java 基于链表的队列

public class QueueLinked<T> {

	private class Node<T> {
		T data;
		Node<T> next;
		Node<T> pre;
		
		public Node(T data) {
			this.data = data;
		}
	}
	
	Node<T> head = null;
	Node<T> tail = null;
	
	public void add(T data) {
		Node<T> node = new Node<T>(data);
		if(head==null) {
			head = node;
			tail = node;
		}else {
			tail.next = node;
			node.pre = tail;
			tail = node;
		}
	}
	
	public T popTail() {
		if(tail==null) {
			return null;
		}else {
			Node<T> node = tail;
			tail = node.pre;
			if(tail!=null) {				
				tail.next = null;
			}
			node.pre = null;
		    return node.data;
		}
	}
	
	public T popHead() {
		if(head==null) {
			return null;
		}else {
			Node<T> node = head;
			head = node.next;
			if(head!=null) {				
				head.pre = null;
			}
			node.next = null;
			return node.data;
		}
	}
	
	public int size() {
		int count = 0;
		Node<T> node = head;
		while(node!=null) {
			count++;
			node = node.next;
		}
		return count;
	}
	
	public static void main(String[] args) {
		QueueLinked<Message> queue = new QueueLinked<Message>();
		
		Message message1 = new Message();
		message1.setMethod("信息1");
		queue.add(message1);
		
		Message message2 = new Message();
		message2.setMethod("信息2");
		queue.add(message2);
		
		System.out.println(queue.size());
		
		Message message = null;
		
		while((message=queue.popTail())!=null) {
			System.out.println(message.getMethod());
		}
		
	}
}