1. UML State Diagrams

2. UML State Diagrams
Describe how an object’s state changes in response to external events
Similar to State Transition Diagrams / FSMs
Nodes are states
Arrows are events/actions/stimulus
Ideal for describing behavior of a single object
Sequences of states that an object goes through
Events that cause a transition from one state to another
Actions that result from a state change

3. State Diagram Key Terms
Transition
Activity
entry, exit, do, entry
Action
Event
State – The current status of an object based upon the total set of events it has responded to so far.
Transition – Associations between states. Transitions are fired (taken) based upon events that the object responds to.
Activity – Nonatomic sequences of actions that have some duration. (assumed to take time)
Action – Atomic executable statements that invoke an operation. (assumed to happen almost instantly)
Entry Action – Always performed on entering a state.
Exit Action – Always performed when the state is exited.
Do Action – Performed continuously as long as the object is in the state
Include Action - Invoke the states internal state machine (Do not reference operations).
Event – Occurrences that may trigger an object to change states.

4. State Machine Syntax Name Normal State Activities Start/Initial State
Stop/Accepting/Final State
Transition
EventName [Guard] / Action

5. Actions vs Activities
Actions are associated with transitions, are considered to be processes that occur quickly and are not interruptible
Activities are associated with states, can take longer, and can be interrupted by events
“do” events can iterate
“entry” events happen only on entry to state

6. Basic State Diagram: Order Entry
[Not all items checked]
/ getNextItem
[All items checked &&
All items available]
dispatching
checking
do/initiate
delivery
do/check
item
Item Received
[All items available]
[All items checked &&
Some items not in stock]
shipped
waiting
delivered
Item Received
[Some items not
In stock]
cancelled
cancelled
cancelled
cancelled

7. State Diagram with Superstates
Active
[Not all items checked]
/ getNextItem
[All items checked &&
All items available]
dispatching
checking
do/initiate
delivery
do/check
item
[All items checked &&
Some items not in stock]
waiting
Item Received
[All items available]
Item Received
[Some items not
In stock]
shipped
cancelled
cancelled
delivered

8. Order Entry With Concurrency
Ordering
waiting
checking
shipping
authorizing
authorized

9. State Diagram Exercise
Draw a statechart diagram modeling a vacuum cleaner robot’s behavior.
If activated, it starts vacuuming offices, afterwards the canteen, and finally the laboratory. If it finishes cleaning the laboratory, it starts again in the offices.
If deactivated, it returns to its locker and stops there.
Every time on changing its operational area, it empties its dust bin into some container.
If it runs low on energy, it goes to a recharging station and recharges its batteries. Afterwards it continues the cleaning where it has stopped.

10. State Diagram Exercise (1)
These arrows
indicate that when
the superstate is
entered, the first
state reached is
“Clean Offices”
Problem: when you
go to “Charge Robot”,
how do you know
where to return to?

11. Encoding “History” Information
In Statecharts, sometimes, you’d like to remember “history”, such as when you want to return to a specific state within a substate/superstate.
One way to do this is to set/check a variable, and UML lets you do this within activities. But you’d have to do this in each state and add proper transitions.
An alternative is to use a “history state”. In such a state, your first entry goes to the state denoted, inside the substate/superstate, with an arrow. On any subsequent entry it is assumed that you return to the state from which you last left.

12. State Diagram Exercise (2)
The “H” indicates
that this is a
“history” superstate

13. Basic State Diagram: Order Entry
Dial(n) [valid && incomplete]
DialTone
Dialing
Caller lifts handset
do/play dial
tone
do/dial
Idle
Dial(n) [invalid]
Dial(n) [complete] / connect
Invalid
Connected
do/play
message
busy
connected
Busy
Ringing
do/play busy
tone
do/play ring
tone
Caller replaces handset

14. Questions
What does the positioning of the “Idle” state and the edges into and out of it indicate?
Provide an interpretation of the meaning of and actions associated with the “Dialing” state and the edges out of it.
Modify the diagram to indicate that, when the system is in the Connecting state and a busy signal is detected, if the callee has an answering service, the call is directed to the service instead of to a busy tone.