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);
    }
}

a,b,c类型不对，例：a = a.nextInt();第一个a是scanner，第二个a是int。不能转换类型

package wt1;

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 anum = a.nextInt();
         Scanner b = new Scanner(System.in);
         System.out.println("请输入加减乘除符号。(1 = +,2 = -,3 = *,4 = /.)");
         int bnum = b.nextInt();
         System.out.println("请输入第二个数字。");
         Scanner c = new Scanner(System.in);
         int cnum = c.nextInt();
         switch (bnum)
         {
         case 1:
             num = anum + cnum;
             break;
         case 2:
                 num = anum - cnum;
                 break;
         case 3:
             num = anum * cnum;
             break;
         case 4:
             if (anum> cnum)
             {
                 num = anum / cnum;
                 break;
             }
        
         }
         System.out.println("输出结果。");
         System.out.println(num);
     }
 }

已经解决。