数据结构----队列

public class Queue<Item> implements Iterable<Item> { //实现迭代器接口
    //持有一个头指针一个尾指针，方便进队和出队。
    private Node<Item> first，last; 
    private int n;  //保存队列中的元素个数       
    //节点类
    private static class Node<Item> {           
        private Item item;
        private Node<Item> next;
    }
    //构造器
    public Queue() {first = null;last  = null;n = 0;}
    //是否为空    
    public boolean isEmpty() {  return first == null;  }
    //当前元素个数
    public int size() {return n;}
    //入队列
    public void enqueue(Item item) { 
        Node<Item> oldlast = last;
        last = new Node<Item>();
        last.item = item;
        last.next = null;
        if (isEmpty()) first = last;
        else           oldlast.next = last;
        n++;
    }
    //出队列
    public Item dequeue() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        Item item = first.item;
        first = first.next;
        n--;
        if (isEmpty()) last = null;  
        return item;
    }

    //创建迭代器
    public Iterator<Item> iterator()  {
        return new ListIterator<Item>(first);  
    }
    //实现迭代器
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;
        public ListIterator(Node<Item> first) {
            current = first;
        }
        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }
        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }
}