标准偏差输出为0，如何修复？

Question

我在这个程序中的标准差输出为0，我不知道我做错了什么。我被困在这几个小时，请帮忙。

public static void main(String args[])
{
    Scanner scan = new Scanner(System.in);

    System.out.println("How many numbers do you want to calculate?");
    int n = scan.nextInt();

    double a[] = new double[(int) n];       // casting n to a double
    double sum = 0.0;
    double sd = 0.0;

    System.out.println("Fill in the values for all " + n + " numbers.");
    for(int i = 0; i < a.length; i++)
    {
        a[i] = scan.nextDouble();
        sum = sum + a[i];
    }

    double average = sum/a.length;

    for(int i = 0; i < a.length; i++)
    {
           a[i] = Math.pow((a[i]-average),2);       // need help here
    }


    System.out.println("The Mean of the " + n +" numbers is " + average);
    System.out.println("The Standard Deviation of the " + n + " numbers is " + sd);
}