Iterator模式

话不多说，来写一段Iterator实例程序

//一本简单的书
public class Book {

    private String name;

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

//简单的迭代器
public interface Iterator {
    boolean hastNext();
    Object next();
}

// 简单的集合
public interface Aggregate {
    Iterator iterator();
}

// 简单的书架
public class BookShelf implements Aggregate {

    private Book[] books;

    private int last = 0;

    public BookShelf(int maxSize) {
        this.books = new Book[maxSize];
    }

    public Book getBookAt(int index) {
        if (index < 0 || index > books.length - 1) {
            throw new IllegalArgumentException("index error");
        }
        return books[index];
    }

    public void appendBook(Book book) {
        this.books[last] = book;
        last++;
    }

    public int getLength() {
        return last;
    }

    @Override
    public Iterator iterator() {
        return new BookShelfIterator(this);
    }
}

// 迭代器的实现，“真迭代器”
public class BookShelfIterator implements Iterator {

    private BookShelf bookShelf;

    private int index;

    public BookShelfIterator(BookShelf bookShelf) {
        this.bookShelf = bookShelf;
        this.index = 0;
    }

    @Override
    public boolean hastNext() {
        return index < bookShelf.getLength();
    }

    @Override
    public Object next() {
        return bookShelf.getBookAt(index++);
    }
}

// 主类
public class Main {
    public static void main(String[] args) {
        BookShelf bookShelf = new BookShelf(4);
        bookShelf.appendBook(new Book("三国演义"));
        bookShelf.appendBook(new Book("水浒传"));
        bookShelf.appendBook(new Book("西游记"));
        bookShelf.appendBook(new Book("红楼梦"));
        Iterator iterator = bookShelf.iterator();
        while (iterator.hastNext()) {
            Book book = (Book) iterator.next();
            System.out.println(book.getName());
        }
    }
}

类图

总结一下示例代码优雅之处

• Iterator模式在集合外引入迭代器这个角色，将集合的遍历与集合本身进行了分离，循环不依赖BookShelf的实现
• 面向接口编程，返回Iterator，而不是具体的BookShelfIterator，屏蔽具体实现细节，弱化了类之间的耦合。

类扮演的角色

• Iterator 迭代器，其中BookShelfIterator是实现它的具体迭代器
• Aggregate 集合，负责定义创建迭代器Iterator的api，BookShelf是具体的集合类，负责创建具体的迭代器BookShelfIterator

注：

next方法并非返回下一个元素，而是返回当前元素，并将指标（游标）指向下一个，其实合适的命名应该为 returnCurrentElementAndAdvanceToNextPosition

扩展

这里只是个简单的Iterator程序，从前向后迭代，还可以引申出从后向前迭代，两者兼具（previous+next），跳跃式等等