The Java Embedded Aggregator Class VarAggregate

 
    public static class VarAggregate implements Aggregate<Number>
    {
        public void initialize(Number val) 
        {
            double v = val.doubleValue();
            sum = v;
            sum2 = v*v;
            count = 1;
        }
 
        public void accumulate(Number val) 
        {
            double v = val.doubleValue();
            sum += v;
            sum2 += v*v;
            count += 1;
        }
         
        public Object result() 
        {
            return new Double((sum2 - sum*sum/count)/count);
        }
 
        public void merge(Aggregate<Number> other) 
        {
            VarAggregate otherVar = (VarAggregate)other;
            sum += otherVar.sum;
            sum2 += otherVar.sum2;
            count += otherVar.count;
        }
 
        double sum;
        double sum2;
        long count;
    }