Section 1.3. Models and Diagrams

1.3. Models and Diagrams

Many newcomers to UML focus on the different types of diagrams used to model their system. It's very easy to assume that the set of diagrams that have been created actually are the model. This is an easy mistake to make because when you are using UML, you will normally be interacting with a UML tool and a particular set of diagrams. But UML modeling is not just about diagrams; it's about capturing your system as a modelthe diagrams are actually just windows into that model.

A particular diagram will show you some parts of your model but not necessarily everything. This makes sense, since you don't want a diagram showing everything in your model all at onceyou want to be able to split contents of your model across several diagrams. However, not everything in your model needs to exist on a diagram for it to be a part of your model.

So, what does this mean? Well, the first thing to understand is that your model sits behind your modeling tool and diagrams as a collection of elements. Each of those elements could be a use case, a class, an activity, or any other construct that UML supports. The collection of all the elements that describe your system, including their connections to each other, make up your model.

However, if all you could do was create a model made up of elements, then you wouldn't have much to look at. This is where diagrams come in. Rather than actually being your model, diagrams are used merely as a canvas on which you can create new elements that are then added to your model and organize related elements into a set of views on your underlying model.

So, when you next use your UML tool to work with a set of diagrams in UML notation, it is worth remembering that what you are manipulating is a view of the contents of your model. You can change elements of your model within the diagram, but the diagram itself is not the modelit's just a useful way of presenting some small part of the information your model contains.