A Student Guide to ObjectOriented Development

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A Student Guide to Object-Oriented Development

• Chapter 7 State Diagrams

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State diagram
A state diagram shows the effects of all use
cases on one class
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State diagram

• Increasingly used to model ways a system can
  respond to external events
• All objects of the class (the individual
  customers or products) have the same set of
  responses, but each responds differently
  depending on the state it is in.

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State Diagram

• Describes the behaviour of a single class
  illustrating how its objects behave during their
  lifetimes ( all uses cases )
• Model the order that events affect an object
• The bike must be assigned a number before it can
  be rented
• In most systems only a few of the classes are
  complicated enough to merit a state diagram.
• State diagrams are also referred to as state
  charts and state transition diagrams

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States and Events

• State - The situation an object 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).
  
• State has duration.
• Event - An event is something that happens which
  has significance for the system and affects an
  object of at least one of the systems 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.
• Events are instantaneous.

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The state of an object

• 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 rented 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
stop state
Self transition
Guard
state diagram for a bank account class
Event fires a transition
Start state
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Event/guard/action
superstate
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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