课上动手东脑问题

import java.util.Random;
import java.util.Scanner;
public class RandomExample
{
public static void main(String[] args)
{
int count=0;

    for(int i=0;i<30;i++)
    {//生成随机两个随机数：
        Random random = new Random();
        Scanner scanner = new Scanner(System.in);
        int number1 = random.nextInt(99) + 1;
        int number2 = random.nextInt(99) + 1;
        int operator = random.nextInt(4);
        String opSymbol = "";
        int result = 0;
        boolean valid = true;

//进行四则运算：
switch (operator) {
case 0:
opSymbol = "+";
result = number1 + number2;
break;
case 1:
opSymbol = "-";
result = number1 - number2;
break;
case 2:
opSymbol = "*";
result = number1 * number2;
break;
case 3:
opSymbol = "/";
result = number1 / number2;
break;
}

//输入结果并检验
if (valid)
{
System.out.printf("请计算：%d %s %d = ?%n", number1, opSymbol, number2);
System.out.print("请输入你的答案：");
int userAnswer = scanner.nextInt();

            if (userAnswer == result) {
                System.out.println("回答正确！");
            } else {
                System.out.printf("回答错误，正确答案是：%d%n", result);
                count++;
            }
        }
    }
    System.out.printf("错误的题数为：%d\n",count);
    System.out.printf("正确率是:%0.2f%%\n",(30-count)/30.0*100) ;
}

}