presto-main/src/main/java/io/prestosql/operator/aggregation/AggregationImplementation.java/AggregationImplementation/findAggregationStateParamType
class AggregationImplementation:        public static Class<?> findAggregationStateParamType(Method inputFunction)
        {
            return inputFunction.getParameterTypes()[findAggregationStateParamId(inputFunction)];
        }