Lesson: Understanding the UML model hierarchy

A UML diagram provides a visual representation of a specific aspect or behavior of a system.

A Unified Modeling Language (UML) diagram describes the quantifiable aspects of a system that can be visually described, such as relationships, behavior, structure, and functionality. For example, a use-case diagram describes the interactions between the users of the system, called the actors, and the system. A class diagram is often used to describe the structure of a system, or the details of an implementation. A UML diagram contains diagram elements that visually describe the classifiers in a diagram. These classifiers are the diagrammatic representation of a model element. UML diagrams provide views of UML models; however, diagram elements are not considered part of the semantic model.

A UML model might or might not contain a UML diagram, but most UML models contain several diagrams. A UML model can also contain one or more submodels.

IBM® Rational® modeling tools provide a flexible development environment for creating and editing UML models. You can use the Project Explorer view to edit the source code and model structure of the semantic model, and you can use the diagram editor to edit the diagrams of the visualized model. When you edit models, you should understand the differences between model elements and diagram elements. For example, when you right-click an element in the diagram editor, there are two deletion options. If you click Delete from Diagram, the element is deleted from the diagram only, because the diagram element is only a visualization of a model element. Conversely, if you click Delete from Model, the element is deleted from both the model and the diagram. By default, when the user selects a diagram element and presses the Delete key or when the user selects a diagram element and clicks File > Delete, the element is removed from only the diagram.


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