Object-Oriented Software Development with UML - UNU-IIST ...

Creating Collaboration Diagrams 103ExampleIn the POST application, consider the part of the conceptual model shown in Figure 7.20.SaledatetimeContains1..*SalesLineItemquantity*Described-byProductSpecificationdescriptionpriceupcFigure 7.20: Partial conceptual modelSince ¿+ÀÁ[Æ a contains (in fact, aggregates) ¿+À§Á†ÆDÂ;à ÄLÅ+ÆSÇWÈXÆDÉ many objects, the Creator pattern ¿+ÀÁ[Æsuggestsis a good candidate to be responsible for ¿+ÀÁ[ÆWÂ:à ÄLÅ+Æ9ÇWÈXÆÉ creating objects. This leads to a design of objectinteractions in a collaboration diagram, as shown in Figure 7.21.makeLineItem(quantity)1:create(quantity):SaledatetimeSale:SalesLineItemmakeLineItem()total()Figure 7.21: Create a SalesLineItemThe assignment of this responsibility requires that aɈÀBØ¥Æ9Ã]Ä^Å+ÆSÇWÈXÆDÉ method be defined in¿+À§Á†Æ . In general,a method has to be defined in a class to fulfill each responsibility assigned to that class.Sometimes a creator is found by looking for the class that has the initialising data that will be passed induring the creation of an object. This is actually an example of the Expert pattern. For example, assumethat a Î À§ÍBÉdÆÅsÈ instance needs to be initialised, when created, with the ¿+ÀÁ[Æ total. Since ¿+ÀÁ[Æ knows thetotal,¿-ÀÁ†Æ is a candidate creator of the Î ÀÍÉdÆDÅ_È object.Low CouplingCoupling is a measure of how strongly one class is connected to, has knowledge of, or relies upon otherclasses. A class with low (or weak) coupling is not dependent on too many other classes.A class with high (or strong) coupling relies upon many other classes. Such classes are undesirable; theysuffer from the following problems:Report No. 259, July 2002UNU/IIST, P.O. Box 3058, Macau