设计模式 - 迭代模式(iterator pattern) Java 迭代器(Iterator) 详细解释

迭代模式(iterator pattern) Java 迭代器(Iterator) 详细解释

 

Java的标准库(util)中包括迭代器接口(iterator interface), import java.util.Iterator;

继承(implements)迭代器接口(Iterator)须要重写(override)三个函数: hasNext(), next()和remove();

Java的聚合类型, 如ArrayList包括迭代器.

可是数组类型, 须要重写对应的迭代器(iterator).

详细方法:

1. ArrayList类型, 包括迭代器的方法, 能够直接返回.

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

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

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

  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<MenuItem> createIterator() {
    return menuItems.iterator();
  }

}

2. 数组类型, 创建对应的迭代器类, 继承(implements)迭代器(Iterator), 重写迭代器的方法.

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

import java.util.Iterator;

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

  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<MenuItem> createIterator() {
    return new DinerMenuIterator(menuItems);
  }

}


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

import java.util.Iterator;

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

  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 MenuItem next() {
    // TODO Auto-generated method stub

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

    return menuItem;
  }

  @Override
  public void remove() {
    if (position <= 0) {
      throw new IllegalStateException
      ("You can't remove an item until you've done at least one next()");
    }

    if (items[position-1] != null) {
      for (int i=position-1; i<(items.length-1); ++i) {
        items[i] = items[i+1];
      }
      items[items.length-1] = null;
    }
  }

}

3. 菜单接口(interface), 包括创建迭代器(createIterator)的方法.

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

import java.util.Iterator;

/**
 * @author C.L.Wang
 *
 */
public interface Menu {
  public Iterator<MenuItem> createIterator();
}

4. 详细的菜单项, 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;
  }

}

5. 客户类(client), 调用迭代器(iterator)方法.

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

import java.util.Iterator;

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

  Menu pancakeHouseMenu;
  Menu dinerMenu;

  /**
   * 
   */
  public Waitress(Menu pancakeHouseMenu, Menu dinerMenu) {
    // TODO Auto-generated constructor stub
    this.pancakeHouseMenu = pancakeHouseMenu;
    this.dinerMenu = dinerMenu;
  }

  public void printMenu() {
    Iterator<MenuItem> pancakeIterator = pancakeHouseMenu.createIterator();
    Iterator<MenuItem> dinerIterator = dinerMenu.createIterator();
    System.out.println("MENU\n----\nBREAKFAST");
    printMenu(pancakeIterator);
    System.out.println("\nLUNCH");
    printMenu(dinerIterator);

  }

  private void printMenu(Iterator<MenuItem> 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:

/**
 * @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