private Object processNoAttribute(ComplexEvent event) {
        switch (event.getType()) {
            case CURRENT:
                return processAdd(null);
            case EXPIRED:
                return processRemove(null);
            case RESET:
                return processReset();
        }
        return null;
    }
Abstract parent class for attribute aggregators. Attribute aggregators are used to perform aggregate operations
such as count, average, etc.

@param <S> current state for the Attribute Executor
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/attribute/aggregator/AttributeAggregatorExecutor.java