迭代器模式

Iterator Pattern 对象行为型模式

意图

提供一种方法顺序访问一个聚合对象中的各个元素，且不需要暴露该对象的内部表示

结构

其中：

• Iterator (迭代器) 定义访问和遍历元素的接口
• 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
• Aggregate (聚合) 定义创建相应迭代器对象的接口
• 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

适用性

Iterator 模式适用于：

• 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

利用 Java 已实现迭代器

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

public class IteratorPattern {
    public static void main(String[] args) {
        List<Book> bookList = new ArrayList<>();

        String[] books = {"Linear Algebra", "Algorithm"};
        double[] prices = {25.8, 29.8};

        for(int i = 0; i < 2; i++){
            bookList.add(new Book(books[i], prices[i]));
        }

        // 访问元素 1
        for(int i = 0; i < bookList.size(); i++){
            Book book = bookList.get(i);
            System.out.println("name: " + book.getName() + " price: " + book.getPrice());
        }

        // 访问元素 2
        System.out.println("=====222=====");
        for(Book book : bookList){
            System.out.println("name: " + book.getName() + " price: " + book.getPrice());
        }

        // 访问元素 3 迭代器
        System.out.println("=====333=====");
        Iterator iterator = bookList.iterator();
        while (iterator.hasNext()){
            Book book = (Book) iterator.next();
            System.out.println("name: " + book.getName() + " price: " + book.getPrice());
        }
    }
}

class Book{
    private String name;
    private double price;

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

    public double getPrice() {
        return price;
    }

    public String getName() {
        return name;
    }
}

例子

import javax.sound.midi.Soundbank;
import java.util.ArrayList;
import java.util.List;

public class IteratorPattern {
    public static void main(String[] args) {
        BookAggregate bookAggregate = new BookAggregate();

        String[] books = {"Linear Algebra", "Algorithm"};
        double[] prices = {25.8, 29.8};

        for(int i = 0; i < 2; i++){
            bookAggregate.Add(new Book(books[i], prices[i]));
        }

        Iterator iterator = bookAggregate.CreateIterator();
        while(iterator.hasNext()){
            Book book = (Book)iterator.next();
            System.out.println("name: " + book.getName() + " price: " + book.getPrice());
        }
    }
}

interface Iterator{
    public boolean hasNext();
    public Object next();
}

class BookIterator implements Iterator{
    private int index;
    private BookAggregate bookAggregate;

    public BookIterator(BookAggregate bookAggregate){
        this.index = 0;
        this.bookAggregate = bookAggregate;
    }

    @Override
    public boolean hasNext(){
        if (index < bookAggregate.getSize()) return true;
        return false;
    }

    @Override
    public Object next(){
        Object obj = bookAggregate.get(index);
        index++;
        return obj;
    }
}

interface Aggregate{
    public Iterator CreateIterator();
}

class BookAggregate implements Aggregate{
    private List<Book> list = new ArrayList<>();

    public void Add(Book book){
        list.add(book);
    }

    public Book get(int index){
        return list.get(index);
    }

    public int getSize(){
        return list.size();
    }

    @Override
    public Iterator CreateIterator(){
        return new BookIterator(this);
    }
}

class Book{
    private String name;
    private double price;

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

    public double getPrice() {
        return price;
    }

    public String getName() {
        return name;
    }
}