使用两个栈模拟一个队列

 1     public class Queue<E>{
 2         private Stack<E> s1 = null; //s1作为插入栈
 3         private Stack<E> s2 = null; //s2作为弹出栈
 4         
 5         public Queue(){
 6             s1 = new Stack<E>();
 7             s2 = new Stack<E>(); 
 8         }
 9         //判断队列是否为空
10         public synchronized boolean isEmpty(){
11             return s1.isEmpty()&&s2.isEmpty();
12         }
13         
14         //统计队列中元素个数
15         public synchronized int size(){
16             return s1.size()+s2.size();
17         }
18         
19         //入队列
20         public synchronized void put(E data){
21             s1.push(data);
22         }
23         
24         //出队列(记得要将出完队列
25         public synchronized E pop(){
26             if(s2.isEmpty())
27                 while(!s1.isEmpty())
28                     s2.push(s1.pop());
29             return s2.pop();
30         }
31     }