迭代器模式的应用

类图：

代码示例：

package iterator;

public interface Aggregate {
    public Iterator iterator();
}

package iterator;

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

package iterator;

public abstract class BookShelfs {
    
    public abstract void appendBook(Book book);
    
    public abstract Book getBookAt(int index);
    
    public abstract int getLength();
    
    
}

package iterator;
/**
 * 具体的聚合器
 */
public class BookShelf extends BookShelfs implements Aggregate {
    
    private Book[] books;
    //标记目前书架上有多少书
    private int last;
    
    public BookShelf(int maxsize){
        books = new Book[maxsize];
        last = 0;
    }
    
    @Override
    public Book getBookAt(int index){
        return books[index];
    }
    
    @Override
    public void appendBook(Book book){
        /*实际上应该有一个最大容量的判断，如果超出了数组边界，会抛出越界异常
        (就看这里是由编程者自己做处理还是让编译器抛异常，上层程序再来处理)
        如果是类的开发者，应该是会选择让上层程序来处理
        */
        books[last]=book;
        last++;
    }
    
    @Override
    public int getLength(){
        return last;
    }
    
    @Override
    public Iterator iterator() {
        //直接创建使用一个迭代器
        return new BookShelfIterator(this);
    }

}

package iterator;

import java.util.Vector;

public class BookShelf2 extends BookShelfs implements Aggregate {
    
    private Vector<Book> books;
    
    public BookShelf2(){
        books = new Vector<Book>();
    }
    
    @Override
    public void appendBook(Book book){
        books.add(book);
    }
    
    @Override
    public Book getBookAt(int index){
        return books.get(index);
    }
    
    @Override
    public int getLength() {
        return books.size();
    }
    
    
    @Override
    public Iterator iterator() {
        return new BookShelfIterator(this);
    }


}

package iterator;

public class BookShelfIterator implements Iterator {
    
    private BookShelfs bookShelf;
    //记录下当前迭代的位置
    private int index;
    
    public BookShelfIterator(BookShelfs bookShelf){
        this.bookShelf = bookShelf;
        index = 0;
    }
    
    @Override
    public boolean hasNext() {
        if(index<bookShelf.getLength()){
            return true;
        }
        return false;
    }

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

}

package iterator;

public class Book {
    private String name;
    public Book(){}
    
    public Book(String name){
        this.name = name;
    }
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

package iterator;

public class Client {
    public static void main(String[] args) {
        //数组存储书籍
        BookShelf myShelf = new BookShelf(10);
        //以一种不同的组织方式组织书的书架(Vector存放书籍)
        BookShelf2 otherShelf = new BookShelf2();
        
        myShelf.appendBook(new Book("thinking in java"));
        myShelf.appendBook(new Book("effective java"));
        myShelf.appendBook(new Book("design patterns"));
        
        otherShelf.appendBook(new Book("head first java"));
        otherShelf.appendBook(new Book("head first design pattern"));
        
        Iterator it = new BookShelfIterator(myShelf);
        while(it.hasNext()){
            Book b = (Book) it.next();
            System.out.println(b.getName());
        }
        
        it = new BookShelfIterator(otherShelf);
        while(it.hasNext()){
            Book b = (Book) it.next();
            System.out.println(b.getName());
        }
        
        
        
        
    }
}

运行结果：

thinking in java
effective java
design patterns
head first java
head first design pattern