Concurrent States

States can be broken into several orthogonal state diagrams that run concurrently. Figure 10.5 shows a pathetically simple alarm clock that can play either CDs or the radio and show either the current time or the alarm time.

The choices CD/radio and current/alarm time are orthogonal choices. If you wanted to represent this with a nonorthogonal state diagram, you would need a messy diagram that would get very much out of hand should you want more states. Separating out the two areas of behavior into separate state diagrams makes it much clearer.

Figure 10.5 also includes a history pseudostate. This indicates that when the clock is switched on, the radio/CD choice goes back to the state the clock was in when it was turned off. The arrow from the history pseudostate indicates what state to be in on the first time when there is no history.