@Override
    public Object processAdd(Object data, AggregatorState state) {
        if (data == null) {
            return state.currentValue();
        }
        return state.processAdd(data);
    }
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 SumAttributeAggregatorExecutor} configuration reader.
@param siddhiQueryContext           Siddhi query runtime context
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/attribute/aggregator/SumAttributeAggregatorExecutor.java