UML Weekend Crash Course™ - To Parent Directory

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Sunday Morning
A substate is a state within a state, a lower level of detail within a state. For example, a
car can be in the state of moving forward. Within the moving forward superstate are substates
of moving forward in first gear, moving forward in second gear, and so on. The substates
provide a low-level view of a model element so that you can address specific issues
individually and in terms of their interactions and interdependencies. This detailed view
also allows you to highlight concurrent states and focus on how to control the splitting and
merging of concurrent states.
To illustrate these concepts, I’m going to use what I hope is a familiar example, a thermostat.
I’m going to simplify it even further and only look at the cooling side of its responsibilities.
The thermostat is a typical control-type object. Its job is to direct the work of
other objects, much like an application directs the behavior of the screen and the access to
the database.
Figure 22-7 models the superstate Cooling with two concurrent substates. In this example,
the two substates are both initial states. The diagram says that when the Thermostat
enters the Cooling state, it splits into two concurrent substates, that is, it is now doing two
things at the same time: It is monitoring the progress of the cooling process, and it is monitoring
the cooling device for any problems. The two substates start immediately upon entering
the Cooling state.
Cooling
Monitor Cooling
Monitor Device
Figure 22-7 Superstate Cooling with two substates, Monitor Cooling and Monitor
Device
It is also possible to trigger the substates by transition events like a call or time event.
In this case, the transition event would extend all the way into the superstate and point
directly to the substate. This often involves split of control, which I cover next.
Split of control
Split of control means that, based on a single transition, you want to proceed with multiple
tasks. This is exactly the same concept you learned in the Activity diagram session. Split of