Learning UML 2.0

Chapter 14. Modeling an Object’s State: State Machine Diagrams

Activity diagrams and interaction diagrams are useful for describing behavior, but there’s still a missing piece. Sometimes the state of an object or system is an important factor in its behavior. For example, if the CMS required potential users to submit an application for an account, which could be approved or rejected, then the AccountApplication object may act differently depending on whether it is pending, accepted, or rejected.

In such situations, it’s helpful to model states of an object and the events causing state changes—this is what state machine diagrams do best. Continuing the above example, the AccountApplication object could have the states pending, accepted, and rejected as possible values of an attribute, and change states upon events such as approve or reject. A state machine diagram allows you to model this behavior.

State machine diagrams are heavily used in special niches of software and hardware systems, including the following:

• Real-time/mission-critical systems, such as heart monitoring software

• Dedicated devices whose behavior is defined in terms of state, such as ATMs

• First-person shooter games, such as Doom or Half-Life

To reflect these common uses, this chapter will deviate from the CMS example used throughout the rest of this book.

Most of this chapter focuses on behavioral state machines , which can show states, transitions, and behavior (inside states and along transitions). There’s another type ...