10.2 链接到外部类

内部类自动拥有对外围类所有成员的访问权

interface Selector{
    boolean end();
    Object current();
    void next();
}
public class Sequence {
    private  Object[] items;
    private int next = 0;
    public Sequence(int size){
        items = new Object[size];
    }
    public void add(Object x){
        if (next < items.length){
            items[next++] = x;
        }
    }
    private  class SequenceSelector implements Selector{
        private int i = 0;
        @Override
        public boolean end() {
            return i==items.length;
        }
        @Override
        public Object current() {
            return items[i];
        }
        @Override
        public void next() {
            if (i<items.length){
                i++;
            }
        }
    }
    public Selector selector(){
        return new SequenceSelector();
    }
    public static void main(String[] args) {
        Sequence s = new Sequence(5);
       for (int i = 0; i<5 ; i++){
           s.add(Integer.toString(i));
       }
       Selector selector = s.selector();
       while (!selector.end()){
           System.out.println(selector.current()+" ");
           selector.next();
       }
    }
}
输出： 0 1 2 3 4