迭代器模式

定义迭代器模式

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。

实例：

三个餐厅要合并，每个餐厅的菜单采用不同和数据结构实现，服务员需要获取所有的菜单信息，但是处理逻辑又不能过于复杂。

/**
 * 菜单项
 */
public class MenuItem {
    String name;
    String description;
    boolean vegetarian;
    double price;
    public MenuItem(String name,String description,boolean vegetarian,double price){
        this.name=name;
        this.description=description;
        this.vegetarian=vegetarian;
        this.price=price;
    }

    public double getPrice() {
        return price;
    }

    public boolean isVegetarian() {

        return vegetarian;
    }

    public String getDescription() {

        return description;
    }

    public String getName() {

        return name;
    }
}

/**
 * 菜单接口
 */
public interface Menu {
    Iterator<MenuItem> createIterator();
}

/**
 * 午餐菜单
 */
public class DinerMenu implements Menu{
    @Override
    public Iterator<MenuItem> createIterator() {
        return new DinerMenuIterator(menuItems);
    }
    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);
    }
    public void addItem(String name,String description,boolean vegerarian,double price){
        MenuItem menuItem=new MenuItem(name,description,vegerarian,price);
        if(numberOfItems>=MAX_ITEMS){
            System.out.println("Sorry,menu is full! Can't add item to menu");
        }else{
            menuItems[numberOfItems]=menuItem;
            numberOfItems++;
        }
    }
}

/**
 * 早餐菜单
 */
public class PancakeHouseMenu implements Menu{
    @Override
    public Iterator<MenuItem> createIterator() {
        return menuItems.iterator();
    }

    ArrayList<MenuItem> 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 make 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);
    }
}/**
 * 咖啡菜单
 */
public class CafeMenu implements Menu{
    @Override
    public Iterator<MenuItem> createIterator() {
        return menuItems.values().iterator();
    }

    Hashtable<String,MenuItem> menuItems=new 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);
    }

}

/**
 * 迭代器具体实现类，用于遍历午餐
 */
public class DinerMenuIterator implements Iterator<MenuItem>{
    private MenuItem[] menuItems;
    private int position=0;
    public DinerMenuIterator(MenuItem[] menuItems){
        this.menuItems=menuItems;
    }
    @Override
    public boolean hasNext() {
        if(position>=menuItems.length||menuItems[position]==null){
            return false;
        }else {
            return true;
        }
    }

    @Override
    public MenuItem next() {
        MenuItem menuItem=menuItems[position];
        position++;
        return menuItem;
    }
}

/**
 * 服务员类
 */
public class Waitress {
    ArrayList<Menu> menus;
    public Waitress(ArrayList<Menu> menus){
        this.menus=menus;
    }
    public void printMenu(){
        Iterator<Menu> menuItemIterator=menus.iterator();
        while(menuItemIterator.hasNext()){
            Menu menu=menuItemIterator.next();
            printMenu(menu.createIterator());
        }
    }
    private void printMenu(Iterator<MenuItem> iterator){
        System.out.println();
        while (iterator.hasNext()){
            MenuItem menuItem=iterator.next();
            System.out.print(menuItem.getName()+", ");
            System.out.print(menuItem.getPrice()+" -- " );
            System.out.println(menuItem.getDescription());
        }
        System.out.println();
    }
}

测试：

public class Test {
    public static void main(String[] args){
        Menu dinerMenu=new DinerMenu();
        Menu pancakeMenu=new PancakeHouseMenu();
        Menu cafeMenu=new CafeMenu();
        ArrayList<Menu> menus=new ArrayList<>();
        menus.add(dinerMenu);
        menus.add(pancakeMenu);
        menus.add(cafeMenu);
        Waitress waitress=new Waitress(menus);
        waitress.printMenu();
    }
}

Vegetarian BLT, 2.99 -- Fakin Bacon with lettuce & tomato on whole wheat

BLT, 2.99 -- Bacon with lettuce & tomato on 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

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 make with fresh blueberries

Waffles, 3.59 -- Waffles,with your choice of blueberries or strawberries

Soup of the day, 3.69 -- A cup of the soup of the day, with a side salad

Burrito, 4.29 -- A large burrito ,with whole pinto beans,salsa,guacamole

Veggie Burger and Air Fries, 3.99 -- Veggie burger on a whole wheat bun, lettuce,tomato,and fries