A Construct for Effectively Implementing Semantic Associations

Associations are a key concept in object-oriented modeling. Implementing purely semantic associations with direct containment can lead to reduced cohesion and increased coupling as well as difficulties with referential integrity. Implementing semantic associations using the constructs shown in this paper will lead to domain objects that are more flexible and reusable. Adding container classes for domain objects and their association objects leads to a high level of traceability between the conceptual model and it’s implementation. 1 THE IMPORTANCE OF ASSOCIATIONS Object-oriented (OO) techniques were specifically developed in order to reduce domain complexity and communication between domain experts and systems developers can understand. OO models the user’s perspective of the system in a semantically meaningful manner that follows human conceptualization. “Object-oriented systems allow the real world to be represented more directly than do conventional ones” [Gottlob96]. Associations play a key role in object-oriented domain modeling. They capture the nature of the domain by depicting relationships among objects. An association is a group of links between instances with common semantics and structure, the key point being an association involves a semantic relationship between two or more classes. Associations can be either static or dynamic in nature. Dynamic interactions depict sending messages or signals between classes. Static structures such as inheritance, aggregation, and composition are also forms of associations. A third category of associations is a purely semantic association. Semantic constructs such as roles and relationships comprise this group. Associations that support semantic constructs are easily modeled in UML, but inconsistently implemented. Semantic associations enrich the understanding of the system by capturing the nature of the domain and, in certain domain constructs, play a key role in the understanding of the system’s conceptual model. A CONSTRUCT FOR EFFECTIVELY IMPLEMENTING SEMANTIC ASSOCIATIONS 102 JOURNAL OF OBJECT TECHNOLOGY VOL. 2, NO. 3 Purely semantic associations are the focus of this article. Dynamic associations and static associations are well understood and implementation is handled through native constructs in nearly all object-oriented languages. Semantic associations are implemented inconsistently, if at all, in development languages which necessarily puts their implementation in the hands of the developer. We will focus on implementing semantic associations and how container classes play a key role in associations. 2 STRUCTURE OF ASSOCIATIONS General Association Concepts An association is by default bidirectional meaning that it can be read from either end with significance. An example is an "is_married_to" association between a class Man and a class Woman. Adding an arrowhead at one end specifies that the association is only navigable in one direction as in Fig. 1. Given a Radar object, the associated Beam objects can be identified, but a Beam object has no reference to the Radar object emitting it.