迭代器模式(Iterator)

情景：乘车买票，不管你是谁！

定义：提供一种方法顺序访问一个聚合对象中各个元素，而又不暴露该对象的内部表示。

结构图：

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

interface Aggreagate {//创建迭代器Iterator createIterator();}public class ConcreteIterator implements Iterator{private ConcreteAggregate ca; //定义一个具体聚合对象private int currentIndex = 0;public ConcreteIterator(ConcreteAggregate ca) {//初始化将具体的聚合对象传入this.ca = ca;}@Overridepublic boolean hasNext() {if (currentIndex < ca.getList().size()) {return true;}return false;}@Overridepublic Object next() {Object o = ca.getList().get(currentIndex);currentIndex++;return o;}}public class ConcreteAggregate implements Aggreagate{//用list存放聚合对象private List<Object> list = new ArrayList<Object>();public List<Object> getList() {return list;}public void setList(List<Object> list) {this.list = list;}@Overridepublic Iterator createIterator() {return new ConcreteIterator(this);}}//反向遍历public class ConcreteIteratorDesc implements Iterator{private ConcreteAggregate ca;private int current = 0;public ConcreteIteratorDesc(ConcreteAggregate a) {this.ca = a;current = ca.getList().size() - 1;}@Overridepublic boolean hasNext() {if (current >= 0) {return true;}return false;}@Overridepublic Object next() {Object o = ca.getList().get(current);current--;return o;}}public class Client {public static void main(String[] args) {ConcreteAggregate a = new ConcreteAggregate();a.getList().add("1");a.getList().add("2");a.getList().add("3");//Iterator iterator = new ConcreteIteratorDesc(a);Iterator iterator = new ConcreteIterator(a);while (iterator.hasNext()) {Object o = iterator.next();System.out.println(o + " ");}}}

     总的来说，迭代器模式就是分离了集合对象的遍历行为，抽象出一个迭代器类来负责，这样不仅可以做到不暴露集合内部结构，又可以让外部代码透明地访问集合内部的数据。