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State Machine Model

State Machine Model. State Machine View. describes the dynamic behavior of objects over time each object is treated in isolation the view describes the events and operations that manipulate the object

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State Machine Model

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  1. State Machine Model

  2. State Machine View • describes the dynamic behavior of objects over time • each object is treated in isolation • the view describes the events and operations that manipulate the object • the state view of the entire system is described by a collection of state diagrams, one corresponding to each class in the system C-S 546

  3. About State Diagram • one state diagram per class in the class diagram • one logical state diagram may be spread out into one or more physical diagrams for space considerations • enrichment of the finite state machine model • based on David Harel’s State charts, tailored towards object-orientation C-S 546

  4. Main components of a state diagram • a collection of states of the object under consideration • a collection of actions while entering a state, existing a state or within a state • a collection of transitions between the states of the object • a collection of events that trigger transitions • a collection of conditions (optional) that might constrain the triggering of events, or implementations of transitions C-S 546

  5. State diagram – basic syntax event (params) [guard] / action (params) initial State name actions entry/ … exit/ … do/ event (params) event (params) actions final Unnamed states C-S 546

  6. State diagram - semantics • there must be exactly one initial state and one or more final states • a transition between states is represented by the event that triggers the transition (a more concrete definition given later) • transitions may have guards or conditions under which the transitions fire • the actions associated with a transition will be executed as soon as the transition fires C-S 546

  7. 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; may be ongoing until the object leaves the state C-S 546

  8. State – a formal definition • is a condition during the life of an object during which the object performs an action or waits for some event • is represented by the collection of attributes and their corresponding values • an object after being created must be 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) C-S 546

  9. State – a formal definition (continued) • (directly or indirectly) includes links (instances of associations) connected with the object at that instant • may be decomposed into concurrent substates (AND relationship) • may be composed using mutually exclusive disjoint substates (OR relationship) C-S 546

  10. Event – a formal definition • a noteworthy occurrence • UML manual • 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 • … C-S 546

  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 C-S 546

  12. Representation of events • events are represented by unique labels in the diagram • sometimes the transitions that are fired by the occurrence of the event is also represented by the same label • change event labels are preceded by the keyword “when” • time event labels are preceded by the keyword “after” C-S 546

  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 C-S 546

  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 • “opening an account” precedes “depositing into account” • “graduation” occurs only after “finishing all requirements” and “clearing pending dues” C-S 546

  15. Transition • represents the change of states of an object • switch from “Joined University” to “Registered for Fall” • is an abstraction of an operation • registering for a course • has finite and significant duration • time taken to complete registration of one course C-S 546

  16. Transition (continued) • may have parameters • a transition corresponding to the registration process may have “course name” and “prerequisites” as parameters. • is triggered/invoked/fired by the occurrence of an event • change of semester from “Summer” to “Fall” may initiate the registration process • request by administration or department may initiate the registration process C-S 546

  17. Transition (continued) • may have a guard/condition • transition for registration may require that the student must have his/her ID validated • transition for graduation may require previous library dues to be paid off • transition to admit new students may require that the students can register only after officially admitted into the program • an event may cause several transitions to fire • completion of registration process may cause the course object to update its enrolment and at the same time, the account object to update the tuition fees to be paid C-S 546

  18. Transition (continued) • is normally identified by the same label as the event that fires the transition • “register”, “withdraw”, … • may be associated with an action • different from actions associated with the states • “register” may have an action to check the validity of parameters, to check that no previous registration has been done for the same set of parameters etc. C-S 546

  19. Example 1 – State of a “Student” : Student Student name : String id : Integer courses : set of CourseID name = “John” id = 1514601 courses = {CS546, CS742} Class definition The state of a student object : Student name = “John” id = 1514601 courses = {CS546, CS742, CS551} Another state of the student object C-S 546

  20. 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 C-S 546

  21. 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 C-S 546

  22. Example 1 - Exercise • The given state diagram for the student class is incomplete. Complete the diagram to include all the transitions and also include transitions that correspond to withdrawing from a course C-S 546

  23. Example 2 – State diagram for Account class in ATM withdrawNormal [amt < balance] / withdraw() deposit/ deposit() Normal initial deposit / deposit() open/ open() withdraw [amt = balance] / withdraw() depositOP [amt < -balance] / deposit() withdrawInitial / withdraw() close/ close() depositOP [amt=-balance]/ deposit() Overdraft withdrawOP / withdraw() C-S 546 closeAC [amt=-balance] / deposit()

  24. Example 2 - Exercise • Complete the state diagram for the Account class in ATM, by including any missing transitions. • The diagram assumes that there is no limit on overdraft protection. Make changes to the diagram assuming that there is an overdraft limit of $N. C-S 546

  25. Communications between objects • Objects communicate by sending messages to each other. • A message is realized as an event/transition in a state diagram. • The object that sends the message is said to generate an event. • Modeled by the action associated with the transition • The object that receives the message is said to realize / accept that event. C-S 546

  26. Example – A user object in ATM Deposit / deposit_User() Using Machine Withdraw / withdraw_User () C-S 546

  27. Documentation • State: Using Machine • This is the only state for this object and hence no need to enumerate the variables by which this state is defined • Transitions • Event : Deposit • Parameters : amount • Conditions : None • Action: deposit_User (amount) • Events generated: deposit OR depositOP • Communicating objects : Account • Event : Withdraw • Parameters : amount • Conditions : None • Action: withdraw_User (amount) • Events generated : withdraw OR withdrawOP OR withdrawInitial or withdrawNormal • Communicating objects : Account C-S 546

  28. Documentation – alternate style • State: Using Machine • This is the only state for this object and hence no need to enumerate the variables by which this state is defined • Transitions • Deposit (amount) / deposit_User (amount); Account.deposit (amount) OR Account.depositOP (amount) • Withdraw (amount) / withdraw_User (amount); Account.withdraw (amount) OR Account.withdrawOP (amount) Account.withdrawNormal (amount) Account.withdrawInitial (amount) C-S 546

  29. Simple and composite states • A state is composite, in contrast to a simple state, if it has a graphical decomposition • UML Manual version 1.5 • A diagram for a composite state has two or more sub-diagrams connected by simple and/or concurrent transitions • Example: concurrent states • The word “superstate” is used sometimes to refer to a composite state C-S 546

  30. Composite State – basic syntax Simple State Simple State C-S 546

  31. Composite State – Transparent Transitions Simple State Simple State C-S 546

  32. No CD Loaded Composite State - Example Eject CD drawer closed CD drawer open Power OFF Power ON [no CD] Eject [no CD] Eject Eject [CD in] CD loaded Power ON [CD in] CD playing Pause CD paused Pause or Play Power OFF Stop Stop CD stopped Play C-S 546

  33. Observations • The state diagram for the CD player example indicates a composite state that includes two MUTUALLY EXCLUSIVE states • The player will be in only one of these two substates at any time, but not in both at the same time • The guard condition on the initial state chooses the appropriate substate upon Power ON • There is only one terminal state that is common to both the substates C-S 546

  34. Abstract view of the CD player Eject No CD loaded Power ON [no CD] Eject [CD in] Power OFF Power ON [CD in] CD loaded C-S 546

  35. Composite State with Concurrent Transitions Concurrent transition Concurrent transition Composite State C-S 546

  36. Concurrent States - example Student attending a course Incomplete Lab done Lab done Lab 1 Lab 2 passed Project done Project Final exam pass fail failed C-S 546

  37. Observations • If a composite state has concurrent substates, • an entry point to the composite state represents a concurrent transition, even if it is not represented • an exit point from the composite state represents a concurrent transition, even if it is not represented • all concurrent substates start at the same time • the composite state terminates only when all the concurrent substates terminate C-S 546

  38. Observations (continued) • if there is a transition from any one of the substates that lead to a state outside the composite state, then all the other concurrent substates terminate prematurely • the transition “fail” in the third substate illustrates this situation C-S 546

  39. Composite State - Exercise • Draw a composite state diagram for the following problem: • Show the state of 3 phones when all of them are in use in a conference call • Hint: First identify the various values for the status of a phone C-S 546

  40. State diagrams and specialization • Inheritance mechanism allows to redefine only behaviors of a superclass and not the structure of the superclass • The structure of a subclass must be the same or a superset of the structure of the superclass • Every state of a superclass object is also a valid state of the subclass object • The state diagram of a superclass can be inherited into the state diagram of a subclass C-S 546

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