Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process, Second Edition

12.6. Modeling Attribute Quantities and Units

Most numeric quantities should not be represented as plain numbers. Consider price or velocity. These are quantities with associated units, and it is common to require knowing the unit, and to support conversions. The NextGen POS software is for an international market and needs to support prices in multiple currencies. In the general case, the solution is to represent Quantity as a distinct conceptual class, with an associated Unit [Fowler96]. Since quantities are considered data types (unique identity of instances is not important), it is acceptable to collapse their illustration into the attribute section of the class box (see Figure 12.6). It is also common to show Quantity specializations.