1 a student guide to object- oriented development chapter 7 state diagrams

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A Student Guide to Object-A Student Guide to Object-Oriented DevelopmentOriented 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 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.

State diagramState diagram

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State DiagramState 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 EventsStates 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 objectThe 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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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)

state

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state diagram for a bank account class

State diagram for a bank account class

In credit

Overdrawn

open account and deposit money

deposit money

withdraw money [new balance < 0 and within overdraft limit]

deposit money [new balance >= 0]

deposit money [new balance < 0 and within overdraft limit]

withdraw money [new balance >= 0]

withdraw balance and close account

withdraw money [new balance < 0 and within overdraft limit]

Start state

stop state

Event fires a transition

Self transition

Guard

In the "In credit" stateIn the "In credit" state

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UML and C++ A Practical Guide To Object-Oriented Development

State ModelState ModelState Model describes the sequence of

states that an object goes through during its lifetime in response to events.

Includes the objects responses to events– Actions

• Single act performed by the object

– Activities• Continuous set of actions performed by an object


StateState

This represents a stage in the lifecycle of an object.

Technically the period of time during which an object is waiting for an event to occur.

The internal state of an object is the combination of the data values of the attributes of object.


EventEvent

An event is a condition that can be detected by an object.

From an programmatic perspective– An event is a one way asynchronous

transmission of information from one object to another.

– The information may have parameters with names and types as part of the message sent


TransitionTransition

A transition is a response by an object to an event received by it. – The response can be

• a change to a new object state

• a change to the same object state

• the performance of some action

The ability of the object to respond to a given event is described by guard conditions.


ActionAction

An action is an activity or operation that is done inside a state or on a transition between states

An action is– atomic– instantaneous

An action can occur– during a transition– on entry to a state, – during the entire period an object is in a state,– on exit from a state,– on arrival of an event that does not cause a state

transition


ActivityActivity

An activity is an operation or set of operations that executes during the entire time an object is in a state.

An activity is not atomic.

An activity can be interrupted by an event while it is executing

Events for Bike ObjectEvents for Bike Object

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Completed state diagram for Bike Completed state diagram for Bike ObjectObject

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State diagrams – key wordsState 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 eventsState diagrams – entry and exit events

offHook

entry / start dialling toneexit / stop dialling tonedo / 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’


Nested State DiagramNested State Diagram

Simple State

Complex State

Substate1

entry: entry action

Substate2

Substate1

entry: entry action

Substate2

event( args )[ cond ] / action t̂arget.event(args)event( args )[ cond ] /

action t̂arget.event(args)

event( args )[ cond ] / action t̂arget.event(args)

event( args )[ cond ] / action t̂arget.event(args)


Nested Concurrent SubstatesNested Concurrent Substates

filled out

Course registration request

partially filled out

SSN empty

validatingentry / check SSN

userentersSSN

SSNvalid

SSN notvalid /clear SSN

Course id empty

validatingentry / check cid

userenterscid

cidvalid

cid notvalid /clear cid

emptyentry / clear SSN and course id This state will not

be entered untilboth the SSN andCourse id fields arefilled in and valid


Nested Concurrent SubstatesNested Concurrent Substates

waiting for one field to be filled in

SSN empty validatingentry / check SSN

user enters SSN

SSN valid

HRid valid

HRid not valid /clear HRid

SSN not valid/ clear SSN

HRid empty validatingentry / check HRid

user enters HRid

filled out

emptyentry / clear SSN and HRid


Mealy model for Microwave OvenMealy model for Microwave Oven


Moore Moore model model

1 a student guide to object- oriented development chapter 7 state diagrams

Documents

state diagrama state

particular state

credit state

state charts

state transition diagrams

time state diagrams

use casein state diagrams

bike object