wang
 *
 */
public class PancakeHouseMenu {

	ArrayList<MenuItem> menuItems;

	/**
	 *
	 */
	public PancakeHouseMenu() {
		// TODO Auto-generated constructor stub
		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 Iterator createIterator() {
		return new PancakeHouseMenuIterator(menuItems);
	}

}

3. 迭代器接口(iterator interface), 包括hasNext()方法和next()方法.

/**
 * @time 2014年6月27日
 */
package iterator;

/**
 * @author C.L.Wang
 *
 */
public interface Iterator {

	boolean hasNext();
	Object next();

}

4. 聚合对象的详细的迭代器(concrete iterator), 继承(implements)迭代器接口(iterator interface), 实现hasNext()方法和next()方法;

/**
 * @time 2014年6月27日
 */
package iterator;

/**
 * @author C.L.Wang
 *
 */
public class DinerMenuIterator implements Iterator {

	MenuItem[] items;
	int position = 0;

	/**
	 *
	 */
	public DinerMenuIterator(MenuItem[] items) {
		// TODO Auto-generated constructor stub
		this.items = items;
	}

	/* (non-Javadoc)
	 * @see iterator.Iterator#hasNext()
	 */
	@Override
	public boolean hasNext() {
		// TODO Auto-generated method stub

		if (position >= items.length || items[position] == null) {
			return false;
		}

		return true;
	}

	/* (non-Javadoc)
	 * @see iterator.Iterator#next()
	 */
	@Override
	public Object next() {
		// TODO Auto-generated method stub

		MenuItem menuItem = items[position];
		++position;

		return menuItem;
	}

}

/**
 * @time 2014年6月27日
 */
package iterator;

import java.util.ArrayList;

/**
 * @author C.L.Wang
 *
 */
public class PancakeHouseMenuIterator implements Iterator {

	ArrayList<MenuItem> menuItems;
	int position;

	/**
	 *
	 */
	public PancakeHouseMenuIterator(ArrayList<MenuItem> menuItems) {
		// TODO Auto-generated constructor stub
		this.menuItems = menuItems;
	}

	/* (non-Javadoc)
	 * @see iterator.Iterator#hasNext()
	 */
	@Override
	public boolean hasNext() {
		// TODO Auto-generated method stub
		if (position >= menuItems.size() || menuItems.get(position) == null) {
			return false;
		}

		return true;
	}

	/* (non-Javadoc)
	 * @see iterator.Iterator#next()
	 */
	@Override
	public Object next() {
		// TODO Auto-generated method stub

		MenuItem menuItem = menuItems.get(position);
		++position;

		return menuItem;
	}

}

5. 客户类使用详细聚合类(concrete aggregate)的迭代器(iterator).

/**
 * @time 2014年6月27日
 */
package iterator;

/**
 * @author C.L.Wang
 *
 */
public class Waitress {

	PancakeHouseMenu pancakeHouseMenu;
	DinerMenu dinerMenu;

	/**
	 *
	 */
	public Waitress(PancakeHouseMenu pancakeHouseMenu, DinerMenu dinerMenu) {
		// TODO Auto-generated constructor stub
		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 = (MenuItem)iterator.next();
			System.out.print(menuItem.getName() + ": ");
			System.out.print(menuItem.getPrice() + " -- ");
			System.out.println(menuItem.getDescription());
		}
	}

}

6. 測试代码:

/**
 * @time 2014年6月27日
 */
package iterator;

/**
 * @author C.L.Wang
 *
 */
public class MenuTestDrive {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		PancakeHouseMenu pancakeHouseMenu = new PancakeHouseMenu();
		DinerMenu dinerMenu = new DinerMenu();

		Waitress waitress = new Waitress(pancakeHouseMenu, dinerMenu);

		waitress.printMenu();
	}

}

7. 输出:

MENU
----
BREAKFAST
K&B's Pancake Breakfast: 2.99 -- Pancakes with scrambled eggs, and toast
Regular Pancake Breakfast: 2.99 -- Pancakes with fried eggs, sausage
Blueberry Pancakes: 3.49 -- Pancakes made with fresh blueberries
Waffles: 3.59 -- Waffles, with your choice of blueberries or strawberries

LUNCH
Vegetarian BLT: 2.99 -- (Fakin') Bacon with lettuce & tomato on whole wheat
BLT: 2.99 -- Bacon with lettuce & tomato on the whole wheat
Soup of the day: 3.29 -- Soup of the day, with a side of potato salad
Hotdog: 3.05 -- A hot dog, with saurkraut, relish, onions, topped with cheese