新手求解.1~30的阶乘求和 ，用方法编写 哪错了？解决办法

新手求解..1~30的阶乘求和 ，用方法编写 哪错了？
double sum = 1.00;
for(int i = 1; i <= 30; i++){
int num = F(a);
sum += num;
}
System.out.println("sum = " + sum);
}
public static double F(int num){
for(int i = 1; i <= num; i++){
int a = 1;
a = a * i;
}
return a;

[解决办法]
求阶乘要用BigInteger，30！用long早就越界了

Java code

import java.math.BigInteger;public class Test1 {    public static void main(String[] args) {        BigInteger sum = new BigInteger("0");        for (int i = 1; i <= 30; i++) {            sum = sum.add(getResult(i));        }        System.out.println("sum = " + sum);    }    public static BigInteger getResult(int num) {        BigInteger result = new BigInteger("1");;        for (int i = 1; i <= num; i++) {            result = result.multiply(new BigInteger(String.valueOf(i)));        }        return result;    }}