9 - UML State Machine Diagrams

Summary

UML state machine diagrams depict the dynamic behavior of an entity based on its response to events, showing how the entity reacts to various events based on its current state. Create a UML state machine diagram to

1. ■ explore the complex behavior of a class, actor, subsystem, or component;

2. ■ model real-time systems.

General Guidelines

Create a State Machine Diagram When Behavior Differs Based on State

If an entity, such as a class or a component, exhibits the same sort of behavior regardless of its current state, then drawing a UML state machine diagram will be of little use. For example, a SurfaceAddress class is fairly simple, representing data that you will display and manipulate in your system. Therefore, a UML state machine diagram would not reveal anything of interest. On the other hand, a Seminar object is fairly complex, reacting to events such a enrolling a student differently depending on its current state, as you can see in Figure 38.

Place the Initial State in the Top Left Corner

An initial state is modeled with a filled in circle, as you can see in Figure 38. Placing an initial state in the top left corner reflects the way that people in Western cultures read. Another good option is to place the initial state at the top of your diagram in the center.