A method for obtaining a value via a rule engine implemented by a collection object associated with an object-oriented application. A request for the value includes a key, is received from the application, and is directed to a method of the collection object. The collection object is capable of storing the key and associated data, and providing the data in response to receiving the request. An overriding of the method of the collection object replaces the provision of the data by the collection object with a processing of the request by a rule engine external to the application. The rule is identified in a rule definition file external to the application based on an association between the rule and the key. An algorithm associated with the rule and included in the rule definition file is executed to provide the requested value.