Learning UML

Advanced State Diagrams

To be flexible and powerful, the UML allows you to put one or more state diagrams inside a single state to indicate that when an element is in that state, other elements inside of it have their own states. For example, within the Active state of the project management system, each object and subsystem has its own state, and the state diagram for each element may be shown inside the Active state of the project management system or by using the element’s own state diagram. This recursive nature of state diagrams is very powerful, because it allows you to learn the basic notation of state diagrams and reapply it to nested state diagrams without having to learn more notation.

In the UML, a state may be shown as a rectangle with rounded corners and three compartments separated by horizontal solid lines. Figure 7-5 shows the Active state of the project management system with three compartments. The top compartment of a state contains the name of the state or a description of the situation of the element.

The middle compartment of a state may contain transitions for the following special events:

entry

Indicates the action to be performed when the element enters the state

exit

Indicates the action to be performed when the element exits the state

do

Indicates the action to be performed continuously while the element is in the state

Figure 7-5 shows that when the project management system enters the Active state, a message is logged using the LogMessage action. When ...