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Object Model Notation


The object model notation from Object Modeling Technique (OMT) and Unified Modeling Language (UML) is a commonly accepted graphical notation used for drawing diagrams that describe objects and the relations between them. The notation that is used in examples in this volume is shown in Figure 617 through Figure 622. For more details about other, more advanced OMT object model concepts, please consult [21], and for details about the UML notation see [37] and [38].

Class

The most important concept in an object model is the class definition. A class is a description of a group of similar objects that share the properties defined by the class. The object model notation for a class is exemplified in Figure 617, where the second class definition also shows how to define attributes and operations.

Figure 617 : A collapsed class symbol and a class symbol
with attributes and operations

In some cases it is necessary to reference classes from an external module. The notation used for this purpose is ExternalModule::Class.

Classes may inherit attributes and operations from other classes. The object model notation for this is shown in Figure 618.

Figure 618 : Inheritance between classes

Relations and Multiplicity

Classes may be physically or logically related to each other. This is shown in the object model by means of associations as shown in Figure 619. An association may have a name and/or the endpoints of the association may be labeled by the role of this endpoint.

Figure 619 : Associations between classes

Aggregation is special kind of association that has its own notation as shown in Figure 620.

Figure 620 : Aggregation

The endpoints of associations and aggregations may have a multiplicity as shown in Figure 621.

Figure 621 : Multiplicity of associations and aggregations

Module

In practise the complete requirements object model is often too big to fit into one diagram. To solve this problem it is possible to use multiple object model diagrams that can be organized into a module, which simply is a list of diagrams. It is important to notice that a class may be present in more than one diagram and still only represent one logical class.

Objects

Besides class definitions, object models may of course also contain objects and their relations. The relation that exists between objects are links, which corresponds to the associations for classes. The object symbol has one field containing the name of the object together with a reference to the class, and an attribute field where constant or default values can be assigned to the object attributes. See Figure 622.

Figure 622 : Objects related by links


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