public Object execute(ComplexEvent event) {
        if (attributeSize > 1) {
            return processAttributeArray(event);
        } else if (attributeSize == 1) {
            return processAttribute(event);
        } else {
            return processNoAttribute(event);
        }
    }
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