Iterator Pattern

Traverse a container and access the container's elements. The iterator pattern decouples algorithm from containers.

主要是call next 的时候，iterator如果现在不valid，需要往前走看是不是valid，但这个时候需要cache当前valid的数据，因为iterator是只能往前走。。。

Goal

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. 只能往前走，不能回头，这是思考iterator的关键！ 常见trick；next要return什么，把iterator put one step ahead!

Interface Iterator<E> {
    boolean hasNext();  //check if we still have some elements in collection, don't proceed if empty
    E next();           //proceed to next and return
    void remove();      //remove current element, usually you don't need to implement this
}

Another implementation

    boolean moveNext(); //proceed to next position
    E current();  //return element at current position

Flexible for what ?

1. provide a view not a copy, not changing internal data structure.

2. different traversing algorithm can be done on same collection through implementing different iterators without changing internal data structure.