这段程序为什么无法编译？

import java.util.Scanner;
    class couculator
    {
        public static void main(String[] args)
        {
            int num=0;
            Scanner a = new Scanner(System.in);
            System.out.println("请输入第一个数字。");
            int s = a.nextInt();
           // Scanner b = new Scanner(System.in);//有用吗？
            System.out.println("请输入加减乘除符号。(1 = +,2 = -,3 = *,4 = /.)");
            int d = a.nextInt();
            System.out.println("请输入第二个数字。");
           // Scanner c = new Scanner(System.in);//有用吗？
            int q = a.nextInt();
            a.close();
            switch (d)
            {
            case 1:
                num = s + q;
                break;
            case 2:
                if (s < q)
                {
                    num = s - q;
                    break;
                }
            case 3:
                num = s * q;
                break;
            case 4:
                if (s > q)
                {
                    num = s / q;
                    break;
                }
           
            }
            System.out.println(num);
        }
}