老师，为什么这个把tail给head后，head还是空啊

​public class LinkedListQueue<E> implements Queue<E> {
    @Override
    public int getSize() {
        return size;
    }

    @Override
    public void enQueue(E e) {
        if (tail == null) {
            tail = new Node(e);
            head = tail;
        } else {
            tail.next = new Node(e);
            tail = tail.next;
        }
        size++;
    }

    @Override
    public E deQueue() {
        if (size == 0) {
            throw new IllegalArgumentException("deQueue failed.The queue is empty");
        }
        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if (head == null) {
            tail = null;
        }
        size--;
        return retNode.e;
    }

    @Override
    public E getHead() {
        if (size == 0) {
            throw new IllegalArgumentException("getHead failed.The queue is null");
        }
        return head.e;
    }

    @Override
    public E getTail() {
        return tail.e;
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("队首");
        Node cur = head;
        while (head != null) {
            builder.append("<-" + head.e);
            head = head.next;
        }
        builder.append("<-队尾");
        return builder.toString();
    }


    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    private class Node {
        public E e;
        public Node next;

        public Node() {

        }

        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }

        public Node(E e) {
            this(e, null);
        }

        @Override
        public String toString() {
            return this.e.toString();
        }
    }

    private int size;
    private Node head;
    private Node tail;

    public LinkedListQueue() {

    }

}