Calculating the average incrementally

Avg.java

//package ;

/**
 * @see  <a href="https://ubuntuincident.wordpress.com/2012/04/25/calculating-the-average-incrementally/">Calculating the average incrementally</a>
 */
public class Avg {

    private double value = Double.NaN;

    private long count;

    public Avg() {
        // no-args
    }

    public double value() {
        return value;
    }

    public long count() {
        return count;
    }

    public void add(double number) {
        if (count++ == 0) {
            value = number;
        } else {
            value += (number - value) / count;
        }
    }

    public void reset() {
        value = Double.NaN;
        count = 0;
    }

    public static double calc(int[] numbers) {
        if (numbers.length == 0) {
            return Double.NaN;
        }
        double avg = numbers[0];
        for (int i = 1, len = numbers.length; i < len; i++) {
            avg += (numbers[i] - avg) / (i + 1);
        }
        return avg;
    }

    public static void main(String[] args) {
        int[] numbers = { 5, -3, 7 };
        System.out.println(Avg.calc(numbers));

        Avg avg = new Avg();
        for (int i = 0; i < numbers.length; i++) {
            avg.add(numbers[i]);
        }
        System.out.println(avg.value());
    }

}