The Apache StandardDeviation class can be used for computing both values: "Standard Deviation" and "Population Standard deviation".
For computing the second value initialize it with
StandardDeviation sd = new StandardDeviation(false);
Example:
double[] v = {1.0, 2.0, 3.0, 4.0, 5.0};
StandardDeviation sd = new StandardDeviation(false);
sd.evaluate(v);
// returns 1.414
StandardDeviation sd2 = new StandardDeviation();
sd2.evaluate(v);
// returns 1.581