@Override
    protected StateFactory<AggregatorState> init(ExpressionExecutor[] attributeExpressionExecutors,
                                                 ProcessingMode processingMode,
                                                 boolean outputExpectsExpiredEvents, ConfigReader configReader,
                                                 SiddhiQueryContext siddhiQueryContext) {
        if (attributeExpressionExecutors.length != 1) {
            throw new OperationNotSupportedException("stdDev aggregator has to have exactly 1 parameter, currently " +
                    attributeExpressionExecutors.length
                    + " parameters provided");
        }
        returnType = Attribute.Type.DOUBLE;
        Attribute.Type type = attributeExpressionExecutors[0].getReturnType();
        return () -> {
            switch (type) {
                case INT:
                    return new StdDevAttributeAggregatorStateInt();
                case LONG:
                    return new StdDevAttributeAggregatorStateLong();
                case FLOAT:
                    return new StdDevAttributeAggregatorStateFloat();
                case DOUBLE:
                    return new StdDevAttributeAggregatorStateDouble();
                default:
                    throw new OperationNotSupportedException("stdDev not supported for " + returnType);
            }
        };

    }
The initialization method for FunctionExecutor

@param attributeExpressionExecutors are the executors of each attributes in the function
@param processingMode               query processing mode
@param outputExpectsExpiredEvents   is expired events sent as output
@param configReader                 this hold the {@link StdDevAttributeAggregatorExecutor} configuration reader.
@param siddhiQueryContext           Siddhi query runtime context
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/attribute/aggregator/StdDevAttributeAggregatorExecutor.java