Dissolving Ambiguities in UML Whole-Part Relationships

The Unified Modeling Language is the software industry standard for OO graphical modeling languages. However, it does not adequately encompass the meronymic (whole-part) relationships. This paper provides an extension to the UML definition to include the excluded meronymic relationships in a formalized notation and remove the ambiguity of the aggregation definition in UML. Common design patterns are then translated into the ‘extended’ UML to show its clarity. The translations are analyzed to determine the complexity added to UML using the formal notation. This paper concludes that six historical meronymic relationships are actually design patterns based upon the formalized primary and secondary characteristics of whole-part relationships. Introduction Meronymic (whole-part) relationships are an essential part to Object-Orientated Programming and Design because it is one of the primary methods that human beings attempt to relate objects in the world. Correctly depicting these relationships in a modeling language, following the natural cognitive process further facilitates the design of software by assisting the selection of design patterns. Uniformed Modeling Language (UML) is one of the most common tools for the development of software. UML provides graphical representation of ObjectOrientated designs (OOD) and has become the de facto standard modeling language for industry. It has achieved this status primarily from its pragmatic nature of adopting the best modeling ideas from other graphical OO modeling languages [1]. UML’s graphical nature communicates the design information in a concise visual method but not necessarily with precision. This imprecision is due to the informality of its definitions. [2] This shows primarily in its definition and representation in the relationships known as aggregation and association. Aggregation and association are two of the three primary ways that a class can be related to another in UML. Aggregation can be depicted in two different ways. Strong aggregation is similar to the meronymic composite relationship, which is one of the six types of meronymic relationships defined in literature. [3] Weak aggregation does not correspond to any of the other meronymic relationships. UML does not adequately express the cognitive relationships, which can lead to miscommunication of the system design and defects in the product. Design patterns use modeling languages such as UML to express their solution to a design problem. Design patterns are an extension of the reuse concept in the architecture design of software. They express the structure of a solution shown in a graphical modeling and a description of the pattern’s properties as shown in Figure 1. [4] [5] Design patterns have a pragmatic background and their descriptions have been largely informal. [6] As a result of their impreciseness in definition, there is some ambiguity on which design pattern to implement, especially coupled with the whole-part relationship definitions in UML. Figure 1: Example of a Generic (Façade) Design Pattern Client Class Client Class