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

66 Conceptual Model–AttributesWhat about attributes in code?The restriction that attributes in the conceptual model be only of simple data type does not imply that C++,Java, or Smalltalk attributes (data members, instance variables) must only be simple, primitive data types.Pure data valueMore generally, attributes should be pure data value (or DataTypes in UML)–those for which unique identityis not meaningful in the context of our model or system. For example, it is not (usually) meaningful todistinguish between:Separate instances of the Number 5.=Separate instances of the String ‘cat’.=Separate instances of the PhoneNumber that contain the same number.=Separate instances of the address that contain the same address.=However, it is meaningful to distinguish (by identity) between two instances of a Person whose name areboth “Jill Smith” because the two instances can represent separate individuals with the same name.So the second guideline for identifying attributes is:An element of a pure data type may be illustrated in an attribute box section of another concept,although it is also acceptable to model it as a distinct concept.Pure data values are also known as value objects.The notion of pure data values is subtle. As a rule of thumb,stick to the basic test of “simple” attribute types: make it an attribute if it is naturally thoughtof as number, string, boolean, or time (and so on); otherwise represent is as a separate concept.We always have the following rule.If in doubt, define something as a separate concept rather than as an attribute.Report No. 259, July 2002UNU/IIST, P.O. Box 3058, Macau