java.util 
接口 Queue<E>
- 
- 类型参数:
- E- collection 中所保存元素的类型。
- 所有超级接口: 
- Collection<E>, Iterable<E> 
- 所有已知子接口: 
- BlockingDeque<E>, 
BlockingQueue<E>, 
Deque<E> 
- 所有已知实现类: 
- AbstractQueue, ArrayBlockingQueue, 
ArrayDeque, 
ConcurrentLinkedQueue, 
DelayQueue, LinkedBlockingDeque, 
LinkedBlockingQueue, 
LinkedList, 
PriorityBlockingQueue, 
PriorityQueue, SynchronousQueue
 
import java.util.ArrayDeque;
import java.util.Queue;
import java.util.Scanner;
import java.util.Stack;
public class Main {
	public static int[][] mp = new int[55][55];
	public static char[] st = new char[1005];
	public static char[] end = new char[1005];
	
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        // 涉及到大量字符串拼接时不要用String, 而用StringBuilder
        StringBuilder st = new StringBuilder();
//        st.append(cin.next());
        
//        System.out.println(st);
        
        // java中的栈
        Stack<Integer> sta = new Stack<Integer>();
        // java中的队列:queue接口有很多实现类,建议查询jdk文档
        Queue<Integer> que = new ArrayDeque<>();
        
        que.add(1);
        que.add(2);
        que.add(9);
        que.add(2);
        
        System.out.println(que.toString());
        
        System.out.println(que.peek());
        System.out.println(que.toString());
        System.out.println(que.poll());
        System.out.println(que.toString());
        
    }
}
 
/*
abcd
dddd
3
a b
b c
c d
 */