四则运算

import java.util.Random;
public class activity_main{
private static String[] operation = {"+","-","*","/"};
private static Random random = new Random();
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println("*******四则运算：*******");
for(int i = 0; i < 20; i++) {
String str = "";
for (int j = 0; j < 4; j++) {//循环操作  
         int num = random.nextInt(100);
         int index = random.nextInt(4);
         str += num;
         str += operation[index];
        }
        str += random.nextInt(100);
        System.out.println("("+(i+1)+")  "+str);
}     
}