我最近想写个计算机，然后在写的过程中， 不知道怎么定义两个Scanner并使用。请指教。

import java.util.Scanner;
class couculator
 {
     public static void main(String[] args)
     {
         int num;
         Scanner a = new Scanner(System.in);
         System.out.println("请输入第一个数字。");
         a = a.nextInt();
         Scanner b = new Scanner(System.in);
         System.out.println("请输入加减乘除符号。(1 = +,2 = -,3 = *,4 = /.)");
         b = b.nextInt();
         System.out.println("请输入第二个数字。");
         Scanner c = new Scanner(System.in);
         c = c.nextInt();
         switch (b)
         {
         case 1:
             num = a + c;
             break;
         case 2:
             if (a < c)
             {
                 num = a - c;
                 break;
             }
         case 3:
             num = a * c;
             break;
         case 4:
             if (a > c)
             {
                 num = a / c;
                 break;
             }
        
         }
         System.out.println(num);
     }
 }



这个程序是错误的，scanner参照这个用，代码摘自网友