When to Use State Diagrams

State diagrams are good at describing the behavior of an object across several use cases. State diagrams are not very good at describing behavior that involves a number of objects collaborating. As such, it is useful to combine state diagrams with other techniques.

For instance, interaction diagrams (see Chapter 5) are good at describing the behavior of several objects in a single use case, and activity diagrams (see Chapter 9) are good at showing the general sequence of actions for several objects and use cases.

Not everyone finds state diagrams natural. Keep an eye on how people are working with them. It may be that your team does not find state diagrams useful to its way of working. That is not a big problem; as always, you should remember to use the mix of techniques that works for you.

If you do use state diagrams, don't try to draw them for every class in the system. Although this approach is often used by high-ceremony completists, it is almost always a waste of effort. Use state diagrams only for those classes that exhibit interesting behavior, where building the state diagram helps you understand what is going on. Many people find that UI and control objects have the kind of behavior that is useful to depict with a state diagram.