@Override
        public int compareTo(final Number number) {
            // TODO: NumberHelper should provide a compareTo() implementation
            return Double.valueOf(this.doubleValue()).compareTo(number.doubleValue());
        }
Advances the starts until a non-null value is found or simply returns {@code null}. In this way, an all
{@code null} stream will result in {@code null}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MeanGlobalStep.java