二重循环，输出500内的所有质数

 int count = 0;
        int count2 = 0;
        for(int i = 2;i<=500;i++){
            //证明
            count2++;
            boolean is = true;
            for(int j = 2;j<=Math.sqrt(i);j++){
                  count2++;
                if(i%j==0){
                    is = false;
                    break;
                }
            }
            if(is){
                count++;
                System.out.println(i);
            }
        }
        System.out.println(count+"个质数，循环执行了"+count2+"次");