运行错误

package boy;
public class Test {
    public static void main(String[] args) {
        SuShu(1, 1000);
    }

    public static void SuShu(int a, int b) {//a到b之间的素数个数
        int count = 0;
        for (int i = a; i <= b; i++) {
            if (isPrime(i)) {
                count++;
                System.out.println(i + " ");
                if (count % 10 == 0) {
                    System.out.println();
                }
            }
        }
        System.out.println();
        System.out.println("在" + a + "和" + b + "之间有" + count + "个素数");
    }

    public static boolean isPrime(int a) {//判断素数
        boolean flag = true;
        if (a == 1)
            flag = false;
        else {
            for (int i = 2; i <= Math.sqrt(a); i++) {
                if (a % i == 0) {
                    flag = false;
                    break;
                } else if (a == 1) {
                    flag = false;
                    break;
                } else {
                    flag = true;
                }
            }
        }
        return flag;
    }
}