Iterator 迭代器模式

作者: LoserZhao 日期: 2021年6月8日发表评论 (0) 查看评论

Iterator 迭代器模式

Iterator 迭代器模式

1. 概述

1.1. Head First设计模式定义

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。
迭代器模式让我们能游走于聚合内的每一个元素，而又不暴露其内部的表示。
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。

1.2. 自我理解

面向Iterator接口编程，无论底层的数据结构和迭代算法如何变化，调用者都不用修改代码

1.3. 迭代器的组成

迭代器（Iterator）：迭代器定义访问和遍历元素的接口。
具体迭代器（ConcreteIterator）：具体迭代器实现迭代器接口，对该聚合遍历时跟踪当前位置。
聚合（Aggregate）：聚合定义创建相应迭代器对象的接口。
具体聚合（ConcreteAggregate）：具体聚合实现创建相应迭代器的接口，该操作返回ConcreteIterator 的一个适当的实例。

1.4. 迭代器模式适用场景

访问一个聚合对象的内容而无需暴露它的内部表示。
支持对聚合对象的多种遍历。
为遍历不同的聚合结构提供一个统一的接口。

2. UML

3. Example

3.1. 场景

爆炸性新闻：对象村的湘菜和川菜合并了。
但是湘菜菜谱用的List，川菜菜谱用的数组，新菜单合并中发现每次打印菜单需要用对应的打印方法。

3.2. 不使用Iterator设计模式的代码实现

/**
 * Created
 * Date: 2021/05/25 15:05 <br/>
 * Description:
 * 在没有使用迭代器模式时，我们需要针对于各厂商返回的不同数据结构，去适应对应的遍历方式，List的size，Array的length等。
 * @author www.loserzhao.com
 * @see
 */
public class WithoutIteratorPatternDemo {

    public static void main(String[] args) {
        // 集合的遍历
        XiangFootMenuItem xiangFootMenuItem = new XiangFootMenuItem();
        List<MenuItem> xiangFootMenuItems = xiangFootMenuItem.getMenuItems();
        for(int i=0; i<xiangFootMenuItems.size(); i++){
            System.out.println(xiangFootMenuItems.get(i));
        }

        // 数组的遍历
        ChuanFootMenuItem chuanFootMenuItem = new ChuanFootMenuItem();
        MenuItem[] chuanFootMenuItems = chuanFootMenuItem.getMenuItems();
        for(int i=0; i<chuanFootMenuItems.length; i++){
            System.out.println(chuanFootMenuItems[i]);
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 14:17 2021-05-26 <br/>
     * Description: 湘菜菜单
     */
    public static class XiangFootMenuItem{
        List<MenuItem> menuItems = new ArrayList<MenuItem>();
        public XiangFootMenuItem(){
            addMenuItem("辣椒炒肉", 28.00);
            addMenuItem("剁椒鱼头", 48.00);
        }

        public void addMenuItem(String name, Double price){
            MenuItem menuItem = new MenuItem(name, price);
            menuItems.add(menuItem);
        }

        public List<MenuItem> getMenuItems(){
            return menuItems;
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 14:18 2021-05-26 <br/>
     * Description: 川菜菜单
     */
    public static class ChuanFootMenuItem{
        static final int INIT_LENGTH = 2;
        MenuItem[] menuItems = new MenuItem[INIT_LENGTH];
        int index = 0;
        public ChuanFootMenuItem(){
            addMenuItem("麻婆豆腐", 18.00);
            addMenuItem("重庆火锅", 58.00);
            addMenuItem("四川辣子鸡", 38.00);
        }

        public void addMenuItem(String name, Double price){
            MenuItem menuItem = new MenuItem(name, price);
            if(index == menuItems.length){
                MenuItem[] newMenuItems = new MenuItem[this.menuItems.length + INIT_LENGTH];
                System.arraycopy(menuItems, 0, newMenuItems, 0, menuItems.length);
                menuItems = newMenuItems;
            }
            menuItems[index++] = menuItem;
        }

        public MenuItem[] getMenuItems(){
            return menuItems;
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 14:16 2021-05-26 <br/>
     * Description: 菜单
     */
    public static class MenuItem {
        private String name;
        private Double price;

        public MenuItem() {
        }

        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;
        }

        @Override
        public String toString() {
            return "MenuItem{" +
                    "name='" + name + '\'' +
                    ", price=" + price +
                    '}';
        }
    }
}

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/**

* Created

* Date: 2021/05/25 15:05

* Description:

* 在没有使用迭代器模式时，我们需要针对于各厂商返回的不同数据结构，去适应对应的遍历方式，List的size，Array的length等。

* @author www.loserzhao.com

* @see

public class WithoutIteratorPatternDemo {

public static void main(String[] args) {

// 集合的遍历

XiangFootMenuItem xiangFootMenuItem = new XiangFootMenuItem();

List<MenuItem> xiangFootMenuItems = xiangFootMenuItem.getMenuItems();

for(int i=0; i<xiangFootMenuItems.size(); i++){

System.out.println(xiangFootMenuItems.get(i));

}

// 数组的遍历

ChuanFootMenuItem chuanFootMenuItem = new ChuanFootMenuItem();

MenuItem[] chuanFootMenuItems = chuanFootMenuItem.getMenuItems();

for(int i=0; i<chuanFootMenuItems.length; i++){

System.out.println(chuanFootMenuItems[i]);

}

/**

* Author: www.loserzhao.com

* Date: 14:17 2021-05-26

* Description: 湘菜菜单

public static class XiangFootMenuItem{

List<MenuItem> menuItems = new ArrayList<MenuItem>();

public XiangFootMenuItem(){

addMenuItem("辣椒炒肉", 28.00);

addMenuItem("剁椒鱼头", 48.00);

}

public void addMenuItem(String name, Double price){

MenuItem menuItem = new MenuItem(name, price);

menuItems.add(menuItem);

}

public List<MenuItem> getMenuItems(){

return menuItems;

}

/**

* Author: www.loserzhao.com

* Date: 14:18 2021-05-26

* Description: 川菜菜单

public static class ChuanFootMenuItem{

static final int INIT_LENGTH = 2;

MenuItem[] menuItems = new MenuItem[INIT_LENGTH];

int index = 0;

public ChuanFootMenuItem(){

addMenuItem("麻婆豆腐", 18.00);

addMenuItem("重庆火锅", 58.00);

addMenuItem("四川辣子鸡", 38.00);

}

public void addMenuItem(String name, Double price){

MenuItem menuItem = new MenuItem(name, price);

if(index == menuItems.length){

MenuItem[] newMenuItems = new MenuItem[this.menuItems.length + INIT_LENGTH];

System.arraycopy(menuItems, 0, newMenuItems, 0, menuItems.length);

menuItems = newMenuItems;

}

menuItems[index++] = menuItem;

}

public MenuItem[] getMenuItems(){

return menuItems;

}

/**

* Author: www.loserzhao.com

* Date: 14:16 2021-05-26

* Description: 菜单

public static class MenuItem {

private String name;

private Double price;

public MenuItem() {

}

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;

}

@Override

public String toString() {

return "MenuItem{" +

"name='" + name + '\'' +

", price=" + price +

'}';

}

3.3. Iterator模式的代码实现

3.3.1. 当前案例UML

3.3.2. 代码

/**
 * Created
 * Date: 2021/05/25 15:39 <br/>
 * Description:
 * 用了迭代器设计模式后，我们无需关注数据集合类型，迭代器会提供 hasNext(), next()方法来给我们迭代。
 * 官方说法：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。
 * @author www.loserzhao.com
 * @see
 */
public class IteratorPatternDemo {

    public static void main(String[] args) {
        XiangFootMenuItem xiangFootMenuItem = new XiangFootMenuItem();
        Iterator<MenuItem> xiangFootMenuItemIterator = xiangFootMenuItem.createIterator();
        printMenu(xiangFootMenuItemIterator);

        ChuanFootMenuItem chuanFootMenuItem = new ChuanFootMenuItem();
        Iterator<MenuItem> chuanFootMenuItemIterator = chuanFootMenuItem.createIterator();
        printMenu(chuanFootMenuItemIterator);
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 15:08 2021-05-26 <br/>
     * Description: 打印菜单
     * @param menuItemIterator
     */
    public static void printMenu(Iterator<MenuItem> menuItemIterator){
        while(menuItemIterator.hasNext()){
            System.out.println(menuItemIterator.next());
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 15:08 2021-05-26 <br/>
     * Description: 迭代器
     * @param <E>
     */
    public interface Iterator<E>{
        /**
         * Author: www.loserzhao.com <br/>
         * Date: 14:36 2021-05-26 <br/>
         * Description: 是否还有下一个元素
         * @return
         */
        Boolean hasNext();

        /**
         * Author: www.loserzhao.com <br/>
         * Date: 14:37 2021-05-26 <br/>
         * Description: 获取下一个元素
         * @return
         */
        E next();
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 15:08 2021-05-26 <br/>
     * Description: 湘菜迭代器
     */
    public static class XiangFootMenuItemIterator implements Iterator<MenuItem>{
        List<MenuItem> menuItems;
        int index = 0;
        public XiangFootMenuItemIterator(List<MenuItem> menuItems){
            this.menuItems = menuItems;
        }
        public Boolean hasNext() {
            return index < menuItems.size();
        }

        public MenuItem next() {
            return menuItems.get(index++);
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 15:08 2021-05-26 <br/>
     * Description: 川菜迭代器
     */
    public static class ChuanFootMenuItemIterator implements Iterator<MenuItem>{
        MenuItem[] menuItems;
        int index = 0;
        public ChuanFootMenuItemIterator(MenuItem[] menuItems){
            this.menuItems = menuItems;
        }
        public Boolean hasNext() {
            return index < menuItems.length && menuItems[index] != null;
        }

        public MenuItem next() {
            return menuItems[index++];
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 15:08 2021-05-26 <br/>
     * Description: 菜单聚合类
     */
    public interface MenuAggregate {
        /**
         * Author: www.loserzhao.com <br/>
         * Date: 15:07 2021-05-26 <br/>
         * Description: 获取菜单迭代器
         * @return
         */
        Iterator<MenuItem> createIterator();
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 14:17 2021-05-26 <br/>
     * Description: 湘菜菜单
     */
    public static class XiangFootMenuItem implements MenuAggregate {
        List<MenuItem> menuItems = new ArrayList<MenuItem>();
        public XiangFootMenuItem(){
            addMenuItem("辣椒炒肉", 28.00);
            addMenuItem("剁椒鱼头", 48.00);
        }

        public void addMenuItem(String name, Double price){
            MenuItem menuItem = new MenuItem(name, price);
            menuItems.add(menuItem);
        }

        public Iterator<MenuItem> createIterator(){
            return new XiangFootMenuItemIterator(menuItems);
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 14:18 2021-05-26 <br/>
     * Description: 川菜菜单
     */
    public static class ChuanFootMenuItem implements MenuAggregate {
        static final int INIT_LENGTH = 2;
        MenuItem[] menuItems = new MenuItem[INIT_LENGTH];
        int index = 0;
        public ChuanFootMenuItem(){
            addMenuItem("麻婆豆腐", 18.00);
            addMenuItem("重庆火锅", 58.00);
            addMenuItem("四川辣子鸡", 38.00);
        }

        public void addMenuItem(String name, Double price){
            MenuItem menuItem = new MenuItem(name, price);
            if(index == menuItems.length){
                MenuItem[] newMenuItems = new MenuItem[this.menuItems.length + INIT_LENGTH];
                System.arraycopy(menuItems, 0, newMenuItems, 0, menuItems.length);
                menuItems = newMenuItems;
            }
            menuItems[index++] = menuItem;
        }

        public Iterator<MenuItem> createIterator(){
            return new ChuanFootMenuItemIterator(menuItems);
        }
    }

    /**
     * Author: www.loserzhao.com <br/>
     * Date: 14:16 2021-05-26 <br/>
     * Description: 菜系
     */
    public static class MenuItem {
        private String name;
        private Double price;

        public MenuItem() {
        }

        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;
        }

        @Override
        public String toString() {
            return "MenuItem{" +
                    "name='" + name + '\'' +
                    ", price=" + price +
                    '}';
        }
    }
}

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/**

* Created

* Date: 2021/05/25 15:39

* Description:

* 用了迭代器设计模式后，我们无需关注数据集合类型，迭代器会提供 hasNext(), next()方法来给我们迭代。

* 官方说法：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。

* @author www.loserzhao.com

* @see

public class IteratorPatternDemo {

public static void main(String[] args) {

XiangFootMenuItem xiangFootMenuItem = new XiangFootMenuItem();

Iterator<MenuItem> xiangFootMenuItemIterator = xiangFootMenuItem.createIterator();

printMenu(xiangFootMenuItemIterator);

ChuanFootMenuItem chuanFootMenuItem = new ChuanFootMenuItem();

Iterator<MenuItem> chuanFootMenuItemIterator = chuanFootMenuItem.createIterator();

printMenu(chuanFootMenuItemIterator);

}

/**

* Author: www.loserzhao.com

* Date: 15:08 2021-05-26

* Description: 打印菜单

* @param menuItemIterator

public static void printMenu(Iterator<MenuItem> menuItemIterator){

while(menuItemIterator.hasNext()){

System.out.println(menuItemIterator.next());

}

/**

* Author: www.loserzhao.com

* Date: 15:08 2021-05-26

* Description: 迭代器

* @param <E>

public interface Iterator<E>{

/**

* Author: www.loserzhao.com

* Date: 14:36 2021-05-26

* Description: 是否还有下一个元素

* @return

Boolean hasNext();

/**

* Author: www.loserzhao.com

* Date: 14:37 2021-05-26

* Description: 获取下一个元素

* @return

E next();

}

/**

* Author: www.loserzhao.com

* Date: 15:08 2021-05-26

* Description: 湘菜迭代器

public static class XiangFootMenuItemIterator implements Iterator<MenuItem>{

List<MenuItem> menuItems;

int index = 0;

public XiangFootMenuItemIterator(List<MenuItem> menuItems){

this.menuItems = menuItems;

}

public Boolean hasNext() {

return index < menuItems.size();

}

public MenuItem next() {

return menuItems.get(index++);

}

/**

* Author: www.loserzhao.com

* Date: 15:08 2021-05-26

* Description: 川菜迭代器

public static class ChuanFootMenuItemIterator implements Iterator<MenuItem>{

MenuItem[] menuItems;

int index = 0;

public ChuanFootMenuItemIterator(MenuItem[] menuItems){

this.menuItems = menuItems;

}

public Boolean hasNext() {

return index < menuItems.length && menuItems[index] != null;

}

public MenuItem next() {

return menuItems[index++];

}

/**

* Author: www.loserzhao.com

* Date: 15:08 2021-05-26

* Description: 菜单聚合类

public interface MenuAggregate {

/**

* Author: www.loserzhao.com

* Date: 15:07 2021-05-26

* Description: 获取菜单迭代器

* @return

Iterator<MenuItem> createIterator();

}

/**

* Author: www.loserzhao.com

* Date: 14:17 2021-05-26

* Description: 湘菜菜单

public static class XiangFootMenuItem implements MenuAggregate {

List<MenuItem> menuItems = new ArrayList<MenuItem>();

public XiangFootMenuItem(){

addMenuItem("辣椒炒肉", 28.00);

addMenuItem("剁椒鱼头", 48.00);

}

public void addMenuItem(String name, Double price){

MenuItem menuItem = new MenuItem(name, price);

menuItems.add(menuItem);

}

public Iterator<MenuItem> createIterator(){

return new XiangFootMenuItemIterator(menuItems);

}

/**

* Author: www.loserzhao.com

* Date: 14:18 2021-05-26

* Description: 川菜菜单

public static class ChuanFootMenuItem implements MenuAggregate {

static final int INIT_LENGTH = 2;

MenuItem[] menuItems = new MenuItem[INIT_LENGTH];

int index = 0;

public ChuanFootMenuItem(){

addMenuItem("麻婆豆腐", 18.00);

addMenuItem("重庆火锅", 58.00);

addMenuItem("四川辣子鸡", 38.00);

}

public void addMenuItem(String name, Double price){

MenuItem menuItem = new MenuItem(name, price);

if(index == menuItems.length){

MenuItem[] newMenuItems = new MenuItem[this.menuItems.length + INIT_LENGTH];

System.arraycopy(menuItems, 0, newMenuItems, 0, menuItems.length);

menuItems = newMenuItems;

}

menuItems[index++] = menuItem;

}

public Iterator<MenuItem> createIterator(){

return new ChuanFootMenuItemIterator(menuItems);

}

/**

* Author: www.loserzhao.com

* Date: 14:16 2021-05-26

* Description: 菜系

public static class MenuItem {

private String name;

private Double price;

public MenuItem() {

}

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;

}

@Override

public String toString() {

return "MenuItem{" +

"name='" + name + '\'' +

", price=" + price +

'}';

}

参考文献

《Head First设计模式》
Java 设计模式之迭代器模式

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