Matakuliah
Tahun
: Konsep object-oriented
: 2009
STATE DIAGRAMS
Pertemuan 19
A Student Guide to Object-Oriented
Development
Chapter 7 State Diagrams
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State diagram
In sequence diagrams you are looking at all the classes affected by one
use case
In state diagrams you are looking at one class across all use cases and
how it is effected by the events in the system
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State diagram
• State diagrams are increasingly widely used to model
the ways that a system can respond to external events
• All objects of the class (the individual customers or
products) have the same range of ways in which they
can behave, but the actual way an individual object does
behave during the running of the system depends on the
sequence of events that it experiences.
• State diagrams look at how a class is affected by the
different use cases in the system and how the objects of
the class behave in response to events that affect them.
They model the behaviour of one object across several
use cases.
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State Diagram
• A state diagram only ever describes the behaviour of a single class
illustrating how its objects behave during their lifetimes and how they react
to all uses cases in which they are involved.
• State diagrams also model the order that events affect an object in e.g. in
the Wheels system the bike must be assigned a number before it can be
hired
• In most systems only a few of the classes are complicated enough to merit a
state diagram. These are classes where the response of an object of the class
to a particular event varies depending on the state the object is in at the
time
• State diagrams are also referred to as state charts and state transition
diagrams
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States and Events
• State - The state of the object here refers to the situation it
is in while satisfying some condition (such as a bank
account having some money) or waiting for an event (such
as someone trying to withdraw or deposit money).
• Event - An event is something that happens which has
significance for the system and affects an object of at least
one of the system’s classes. We can tell if an object is in a
particular state by looking at the values of some of its
attributes and its links to other objects.
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The state of an object
• We can tell if an object is in a particular state by looking at the
values of its attributes For example, if a Bank Account object
is in credit the value of the balance attribute will be a positive
amount or zero, but if it is overdrawn the value of balance will
be negative.
• In the Wheels case study, we can tell if a bike is hired out
because there will be a link from the Bike object to an active
Hire object.
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state
start state
stop state
transition from one state to
another
self-transition (no
change of state)
event [guard] / action
transition label (each of the three
parts is optional)
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State diagram for a bank account class
Self
transition
depos it money
stop
state
withdraw money [new balance >= 0]
withdraw balance and
close account
open account and
depos it money
In credit
it money [new
state
diagram
Event
fires depos
balance >= 0]
Start a transition
state
withdraw money [new balance
< 0 and within overdraft limit]
Guard
for a bank account class
depos it money [new balance
< 0 and within overdraft limit]
withdraw money [new balance < 0
and within overdraft lim it]
Overdrawn
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car sensed / check s paces
car leaves[ capacity > s paces ] / inc. s paces
car enters[ >= 1 s pace free ] / dec. s paces
car enters / dec.s paces
Em pty
Spaces free
Event/guard/
action
las t car leaves[ s paces = capacity-1 ] / inc. s paces
car leaves / inc. s paces
car takes last space / dec. s paces
superstate
Full
car park closed
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State diagrams – key words
‘after’ – used to show an event taking place after a
specified time period e.g. after [6 months] / delete
‘when’ – event takes place when a condition is satisfied e.g.
when [all items in stock] / fill order
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State diagrams – entry and exit
events
offHook
entry / start dialling tone
exit / stop dialling tone
do / emit dialling tone
Keyword ‘do’ – behaviour that lasts for the duration of a state is called
an activity, it is modelled using the keyword ‘do’
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