State Diagrams Chapter Explained

State Diagrams

Chapter 5

Object-Oriented Software Systems Engineering – Chapter 5 Slide 1

Objectives

In this chapter we will:
 Introduce the terms used with respect to state
diagrams
 Discuss the context in which state diagrams are
used
 Introduce substates
 Discuss concurrent state diagrams


Statechart Diagrams

 State diagrams describe the life of an object using
three main elements:
 States of an object
 Transitions between states
 Events that trigger the transitions

 A state diagram or statechart specifies a state
machine
 A state machine is described for a class
 Each object has it’s own state machine


Why Use Statechart Diagrams?

 Statecharts typically are used to describe state-dependent
behaviour for an object
 An object responds differently to the same event depending on
what state it is in
 Usually applied to objects but can be applied to any element
that has behaviour
 Actors, use cases, methods, subsystems, systems

 Statecharts are typically used in conjunction with interaction
diagrams (usually sequence diagrams)
 A statechart describes all events (and states and transitions for
a single object)
 A sequence diagram describes the events for a single
interaction across all objects involved


States

 Show what the dependencies are between the
state of an object and its reactions to messages
or other events
 State
 is a condition or situation during the life of an object
within which it performs some activity, or waits for some
events
 Has a name
 Has actions -- execute the state
 Has internal transitions -- transitions cause no change in
a state
 substates -- the nested structure of a state involving
disjoint or concurrent substates


States

 For example:

At Work state name
entry/unlock door action performed on entry to state
do/prepare materials activity performed while in state
telephone rings/answer telephone action performed on arrival of named event
include/lecture state name of a sub-state machine
exit/lock door action performed on leaving state


Initial and Final States

 The initial state of a state machine is indicated
with a solid circle
 Known as a pseudo-state
 A transition from this state will show the first real state

 The final state of a state machine is shown as
concentric circles
 A closed loop state machine does not have a final state;
the object lives until the entire system terminates
 An open loop state machine represents an object that
may terminate before the system terminates


Initial and Final States

 An example:

At Work go home At Home

go to work

die die


Actions and Activities

 Action
 is an executable atomic computation
 includes operation calls, the creation or destruction of
another object, or the sending of a signal to an object
 associated with transitions and during which an action is
not interruptible -- e.g., entry, exit
 Activity is associated with states
 Non-atomic or ongoing computation
 May run to completion or continue indefinitely
 Will be terminated by an event that causes a transition
from the state in which the activity is defined


Events

 An event signature is described as
Event-name (comma-separated-parameter-list)
 Events appear in the internal transition
compartment of a state or on a transition between
states
 An event may be one of four types
 Signal event
 Corresponding to the arrival of an asynchronous message
or signal
 Call event
 Corresponding to the arrival of a procedural call to an
operation
 Time event
 Change event


Events

 A time event occurs after a specified time has
elapsed
 Event name is specified as keyword after
 Parameter list is an expression evaluating to a time
interval
 after(10 seconds after state “At Work” is entered)
 No specified start time implies “since entry to the current
state”
 after(2 seconds)


Events

 A change event occurs whenever a specified
condition is met
 Event name is specified as keyword when
 Parameter list is a boolean expression
 The event occurs when both of the following conditions
are met, irrespective of the order when they happen
 The expression evaluates to true
 The object is in the required state
 For example
 when (state = At Work)
 when (date = January 1 2007)


Transitions

A transition is drawn as an arrow between states
annotated with a transition string
 The transition string denotes the event and
consequent action
 Only one form of arrowhead is used on statecharts
 The distinction between call events and signal events must be
deducted from elsewhere e.g. an interaction diagram

A transition string is described as
 Event-signature [guard-condition]/action-expression^object.message
 If the guard condition is met the transition occurs immediately


Transitions

 A transition whose string contains neither an
event signature nor a guard condition is said to be
unlabeled
 Occurs immediately
 May still carry an action expression


Transitions

 A transition is triggered when its event occurs
 If the guard condition is met, the transition is fired
 If the condition is not met the event is discarded
 The guard condition is checked only once

 If there is no guard condition, triggering will
always cause firing
 Note the distinction between a guard condition
and a change event
 A guard condition is evaluated once, when the
associated event occurs
 A change event occurs whenever its associated
condition is met
 Behaviour is as if the condition were being continually
evaluated


State Diagrams notation

Initial state event final state
name

Invoice Invoice
created paying destroyed
Unpaid Paid

transition
state


State Diagram Example

This shows the state of an object myBkCpy from a BookCopy class

on loan return() on the shelf
entry / myBkCpy.borrow() entry / myBkCpy.return()
borrow()

Entry action : any action that is marked as linked to the entry action is
executed whenever the given state is entered via a transition

on loan return() on the shelf
exit / myBkCpy.returned() exit / myBkCpy.borrowed()
borrow()

Exit action : any action that is marked as linked to the exit action is
executed whenever the state is left via a transition


A Class of BookCopy

BookCopy

onShelf : Boolean value := ‘Y’ on the shelf
value := ‘N’ on loan

return() action / onShelf=‘Y’

borrow() action / onShelf=‘N’


State Diagram Example
get first item all items
in stock
[all items checked &&
*[all items checked]
all items available]
get next item Checking Dispatching
Dispatch items
do / check
Item
b le] do / initiate
[all items checked aila delivery
av d
&& some items not
tems eive
in stock] ll i rec
[a m delivery
Order Item Ite

some items Ordering Delivering
not in stock cancelled
Exit/ Item received entry / deliver
do / order Item Items

Canceling
do / Remove
Item

State Diagram - Nested States

event-1 Super-state
A B
event-1
A B

event-2 event-2

C event-2

C


State Diagram Example including substates
get first item

*[all items checked]
[all items checked && all items available]
get next item
Checking Dispatch items Dispatching
do / check do / initiate
Item ble] delivery
a
v ail
[all items checked && ms a ved
some items not in stock] l ite ecei
Order item [al m r delivery
Ite

Ordering
Exit/ Item received
do / order Item

Canceling Delivering
cancelled
do / Remove entry / deliver
Item Items

Concurrent State models

B
s Finishing
Starting
A C

E T
Explicit control
branching(fork)
D G

F

 Orthogonal Components and Concurrency
 shown separated by dashed line
 supports concurrency
 Objects must be in only one state from each of the
orthogonal components

Concurrent State Models

Three different ways for orthogonal components to
communicate:

 Broadcast Events
 Propogated Events
 IN operators



 Broadcast events
 events that more than one orthogonal component
accepts
 For example an event T1 is sent to all active orthogonal
components it need not be acted on by all components
 what happens if component S1 is in state A, S2 is in state E
and S3 is in state G when a T1 event occurs?
 what happens if S1 is in state A and S2 is in state D when
the event T1 occurs?



 Propagated events are indicated with the caret
following the event name (and optional parameters
and guard)

 IN operators are used as a guard on transition T4.
This allows the S3 component to take the
transition T4 only if S2 is currently in state D


S

S1 H S3
T1
B

F
A

T4[IN(D)]
C
T2^T3(x,y,z) T5
G

S2 H T3(a,b,c)
T6
H
D T3(a,b,c)

T1 E


Summary

In this chapter we have:
 Introduced the terms used with respect to state
diagrams
 Discussed the context in which state diagrams are
used
 Introduced substates
 Discussed concurrent state diagrams


State Diagrams Chapter Explained

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