J2EE Design Patterns

Class Diagrams

Representing the domain model for an application is arguably the most important area of object-oriented modeling. Class diagrams , as a result, are the most widely used feature of the UML.

Class diagrams show the relationships between classes in a system. Since object-oriented design blurs the line between data objects and executable application logic, a UML class diagram documents both. UML actually supports both class diagrams and object diagrams. If you’re reading this book, you’re almost certainly familiar with the distinction between a class and an object: a class is a framework containing methods and data that can be instantiated into one or more objects, which contain actual data items. UML class diagrams show the relationship between the frameworks, and object diagrams show the state of a system with actual objects, containing actual values.

Although the names are the same, a class in a UML class diagram does not necessarily map to a Java class—although in more detailed, design-focused UML diagrams, they will. Class diagrams can be used to build conceptual pictures of an application’s domain model, which can then be used to develop more specific system designs that eventually map into code.

It’s nice to be as complete as possible when developing a class diagram, but it is by no means necessary. Generally, a modeler picks a level of completeness that corresponds with the current stage in the software design process. The highest level class diagrams ignore all ...