ObjectOriented Analysis and Design

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Object-Oriented Analysis and Design
Session 3b Behavioral Modeling State machine
diagrams
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Outline

• Introduction to State Machine...... 3
• State Machine Diagrams Syntax and Semantics.11
• State Hierarchy....31
• State History Mechanism .......41
• State Concurrency ..46
• Concluding Example ..57
• State Machine Diagrams and UML ...62
• Summary ....65

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Introduction to State Machine
4

Characterization of reactive systems

• Continuous interaction with the environment
• inputs and outputs are asynchronous in time.
• Respond to interrupts.
• Stringent time requirements.
• Multiple possible scenarios of operation,
  depending on the history of previous behavior.
• Based on interacting processes that operate in
  parallel.

5
Automata

• A machine whose output behavior is not only a
  direct consequence of the current input, but also
  of some past history of its inputs.
• Characterized by an internal state which
  represents this past experience.

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What are Statecharts?

• Statecharts are visual formalism for specifying
  behavior of complex systems.
• Developed by David Harel.

restbreak
Employee
Resting
Working
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State Machine (Automaton) Diagram

• Graphical rendering of automata behavior

Lamp On
Lamp Off
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Outputs and Actions

• As the automaton changes state it can generate
  outputs

9
Extended State Machines (1)

• Addition of variables (extended state)

ctr Integer
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Extended State Machines (2)

• An extended (Mealy) state machine is defined by
• a set of input signals (input alphabet)
• a set of output signals (output alphabet)
• a set of states
• a set of transitions
• triggering signal
• action
• a set of extended state variables
• an initial state designation
• a set of final states (if terminating automaton)

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State Machine Diagrams Syntax and Semantics
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State Machine Diagram
top state
State
Initial pseudostate
top
Trigger
Ready
Transition
/ctr 0
stop
Done
Final state
Action
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Statechart Diagram Elements

• States
• Any component or object within the system can be
  at a specific state in a given time.
• When holding the state, an object (or a
  component) can perform activities (which can be
  interrupted).
• Transitions
• Specification of rules for moving between states.
• A transition may consist of a trigger, condition,
  and an action.

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Initial and Final States
start
Whites turn
checkmate
Black wins
stalemate
white moves
black moves
Draw
stalemate
Blacks turn
White wins
checkmate
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Activity and Action

• An activity
• can be performed within a state.
• can be continuous.
• can be sequential.
• takes time.
• An action
• can be performed within a state or during a
  transition.
• can not be interrupted.
• is atomic.
• can take time.

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Event-Driven Behavior

• Event a type of observable occurrence
• interactions
• object operation invocation (call event).
• asynchronous signal reception (signal event).
• occurrence of time instants (time event)
• interval expiry.
• calendar/clock time.
• change in value of some entity (change event).
• Event Instance an instance of an event (type)
• occurs at a particular time instant and has no
  duration.

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Transitions

• A transition is enabled if the object is at a
  state preceding the transition.
• An enabled transition is activated upon its
  trigger activation.
• An initial transition indicates that the default
  is entering S2.
• A transition can connect the same state.

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Object Behavior - General Model

• Simple server model

InitializeObject
Handling depends on specific request type
Wait forRequest
HandleRequest
TerminateObject
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Object Behavior and State Machines

• Direct mapping

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State Entry and Exit Actions
entry/lamp.on()
exit/lamp.off()
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Order of Actions Simple Case

• Entry actions are performed after (entering)
  transition actions.
• Exit actions are performed before (exiting)
  transition actions.

Resulting action sequence printf(exiting) pr
intf(to off) lamp.off()
printf(exiting) printf(needless) lamp.off()
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Internal Transitions

• Self-transitions that bypass entry and exit.
  actions

Internal transition triggered by an off event
off/null
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State (Do) Activities

• Forks a concurrent thread that executes until
• the action completes or
• the state is exited.

do activity
do/while (true) alarm.ring()
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Guards

• Conditional execution of transitions
• side-effect free

Selling
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Guards and Events (1)
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Guards and Events (2)

• Special operators
• in(a) means in state a.
• en(b) means the occurrence of entering state b.
• ex(c) means the occurrence of exiting state c.
• tm(min) means trigger if min time units have
  passed.

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Traffic Light Example
N/S may go straight
N/S may turn left
tm(1) cars in N/S left lanes
tm(1)no cars in N/S left lanes
tm(1)
tm(1)
tm(1)no cars in E/W left lanes
E/W may turn left
E/W may go straight
tm(1)cars in E/W left lanes
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Phone Line Example
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Static Conditional Branching

• Merely a graphical shortcut for convenient
  rendering of decision trees.

value gt 200 /sell
value lt 100 /reject
(value gt 100) (value lt 200) /sell
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Dynamic Conditional Branching

• Choice pseudostate guards are evaluated only
  when the decision point is reached.

bid /gain calculatePotentialGain(value)
gain gt 200 /sell
gain lt 100 /reject
(gain gt 100) (gain lt 200) /sell
Dynamic choicepoint
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State Hierarchy
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States Hierarchy (1)

• Intended to cluster together alternative states
  of a single aspect / object.
• Results OR states. An OR state is a super-state
  of its sub-states.
• A state that has no sub-states is a basic state.

The states in an IndentationStatus statechart
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States Hierarchy (2)

• States hierarchy is used for describing different
  levels of abstraction.

High Level
Detailed Level
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States Hierarchy (3)

• States hierarchy is used for clustering.

Un clustered
clustered
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States Hierarchy (4)

• Initial States one per state.
• To be in a state is to be in ONE of its sub
  states.

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States Hierarchy (5)

• Graduated attack on complexity
• states decomposed into state machines

LampFlashing
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States Hierarchy (6)
Default transition to the initial pseudostate

• Higher-level transitions

LampFlashing
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States Hierarchy (7)

• Triggered by a completion event
• generated automatically when an immediately
  nested state machine terminates.

completion transition (no trigger)
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States Hierarchy (8)

• Two or more transitions may have the same
  triggered event
• inner transition takes precedence.
• if no transition is triggered, event is discarded.

LampFlashing
FlashOn
off/
FlashOff
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States Hierarchy (9)

• Same approach as for the simple case

initS2
E/actE
Actions execution sequence exS11?? exS1 ??
actE??enS2 ?? initS2 ?? enS21
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State History Mechanism
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History Mechanism (1)
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History Mechanism (2)
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History Mechanism (3)
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History Mechanism (4)

• Return to a previously visited hierarchical state
• deep and shallow history options

Diagnosing
Diagnostic1
Diagnostic2
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State Concurrency
47
Orthogonality and Concurrency (1)

• Multiple simultaneous perspectives on the same
  entity

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Orthogonality and Concurrency (2)

• Describe independent components of a behavior.
• Behavior of parts of an aggregate object.
• Enforce synchronization of concurrent
  activities.
• Concurrency is an AND-decomposition of a state.
• To be in a state S is to be in ALL of its
  components. S is an AND state.

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Orthogonality and Concurrency (3)

• Combine multiple simultaneous descriptions

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Orthogonality and Concurrency (4)
Use of Orthogonal Regions
No Use of Orthogonal Regions
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Orthogonality and Concurrency (5)

• All mutually orthogonal regions detect the same
  events and respond to them simultaneously.

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Orthogonality and Concurrency (6)

• Typically through shared variables or awareness
  of other regions state changes.

sane Boolean
flying Boolean
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Transition Sequence

• A transition might be a trigger
• An event can trigger additional transitions
• Several transition can be triggered within a
  single step
• Solution
• The number of triggered transition should be
  limited

A
C
b/d
a/c
d/a
c/b
B
D
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Transition Forks and Joins

• For transitions into/out of orthogonal regions

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Reducing Multiple Transitions (1)
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Reducing Multiple Transitions (2)
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Concluding Example
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Mini Organ Example (1)
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Mini Organ Example (2)

• The button a is used for switch the organ on and
  off
• The organ enables a song selection and its style.
  When the organ is turned on the display shows the
  mode STYLE or SONG and its number. The default
  is song 1/sytle 1 and the mode is STYLE.
• To select a style one need to press the b button
  and the required style number (1-9). Each style
  has a function that return its name
  StyleName(x).
• To select a song one need to press the c button
  and the required song number(1-9). Each song has
  a function that return its name SongName(x).
• Pressing the start button plays the current song
  following the current style. Another press stop
  the music. It is possible to change the song or
  style during music playing.

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Mini Organ Example (3)
Events a - pressing the a button b - pressing the
b button c - pressing the c button s - pressing
the START button i - pressing the a digit button
(2-9) 1 - pressing the 1 button songend end of
song playing Variables i,j can get values
between 2-9 k,m can get values between 1-9 w
can get a song or style name
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Mini Organ Example (4)
c
b
x
y
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State Machine and UML
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Dishwasher Class Diagram
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Dishwasher State Machine Diagram
AcmeHeater
AcmeJet
Dishwasher
AcmeTank
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Summary

• UML uses a variant of Harels statecharts
• adjusted to software modeling needs
• Used to model event-driven (reactive) behavior
• well-suited to the server model inherent in the
  object paradigm
• Includes a number of sophisticated features that
  realize common state-machine usage patterns
• entry/exit actions
• state activities
• dynamic and static conditional branching
• Also, provides hierarchical modeling for dealing
  with very complex systems
• hierarchical states
• hierarchical transitions
• orthogonality