To calculate the mean, loop through the list/array of numbers, keeping track of the partial sums and the length. Then return the sum/length.
double sum = 0.0;
int length = 0;
for( double number : numbers ) {
sum += number;
length++;
}
return sum/length;
Variance is calculated similarly. Standard deviation is simply the square root of the variance:
double stddev = Math.sqrt( variance );