Characterising aggregations with existence dependency.

Abstract: The concept of aggregation is considered as one of the basic principles in object-oriented analysis. There is however no standard definition of this concept and each object-oriented analysis method has its own definition of aggregation. The aim of this paper is not to discuss the different types of aggregation that exist. However, having assessed the complexity of the concept, we will illustrate how a basic set of formal concepts is sufficient to define of the structural and behavioral aspects of different existing flavours of aggregation. If a development method wants to offer a rich concept such as aggregation, it can define the semantics of the desired flavour of the aggregation using these core formal concepts. Analysts then have the choice to use the aggregation defined by the method or to fall back on the core concepts if a different flavour of aggregation is needed to model the situation at hand.Principles; Model;

Existence dependency: conceptual modelling by contract.

In Object Orientation, the Generalisation/Specialisation hierarchy and the Whole/Part relationship are prevalent classification schemes for object types. This paper presents a new classification scheme for object types, called 'existence dependency'. Existence dependency captures some of the interesting semantics that are usually associated with the concept of aggregation (Part of relation). In fact, the semantics of existence dependency are hidden in the semantics of the Entity Relationship model, but have never been explicitly named. We will demonstrate how the explicit classification of object types according to the existence dependency relation allows for formal and automatic consistency between static and dynamic aspects of object types that goes far beyond mere syntactical consistency.Classification; Dependency; Model;