定義

迭代器模式(iterator pattern)提供了一種方法順序訪問一個集合對象中的各個元素，而又不暴露其內部的表示。

代碼實現

下面以JDK現有的Iterator的例子來說明適配器模式使用。

給出創建Iterator接口的定義：

import java.util.Iterator;

public interface Menu {
    public Iterator createIterator();
}

菜單項的BEAN對象:

public class MenuItem {
    String name; // 菜名
    String description; // 描述
    boolean vegetarian; // 是否是素菜
    double price; // 菜單價

    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;
    }
}

具體集合對象，管理(增加，移除，查找等)集合的元素，實現創建Iterator接口

import java.util.ArrayList;
import java.util.Iterator;

public class PancakeHouseMenu implements Menu {
    // 內部存儲是ArrayList
    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 fried eggs, sausage", 
            false,
            2.99);

        addItem("Blueberry Pancakes",
            "Pancakes made with fresh blueberries, and blueberry syrup",
            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 getMenuItems() {
        return menuItems;
    }

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

    // other menu methods here
}

另一個具體集合對象，內部的存儲是數組

import java.util.Iterator;

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",
            "A medly of 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;
    }

    // 數組需要自己實現迭代器的創建
    public Iterator createIterator() {
        return new DinerMenuIterator(menuItems);
    }

    // other menu methods here
}

數組類型的迭代器的實現：

import java.util.Iterator; // 使用JDK中迭代器接口

public class DinerMenuIterator implements Iterator {
    MenuItem[] list;
    int position = 0;

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

    public Object next() {
        MenuItem menuItem = list[position];
        position = position + 1;
        return menuItem;
    }

    public boolean hasNext() {
        if (position >= list.length || list[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] = list[i+1];
            }
            list[list.length-1] = null;
        }
    }
}

另一個具體集合對象，內部的存儲是Hashtable

import java.util.*;

public class CafeMenu implements Menu {
    Hashtable menuItems = new Hashtable(); //內部的存儲是Hashtable

    public CafeMenu() {
        addItem("Veggie Burger and Air Fries",
            "Veggie burger on a whole wheat bun, lettuce, tomato, and fries",
            true, 3.99);
        addItem("Soup of the day",
            "A cup of the soup of the day, with a side salad",
            false, 3.69);
        addItem("Burrito",
            "A large burrito, with whole pinto beans, salsa, guacamole",
            true, 4.29);
    }

    public void addItem(String name, String description, 
                         boolean vegetarian, double price) 
    {
        MenuItem menuItem = new MenuItem(name, description, vegetarian, price);
        menuItems.put(menuItem.getName(), menuItem);
    }

    public Hashtable getItems() {
        return menuItems;
    }

    public Iterator createIterator() {
        return menuItems.values().iterator();
    }
}

客戶(招待員)代碼，她持有Menu接口，通過這個接口去創建迭代器遍歷菜單，她不需要知道菜單項內部的存儲細節。

import java.util.Iterator;

public class Waitress {
    Menu pancakeHouseMenu; // 煎餅屋菜單
    Menu dinerMenu;  // 餐廳菜單
    Menu cafeMenu; // 咖啡廳菜單

    public Waitress(Menu pancakeHouseMenu, Menu dinerMenu, Menu cafeMenu) {
        this.pancakeHouseMenu = pancakeHouseMenu;
        this.dinerMenu = dinerMenu;
        this.cafeMenu = cafeMenu;
    }

    public void printMenu() {
        // 生成迭代器接口
        Iterator pancakeIterator = pancakeHouseMenu.createIterator();
        Iterator dinerIterator = dinerMenu.createIterator();
        Iterator cafeIterator = cafeMenu.createIterator();

        System.out.println("MENU\n----\nBREAKFAST");
        printMenu(pancakeIterator);
        System.out.println("\nLUNCH");
        printMenu(dinerIterator);
        System.out.println("\nDINNER");
        printMenu(cafeIterator);
    }
    // 遍歷迭代器，遍歷所有迭代器的代碼都可以複用這段代碼
    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());
        }
    }

    public void printVegetarianMenu() {
        System.out.println("\nVEGETARIAN MENU\n---------------");
        printVegetarianMenu(pancakeHouseMenu.createIterator());
        printVegetarianMenu(dinerMenu.createIterator());
        printVegetarianMenu(cafeMenu.createIterator());
    }

    public boolean isItemVegetarian(String name) {
        Iterator pancakeIterator = pancakeHouseMenu.createIterator();
        if (isVegetarian(name, pancakeIterator)) {
            return true;
        }
        Iterator dinerIterator = dinerMenu.createIterator();
        if (isVegetarian(name, dinerIterator)) {
            return true;
        }
        Iterator cafeIterator = cafeMenu.createIterator();
        if (isVegetarian(name, cafeIterator)) {
            return true;
        }
        return false;
    }


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

    private boolean isVegetarian(String name, Iterator iterator) {
        while (iterator.hasNext()) {
            MenuItem menuItem = (MenuItem)iterator.next();
            if (menuItem.getName().equals(name)) {
                if (menuItem.isVegetarian()) {
                    return true;
                }
            }
        }
        return false;
    }
}

測試驅動代碼

import java.util.*;

public class MenuTestDrive {
    public static void main(String args[]) {
        // 
        Menu pancakeHouseMenu = new PancakeHouseMenu();
        Menu dinerMenu = new DinerMenu();
        Menu cafeMenu = new CafeMenu();

        Waitress waitress = new Waitress(pancakeHouseMenu, dinerMenu, cafeMenu);

        // 各種遍歷菜單的方式
        waitress.printMenu();
        waitress.printVegetarianMenu();

        System.out.println("\nCustomer asks, is the Hotdog vegetarian?");
        System.out.print("Waitress says: ");
        if (waitress.isItemVegetarian("Hotdog")) {
            System.out.println("Yes");
        } else {
            System.out.println("No");
        }
        System.out.println("\nCustomer asks, are the Waffles vegetarian?");
        System.out.print("Waitress says: ");
        if (waitress.isItemVegetarian("Waffles")) {
            System.out.println("Yes");
        } else {
            System.out.println("No");
        }
    }
}

從上面的代碼可以看出，迭代器模式讓客戶遍歷集合的代碼更加通用，它不用關心集合內部的存儲細節，因爲集合對象創建的迭代器封裝了遍歷具體數據結構的細節。

該模式體現了哪些OO原則

原則1: 封裝變化原則
這裏變化的部分是，由不同的集合類型所造成的遍歷方式不同。所以需要把遍歷方式封裝起來。
原則2: 針對接口編程，而不是針對實現編程
原則10: 單一責任原則

迭代器模式把遍歷集合內元素的操作交給了迭代器(或者迭代器的子類)，而集合本身專注在管理元素，這符合單一責任原則，讓設計具有高內聚的特性。

本章總結

Java Collection框架裏面很多類都實現了Iterator接口，可細研究
集合類把遍歷集合內元素的責任封裝在Iterator接口，集合只需要專注在管理集合的元素，比如，添加，刪除，查找等。讓一個類只做一件事，遵守單一責任原則，這樣的設計更加容易維護

rex_nie

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《HeadFirst設計模式》讀書筆記-第9章v1-迭代器模式

定義

代碼實現

該模式體現了哪些OO原則

本章總結

《HeadFirst設計模式》讀書筆記-第8章-模板方法模式

《HeadFirst設計模式》讀書筆記-第7章v1-適配器模式

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