OPEN Relationships - Compositions and Containments

نویسنده

  • Brian Henderson-Sellers
چکیده

RELATIONSHIPS Relationships are critical to object modelling. It is generally agreed that associations (of several °avours), generalizations and aggregations are useful, although the exact semantics di®er between authors. It is important that we, the object-oriented community, gain a common understanding and, in that context, recent formalisms have helped to clarify the situation: the OMG-inspired work of various groups | in particular the metamodels of UML (refs. 1,2) and OML (ref. 3). A second approach, based on mathematical logic as embodied in Object-Z, is presented by Kilov and Ross (ref. 4). In this column and the next, we evaluate the various de¯nitions of object modelling relationships and make recommendations for their use in OPEN (ref. 5) and in other methodologies. This month we look at composition (aggregation) and containment. AGGREGATION Aggregation is also often known as composition (refs. 4,6). It is also sometimes called containment (e.g. ref. 7) but this is misleading as we shall show below. It is important to note that, for the aggregation relationship to be valid, an aggregate (a.k.a. composite) must have at least one emergent property (a property of the aggregate that cannot be deduced from the evaluation of the properties of the individual components) (ref. 4). In addition, at least one property of the composite/aggregate should be dependent upon the value of one or more properties of one or more of its components/parts. Furthermore,

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عنوان ژورنال:
  • JOOP

دوره 10  شماره 

صفحات  -

تاریخ انتشار 1997