1. State Machine Model

2. State Machine View Describes the dynamic behavior of objects over time
Each object is treated in isolation
This view describes events and operations that manipulate the object under discussion
The state view of the entire system is described by a collection of state diagrams, one diagram corresponding to each class in the system
Objects
object
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3. Main components of a state diagram
A collection of states of the object under consideration
A collection of actions while entering a state, exiting a state or within a state
A collection of transitions between states
A collection of events that trigger transitions
A collection of conditions (optional) that might constrain firing of transitions
States
Actions
Transitions
Events
conditions
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4. State diagram – basic syntax
event (EventParams) [guard] /
action (ActionParams)
initial
State name
actions
entry/ …
exit/ …
do/
event (params)
event (Eparams) / action (Aparams)
actions
final
Unnamed states
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5. State diagram - semantics
There must be exactly one initial state and one or more final states
Some state diagrams do not show the final states; in this case, there is an implicit final state when the program terminates
A transition between states is represented by the event that triggers the transition
Transitions may have guards or conditions under which the transitions fire
One
One or more
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6. State diagram – semantics (continued)
A state may optionally have a label
Every state may have
An entry action – executed as soon as the state is entered
An exit action – executed just before leaving the state
A “do” action – executed while the object is in this state; it may be ongoing action until the object leaves the state
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7. State diagram for ATM Account Class
Check balance (act)
Deposit (amount)
Check balance (act)
Deposit (amount)
Transfer (act, amount) [balance > amount]
Normal
Initial
Transfer (act, amount) [balance=amount]
Withdraw (amount) [balance > amount]
close
Withdraw (amount) [balance = amount]
State diagram for ATM Account Class
Actions associated with transitions are not shown in this diagram
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8. State – a formal definition
A state is a condition in the life of an object during which the object performs an action or waits for some event
A state is represented by the collection of attributes and their corresponding values
An object after being created must be in at one particular state at any instant
Unless otherwise mentioned, an object remains in a state for a finite time
UML allows modeling of transient states (states that exist only for a very short and insignificant duration)
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9. State – a formal definition (continued)
A state (directly or indirectly) includes links (instances of associations) connected with the object at that instant
A state may be decomposed into concurrent sub-states (AND relationship)
A state may be composed using mutually exclusive disjoint sub-states (OR relationship)
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10. Event – a formal definition
A noteworthy occurrence
UML manual version 1.5
Something that happens within the system or interacting with the system at an instant
Something that has a significant impact on the system
Examples
sending a signal or data
receiving a signal or data
making a request for execution
a Boolean condition becoming true
a timeout condition becoming true
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11. Four types of events in UML
Signal event
occurs when an object sends a signal to another object
Call event
occurs when a method or operation in an object is invoked
Change event
occurs when a Boolean condition is changed
Time event
occurs when a time limit has reached
Kindergarten-first grade-second grade
Job applicaion, interview
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12. Representation of events
Events are represented by unique labels in the diagram
Generally, the transitions that are fired by the occurrence of the event are also represented by the same label
Change event labels are preceded by the keyword “when”
Time event labels are preceded by the keyword “after”
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13. Generating an Event An event is generated by the runtime system
Asking for inputs
Producing outputs
Execution of a method
Transfer control of execution from one object to another (sending messages or receiving messages)
Abnormal termination
Error handling, exceptions
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14. Temporal properties of Events
An event is considered to be instantaneous
Occurrence of an event causes negligible time
This time is not included in the modeling
Events may have precedence relationships among them
In ATM system, “opening an account” precedes “depositing into account”
In course registration system, “enrolling a student in a course” can occur only after “checking the prerequisites completed by the student”
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15. Transition Represents the change of states of an object
switch from “Empty balance” to “Positive balance”
A transition is an abstraction of an operation
The above transition is an abstraction of deposit operation
A transition has finite and significant duration
Observable time taken to complete deposit operation
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16. Transition (continued)
A transition may have parameters
A transition corresponding to the deposit operation will have the “amount” as a parameter
A transition is triggered/invoked/fired by the occurrence of an event
The transition corresponding to deposit will occur by the event “request for deposit”
The transition from “Positive balance” to “Empty balance” occurs by the completion of the operation “withdrawal”
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17. Transition (continued)
A transition may have a guard/condition
The transition corresponding to the withdrawal operation will occur only if the balance is greater than or equal to the amount to be withdrawn
The condition associated with a transition is always the precondition for the transition and hence must be checked before the transition occurs
An event may cause several transitions to fire
The event that triggers the transition to move to “Empty balance” state after withdrawal may also cause a message to be sent to the account holder and at the same time may also cause a note to be recorded in the account log
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18. Example 1 – State of a “Student”
name : String
id : Integer
courses : set of CourseID
name = “John”
id =
courses = {CS546, CS742}
Class definition
The state of a student object
: Student
name = “John”
id =
courses = {CS546, CS742, CS551}
Another state of the student object
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19. Example 1 – Simplified representation
: Student
Student
courses = {CS546,CS742}
courses : set of CourseID
: Student
courses = {CS546, CS742, CS551}
Indicate only those attributes that define change of state
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20. Example 1 – State diagram for Student class
register [#courses < minRequired] / updateCourses()
register[#courses < minRequired] / updateCourses()
Initial
continuing
entry/ initializeCourses()
register [#courses >= minRequired] / updateCourses()
completed
graduated
[complete
Graduation
Requirements]
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21. Exercise 1
The given state diagram for the student class is incomplete. Complete the diagram to include transitions that correspond to withdrawal from a course
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22. Exercise 2
Complete the state diagram for the Account class in ATM, by including additional states and transitions to allow withdrawals below the available balance.
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23. Exercise 3
Draw the state diagram for an object that performs login validation as described in the following example.
A login subsystem validates first whether a username is the right format. Second, it checks whether the password is in the right format. Third, it checks whether there is an account corresponding to the username. Finally, it verifies whether the username and password match with the existing account.
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24. Exercise 4
Draw the state diagrams for objects in a simple three-light traffic light control system.
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