public static <I> Aggregator<I, Double> doubleAvg(String attributePath) {
        return new DoubleAverageAggregator<I>(attributePath);
    }
public static < I > Aggregator < I , Double > doubleAvg ( String attributePath ) { return new DoubleAverageAggregator < I > ( attributePath ) ; }
@param <I> type of the input object.
@return an aggregator that calculates the average of the input values extracted from the given attributePath.
Does NOT accept null input values nor null extracted values.
Accepts only Double input values (primitive and boxed).
Aggregation result type is Double.