@Override
        public Map<String, Object> snapshot() {
            Map<String, Object> state = new HashMap<>();
            state.put("Sum", sum);
            state.put("Mean", mean);
            state.put("stdDeviation", stdDeviation);
            state.put("Count", count);
            return state;
        }
Standard deviation abstrct aggregator for Double values
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/attribute/aggregator/StdDevAttributeAggregatorExecutor.java