迭代器模式

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。

要点：

1、迭代器模式把在元素之间游走的责任交给迭代器，而不是聚合对象。这样简化了聚合的接口和实现，也让责任各得其所。

2、迭代器模式常见的方法有hasNext()、next()、remove()方法等。

HeadFirst中的迭代器模式实现

1、创建迭代器接口

package Iterator;

/**
* 迭代器接口
*/
public interface Iterator {

    public boolean hasNext();

    public Object next();
}

2、创建菜单项

package Iterator;

public class MenuItem {

    String name;
    double price;

    public MenuItem(String name, double price) {
        this.name = name;
        this.price = price;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }
}

3、创建具体的菜单迭代器

package Iterator;

import java.util.ArrayList;

/**
* 煎饼屋菜单迭代器
*/
public class PancakeHouseMenuIterator implements Iterator {

    /**
* 列表形式的菜单项
*/
public ArrayList<MenuItem> menuItems;
    int position = 0;

    public PancakeHouseMenuIterator(ArrayList<MenuItem> items) {
        this.menuItems = items;
    }

    @Override
    public boolean hasNext() {
        if (position < menuItems.size()) {
            return true;
        }
        return false;
    }

    @Override
    public Object next() {
        if (!this.hasNext()) {
            return null;
        }
        MenuItem item = menuItems.get(position);
        position = position + 1;
        return item;
    }
}

package Iterator;

public class DinerMenuIterator implements Iterator{

    public MenuItem[] menuItems;
    int position = 0;

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

    @Override
    public boolean hasNext() {
        if (position < menuItems.length) {
            return true;
        }
        return false;
    }

    @Override
    public Object next() {
        if (!this.hasNext()) {
            return null;
        }
        MenuItem item = menuItems[position];
        position = position + 1;
        return item;
    }
}

4、创建菜单接口

package Iterator;

public interface Menu {

    // 创建迭代器
    public Iterator createIterator();
}

5、创建具体的菜单

package Iterator;

import java.util.ArrayList;

/**
* 煎饼屋菜单
*/
public class PancakeHouseMenu implements Menu {

    public ArrayList<MenuItem> menuItems;

    public PancakeHouseMenu() {
        menuItems = new ArrayList<MenuItem>();
        menuItems.add(new MenuItem("Regular Pancake Breakfast", 2.99));
        menuItems.add(new MenuItem("Blueberry Pancakes", 3.49));
        menuItems.add(new MenuItem("Waffles", 3.59));
    }

    @Override
    public Iterator createIterator() {
        return new PancakeHouseMenuIterator(menuItems);
    }
}

package Iterator;

/*
    餐厅菜单
 */
public class DinerMenu implements Menu {

    public MenuItem[] menuItems;

    public DinerMenu() {
        menuItems = new MenuItem[3];
        menuItems[0] = new MenuItem("BLT", 2.99);
        menuItems[1] = new MenuItem("Soup of the day", 3.29);
        menuItems[2] = new MenuItem("Hotdog", 3.05);
    }

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

6、创建女招待

package Iterator;

/**
* 女招待
*/
public class Waitress {

    Menu pancakeHouseMenu;
    Menu dinerMenu;

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

    public void printMenu() {
        // 获取菜单迭代器
        Iterator pancakeIterator = pancakeHouseMenu.createIterator();
        Iterator dinerIterator = dinerMenu.createIterator();
        // 使用迭代器打印菜单
        System.out.println("----MENU----\n");
        System.out.println("BREAKFASE:");
        printMenu(pancakeIterator);
        System.out.println("\nLUNCH:");
        printMenu(dinerIterator);
    }

    private void printMenu(Iterator iterator) {
        while (iterator.hasNext()) {
            MenuItem menuItem = (MenuItem)iterator.next();
            System.out.println(menuItem.getName() + ", " + menuItem.getPrice());
        }
    }
}

7、使用女招待打印菜单

package Iterator;

public class Test {

    public static void main(String[] args) {
        // 菜单
        Menu pancakeHouseMenu = new PancakeHouseMenu();
        Menu dinerMenu = new DinerMenu();
        // 女招待
        Waitress waitress = new Waitress(pancakeHouseMenu, dinerMenu);
        // 打印菜单
        waitress.printMenu();
    }
}