public class Queue<Item> implements Iterable<Item> {

    private Node first;
    private Node last;
    private int N;
    private class Node{
        Item item;
        Node next;
    }
    public int size(){
        return N;
    }

    public boolean isEmpty(){
        return first == null
 
;
    }

    public void enQueue(Item item){
        Node oldLast = last;
        last = new Node();
        last.item = item;
        last.next = null;
        if(isEmpty()) first = last;
        else oldLast.next = last;
        N++;
    }

    public Item deQueue(){
        Item item = first.item;
        first = first.next;
        if
 
(isEmpty()) last = null;
        N--;
        return item;
    }


    @Override
    public Iterator<Item> iterator() {
        return new ListIterator();
    }

    public class ListIterator implements Iterator<Item>{
        private Node current = first;

        @Override
        public
 
 boolean hasNext() {
            return current != null;
        }

        @Override
        public Item next() {
            Item item = current.item;
            current = current.next;
            return item;
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }

    }
}

這裡要注意特殊情況，當入佇列的時候，如果為空，那要first = last，出佇列的時候，如果為空，要設定last為空。