Chapter 8. Statechart Diagrams

State machine diagrams capture the behavior of a software system. State machines can be used to model the behavior of a class, subsystem, or entire application. They also provide an excellent way of modeling communications that occur with external entities via a protocol or event-based system.

UML has two types of state machines:

Behavioral state machines

Show the behavior of model elements such as objects. A behavioral state machine represents a specific implementation of an element.

Protocol state machines

Show the behavior of a protocol. Protocol state machines show how participants may trigger changes in a protocol's state and the corresponding changes in the system (i.e., the new state of the protocol). Protocol state machines aren't typically tied to a particular implementation, and they show the required protocol behavior.

Behavioral and protocol state machines share common elements; however, protocol state machines are not tied to an implementation and have restrictions on their transitions. Because protocol state machines are a specialization of behavioral state machines, this chapter first discusses behavioral state machines and other topics common to all state machine diagrams; then details of protocol state machines are explained.