数据结构(三)队列(一)链表实现

public class MyQueue<Item> implements Iterable<Item> {    private Node first;    private Node last;    private int N = 0;    private class Node{        Item item;        Node nextNode;    }    public boolean isEmpty(){return first==null;}    public int size(){return N;}    //入队列    public void endequeue(Item item){        Node old = last;        last = new Node();        last.item = item;        last.nextNode = null;        if(isEmpty()){            first=last;        }else {            old.nextNode = last;        }        N++;    }    //出队列    public Item dequeue(){        Item item = first.item;        first = first.nextNode;        if (isEmpty()){last = null;}        N--;        return item;    }    @Override    public Iterator<Item> iterator() {        return null;    }    private class MyIterator<Item> implements Iterator<Item>{        private int i = N;        @Override        public boolean hasNext() {            return N>0;        }        @Override        public Item next() {            return (Item)first.nextNode.item;        }        @Override        public void remove() {        }    }}