迭代器.md

Iterator 迭代器模式

重点等级：:star::star::star::star::star:

迭代器模式提供了一种方法顺序访问一个聚合对象中的各个元素，而又无需暴露该对象的内部实现，这样既可以做到不暴露集合的内部结构，又可让外部代码透明地访问集合内部的数据。

场景

• 访问一个集合对象的内容而无需暴露它的内部表示
• 为遍历不同的集合结构提供一个统一的接口

代码示例

定义几个角色：

• Iterator（抽象迭代器）。这里使用java.util.Iterator代替,也可以自己重新定义
• BookIterator（具体迭代器）
• Aggregate（抽象聚合类）。它用于存储和管理元素对象，充当抽象迭代器工厂角色。
• BookAggregateImpl（具体聚合类）

public class Book {

    private String name;

    public Book(){

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

    public String getName(){
        return name;
    }
}


public class BookIterator implements Iterator<Book> {

    private Aggregate<Book> bookAggregate;
    private int index;

    public BookIterator(Aggregate<Book> bookAggregate){
        this.bookAggregate = bookAggregate;
        index = 0;
    }

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

    @Override
    public Book next() {
        return bookAggregate.get(index++);
    }
}



public interface Aggregate<T> {

    Iterator<T> iterator();

    T get(int i);

    void add (T t);

    int length();
}



public class BookAggregateImpl implements Aggregate<Book> {

    private Book[] books;
    private int position;

    public BookAggregateImpl(int max_length){
        books = new Book[max_length];
        position = 0;
    }

    @Override
    public Iterator<Book> iterator() {
        return new BookIterator(this);
    }

    @Override
    public void add(Book t) {
        books[position++] = t;
    }

    @Override
    public Book get(int i) {
        return books[i];
    }

    @Override
    public int length(){
        return position;
    }
}



public static void main(String[] args) {

    Book book1 = new Book("book1");
    Book book2 = new Book("book2");
    Book book3 = new Book("book3");
    Book book4 = new Book("book4");
    Book book5 = new Book("book5");

    Aggregate aggregate = new BookAggregateImpl(5);
    aggregate.add(book1);
    aggregate.add(book2);
    aggregate.add(book3);
    aggregate.add(book4);
    aggregate.add(book5);
    Iterator<Book> it = aggregate.iterator();
    while (it.hasNext()) {
        Book book = it.next();
        System.out.println(book.getName());
    }
}

JDK源码中的案例

• java.util.Iterator
• java.util.Enumeration