定义：提供一个接口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实现

迭代器模式可以隐藏聚合对象的实现，减少聚合对象的工作

使用迭代器访问元素不保证顺序，除非聚合对象有特别说明保证其顺序，所以千万别假定迭代器遍历出来的元素是有顺序的。

迭代器有内部迭代器和外部迭代器之分，内部迭代器负责迭代，客户不能控制它的迭代算法，只能将操作传递给它，而外部迭代器由客户控制迭代，比如通过next()方法获取下一个元素，所以外部迭代器更具有弹性。

实现：

1.看看Java的聚合对象，如ArrayList

public Iterator
    
      iterator() {        return new Itr();    }
    

private class Itr implements Iterator
    
      {        int cursor;       // index of next element to return        int lastRet = -1; // index of last element returned; -1 if no such        int expectedModCount = modCount;        public boolean hasNext() {            return cursor != size;        }        @SuppressWarnings("unchecked")        public E next() {            checkForComodification();            int i = cursor;            if (i >= size)                throw new NoSuchElementException();            Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length)                throw new ConcurrentModificationException();            cursor = i + 1;            return (E) elementData[lastRet = i];        }        public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                ArrayList.this.remove(lastRet);                cursor = lastRet;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }        @Override        @SuppressWarnings("unchecked")        public void forEachRemaining(Consumer
      consumer) {            Objects.requireNonNull(consumer);            final int size = ArrayList.this.size;            int i = cursor;            if (i >= size) {                return;            }            final Object[] elementData = ArrayList.this.elementData;            if (i >= elementData.length) {                throw new ConcurrentModificationException();            }            while (i != size && modCount == expectedModCount) {                consumer.accept((E) elementData[i++]);            }            // update once at end of iteration to reduce heap write traffic            cursor = i;            lastRet = i - 1;            checkForComodification();        }        final void checkForComodification() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();        }    }
    

这就是外部迭代器，客户获得Iterator，自己控制迭代步数和操作

2.自己实现一个迭代器

一个汽车4S店汽车清单（数组实现的），使用迭代器方式为客户提供遍历的方式

package com.jv.designpattern.iterator;public class Car {	private String name;	private Long price;	public String getName() {		return name;	}	public void setName(String name) {		this.name = name;	}	public Long getPrice() {		return price;	}	public void setPrice(Long price) {		this.price = price;	}	}

package com.jv.designpattern.iterator;import java.util.Iterator;public class CarMenu implements Iterable{	private final static int MAX_LENGTH = 10;	private int position = 0;	Car[] cars ;	public CarMenu(){		cars = new Car[MAX_LENGTH];	}	public void addCar(Car car) {		cars[position++] = car;	}	@Override	public Iterator iterator() {		return new CarMenuIterator(cars);	}	}

package com.jv.designpattern.iterator;import java.util.Iterator;public class CarMenuIterator implements Iterator{		Car[] cars;	private int position = 0;	public CarMenuIterator(Car[] cars) {		this.cars = cars;	}		@Override	public boolean hasNext() {				if(0<=position && position

CarMenu实现Iterable接口得到更好的弹性，方便被客户使用

CarMenuIterator实现Iterator接口，重写HashNext和next方法，和其他Java api保持一样的迭代方式，减少客户的实现复杂度