迭代器模式

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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，而又不暴露器内部的表示。

类图

这里写图片描述

java代码

// 菜单项public class MenuItem {    private String name;    private float price;    public MenuItem(String name, float price) {        this.name = name;        this.price = price;    }    public String getName() {        return name;    }    public float getPrice() {        return price;    }    @Override    public String toString() {        return "MenuItem{" +                "name='" + name + '\'' +                ", price=" + price +                '}';    }}// 菜单接口public interface Menu {    Iterator iterator();}// 煎饼店菜单public class PancakeHouseMenu implements Menu {    private ArrayList<MenuItem> items;    public PancakeHouseMenu() {        items = new ArrayList<>();        addItem("K&B's Pancake", 2.99f);        addItem("Regular Pancake", 2.59f);        addItem("Blueberry Pancake", 3.49f);    }    public void addItem(String name, float price) {        items.add(new MenuItem(name, price));    }    @Override    public Iterator iterator() {        return new PancakeHouseIterator(items);    }}// 餐厅菜单public class DinerMenu implements Menu {    private MenuItem[] items;    private int MAX_ITEMS = 3;    private int position = 0;    public DinerMenu() {        items = new MenuItem[MAX_ITEMS];        addItem("BLT", 2.99f);        addItem("Soup", 3.29f);        addItem("Hot dog", 3.05f);    }    public void addItem(String name, float price) {        if (position < MAX_ITEMS) {            items[position++] = new MenuItem(name, price);        } else {            throw new ArrayIndexOutOfBoundsException("menu is full");        }    }    @Override    public Iterator iterator() {        return new DinerIterator(items);    }}// 迭代器接口public interface Iterator {    boolean hasNext();    MenuItem next();}// 煎饼店菜单迭代器public class PancakeHouseIterator implements Iterator {    private ArrayList<MenuItem> items;    private int position = 0;    public PancakeHouseIterator(ArrayList<MenuItem> items) {        this.items = items;    }    @Override    public boolean hasNext() {        return position < items.size();    }    @Override    public MenuItem next() {        return items.get(position++);    }}// 餐厅菜单迭代器public class DinerIterator implements Iterator {    private MenuItem[] items;    private int position = 0;    public DinerIterator(MenuItem[] items) {        this.items = items;    }    @Override    public boolean hasNext() {        return position < items.length && items[position] != null;    }    @Override    public MenuItem next() {        return items[position++];    }}// 服务员public class Waitress {    private Menu pancakeHouseMenu;    private Menu dinerMenu;    public Waitress(Menu pancakeHouseMenu, Menu dinerMenu) {        this.pancakeHouseMenu = pancakeHouseMenu;        this.dinerMenu = dinerMenu;    }    public void printMenu() {        printMenu(pancakeHouseMenu.iterator());        printMenu(dinerMenu.iterator());    }    public void printMenu(Iterator iterator) {        while (iterator.hasNext()) {            System.out.println(iterator.next());        }    }}// 测试类public class Test {    public static void main(String[] args) {        new Test().test();    }    private void test() {        Menu pancakeHouseMenu = new PancakeHouseMenu();        Menu dinerMenu = new DinerMenu();        Waitress waitress = new Waitress(pancakeHouseMenu, dinerMenu);        waitress.printMenu();    }}

注：

菜单的实现已经被封装起来了。女招待员不知道菜单是如何存储菜单项集合的。
只要实现迭代器，我们只需要一个循环，就可以多态的处理任何项的集合。
女招待员现在只是用一个接口（迭代器）。
迭代器能让我们游走于聚合内的每一个元素，而又不暴露其内部表示。
把游走的任务放在迭代器上，而不是聚合上。这样简化了聚合的接口和实现，也让责任各得其所。

java自带迭代器

// 菜单项public class MenuItem {    private String name;    private float price;    public MenuItem(String name, float price) {        this.name = name;        this.price = price;    }    public String getName() {        return name;    }    public float getPrice() {        return price;    }    @Override    public String toString() {        return "MenuItem{" +                "name='" + name + '\'' +                ", price=" + price +                '}';    }}// 菜单接口public interface Menu {    Iterator iterator();}// 煎饼店菜单public class PancakeHouseMenu implements Menu {    private ArrayList<MenuItem> items;    public PancakeHouseMenu() {        items = new ArrayList<>();        addItem("K&B's Pancake", 2.99f);        addItem("Regular Pancake", 2.59f);        addItem("Blueberry Pancake", 3.49f);    }    public void addItem(String name, float price) {        items.add(new MenuItem(name, price));    }    @Override    public Iterator iterator() {        // 直接使用List的迭代器        return items.iterator();    }}// 餐厅菜单public class DinerMenu implements Menu {    private MenuItem[] items;    private int MAX_ITEMS = 3;    private int position = 0;    public DinerMenu() {        items = new MenuItem[MAX_ITEMS];        addItem("BLT", 2.99f);        addItem("Soup", 3.29f);        addItem("Hot dog", 3.05f);    }    public void addItem(String name, float price) {        if (position < MAX_ITEMS) {            items[position++] = new MenuItem(name, price);        } else {            throw new ArrayIndexOutOfBoundsException("menu is full");        }    }    @Override    public Iterator iterator() {        // 将数组转化为list，获得其迭代器        return Arrays.asList(items).iterator();    }}// 服务员public class Waitress {    private Menu pancakeHouseMenu;    private Menu dinerMenu;    public Waitress(Menu pancakeHouseMenu, Menu dinerMenu) {        this.pancakeHouseMenu = pancakeHouseMenu;        this.dinerMenu = dinerMenu;    }    public void printMenu() {        printMenu(pancakeHouseMenu.iterator());        printMenu(dinerMenu.iterator());    }    public void printMenu(Iterator iterator) {        while (iterator.hasNext()) {            System.out.println(iterator.next());        }    }}// 测试类public class Test {    public static void main(String[] args) {        new Test().test();    }    private void test() {        Menu pancakeHouseMenu = new PancakeHouseMenu();        Menu dinerMenu = new DinerMenu();        Waitress waitress = new Waitress(pancakeHouseMenu, dinerMenu);        waitress.printMenu();    }}

参考文章
1. Head First 设计模式

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