迭代器模式

      迭代器（Iterator）：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。

类图：

代码：

publicinterface Iterator {

    Object first();

    boolean next();

    Object isDone();

    int currentItem();

}

publicabstractclass Aggregate {

    public List<String> messages=new ArrayList<String>();

    publicintindex=0;

    publicabstract Iterator CreateIterator();

}

publicclass ConcreteIterator implements Iterator {

    protected Aggregate aggregate;

    public ConcreteIterator(Aggregate aggregate){

       this.aggregate=aggregate;

    }

    publicint currentItem() {

       returnthis.aggregate.index;

    }

    public Object first() {

       if(this.aggregate.messages.size()>0){

           returnthis.aggregate.messages.get(0);

       }else{

           returnnull;        

       }

    }

    public Object isDone() {

       if(this.aggregate.index<this.aggregate.messages.size()){

           returnthis.aggregate.messages.get(this.aggregate.index++);

       }else{

           returnnull;        

       }

    }

    publicboolean next() {

       if(this.aggregate.index<this.aggregate.messages.size()){

           returntrue;

       }else{

           returnfalse;

       }

    }

}

publicclass ConcreteAggregate extends Aggregate {

    public Iterator CreateIterator() {

       returnnew ConcreteIterator(this);

    }

}

publicstaticvoid main(String[] args) {

       Aggregate aggregate=new ConcreteAggregate();

       List<String> msg=new ArrayList<String>();

       msg.add("abc");

       msg.add("def");

       msg.add("fhi");

       msg.add("jkl");

       aggregate.messages=msg;

      

       Iterator iterator=aggregate.CreateIterator();

       System.out.println(iterator.first());

       while(iterator.next()){

           System.out.println();

           System.out.println(iterator.currentItem()+"_"+iterator.isDone());

       }

      

    }

小结：

      此模式非常常用，Java和C#语言本身已经实现了迭代器模式（Foreach）。我们只需要了解他就可以了！！