calculating standard deviation in java

public class DataSet {

    private final List<Double> values = new ArrayList<Double>();


    public void add(final double value) {
        values.add(value);
    }

    public double getSum() {
        double sum = 0;
        for(double value : values)
            sum += value;
    }

    public double getAverage() {
        if (values.size() == 0)
            return 0;

        return getSum() / values.size();
    }

    public double getPopulationStandardDeviation() {
        final double average = getAverage();
        double sum = 0;

        for(double value : values) {
            final double v = (value - average);
            sum += v * v;
        }

        return sum;
    }

    public double getStandardDeviation() {
        if (values.size() == 0)
            return 0;

        // TODO: You probably want to divide by N-1 here rather than N (where N is values.size()) if values are a random set from
        // a larger population
        return Math.sqrt(getPopulationStandardDeviation() / values.size());
    }
}