超简单四则运算

package test;

import static org.junit.Assert.*;

import java.math.BigDecimal;
import java.util.Random;
import java.util.Scanner;

import org.junit.Before;
import org.junit.Test;

public class CalculatorTest {
    public static void main(String args[]){
        Random random=null;
        System.out.print("请输入需要生成的题目数量：");
        Scanner d = new Scanner(System.in);
        int n=d.nextInt();
        while(n>=0){
            random=new Random();
            int a=random.nextInt(100);
            int b=random.nextInt(100);
            int i=a%4;
            String c=yunSuanFu(i);
            String result=calculator(a,b,i);
            System.out.print(a+c+b+"=");
            d = new Scanner(System.in);
            String answer=d.next();
            if(answer.equals(result)){
                System.out.println("回答正确");
            }else{
                System.out.println("回答错误");
            }
            n--;
        }
    }
    
    private static String calculator(int a1, int b1, int i) {
        int result = 0;
        switch(i){
        case 0:result=a1+b1;break;
        case 1:result=a1-b1;break;
        case 2:result=a1*b1;break;
        case 3:result=a1/b1;break;
        }
        return String.valueOf(result);
        
    }

    public static String yunSuanFu(int i){
        switch(i){
        case 0:return "+";
        case 1:return "-";
        case 2:return "*";
        case 3:return "/";
        }
        return null;
    }
}

请输入需要生成的题目数量：6
46*87=1
回答错误
9-68=-59
回答正确
24+61=85
回答正确
7/3=2
回答正确
23/98=0
回答正确
77-0=77
回答正确
1-24=-23
回答正确