CS12420Sequence Diagrams – a UML

notation for modelling bahaviour

Lynda Thomas

ltt@aber.ac.uk

Images from Wikipedia unless credited or mine

Current UML - recap

• Class diagrams tell you:– the static structure of the system – the relationships between the data– the methods give clues to what can happen

• Object diagrams tell you:– A snapshot of how the system might look at

some point in time

ClassDiagram

ObjectDiagram

How do you model what happens in a software system?

Use Case diagram tells you:– Who the actors are in the system– What they can do

This is really an analysis diagram

But that doesn’t tell you design i.e. in what order things happen

or how to write the methods

Sequence diagrams do that

• Let’s use google

• http://en.wikipedia.org/wiki/Sequence_diagrams

• http://www.ibm.com/developerworks/rational/library/3101.html

• http://www.agilemodeling.com/artifacts/sequenceDiagram.htm

TIME

So the basic idea is that it shows ‘what happens when’

• Phone Example is ‘above’ the specific design level of a program – more like analysis

• Sequence Diagrams can be at any level actually – analysis or design and help you ‘get your head around’ what is happening

• Consider the procedure for applying to university through UCAS – let’s try it

But usually used to look at how objects in an OO system interact

Let’s have a look at the .pdf filehttp://csis.pace.edu/~marchese/CS389/L9/Sequence%20Diagram%20Tutorial.pdf

for the syntax

Cute (but just noticed error) - Three Little Pigshttp://www.tracemodeler.com/articles/pimp-my-diagram-three-little-pigs/index.html?src=rss

Why not just code it?• Sequence diagrams describe single use cases showing

the behaviour across objects of many classes• can see many objects/classes at a time on same page• good sequence diagram is still a bit above the level of

the real code • can be implemented in many different languages • non-coders can do sequence diagrams • easier to do sequence diagrams as a team

An example

CS122 exam: The translation unit. Ordinary users of the unit can look up Welsh and English words and phrases and get their translation. They can also submit documents for translation (in the direction Welsh to English or English to Welsh) and find out whether those translations are complete, in process or not started…….

Let’s add a new functional requirement:

Users can submit and get an emergency translation of a document

0..*

0..*-dictionary

0..*-myDocs

0..1-trans

Application

//runs menu +emerTrans (): void

LinguisticUnit

//a word or phrase

DocumentInfo//information about document

+DocumentInfo(String docName, String whichDirection, String uid, Date dateSubmitted)

+ String getDocName()+ void setCheckedOut(Translator translator)+Translator getTranslator()+ void setCompleted()+ boolean isCompleted()+ Date getDate()+ String toString()

Translator

//stores all info

-String uid-String phone

Model

//holds data1..1

0..*-translators

0..*-allDocs

+doEmerTrans (): void

0..*

0..*-dictionary

0..*-myDocs

0..1-trans

Application

//runs menu +emerTrans (): void

LinguisticUnit

//a word or phrase

DocumentInfo//information about document

+DocumentInfo(String docName, String whichDirection, String uid, Date dateSubmitted)

+ String getDocName()

+ void setCheckedOut(Translator translator)+Translator getTranslator()+ void setCompleted()+ boolean isCompleted()+ Date getDate()+ String toString()

Translator

//stores all info

-String uid-String phone

Model

//holds data1..1

0..*-translators

0..*-allDocs

+getFreeTrans (): Translator

+doEmerTrans (): void

aTrans=

0..*

0..*-dictionary

0..*-myDocs

0..1-trans

Application

//runs menu +emerTrans (): void

LinguisticUnit

//a word or phrase

DocumentInfo//information about document

+DocumentInfo(String docName, String whichDirection, String uid, Date dateSubmitted)

+ String getDocName()

+ void setCheckedOut(Translator translator)+Translator getTranslator()+ void setCompleted()+ boolean isCompleted()+ Date getDate()+ String toString()

Translator

//stores all info

-String uid-String phone

Model

//holds data1..1

0..*-translators

0..*-allDocs

+getFreeTrans (): Translator

+doEmerTrans (): void

+doEmer(DocInfo d): void

Alternatives?Interaction diagrams do the same sort of job

Take an object diagram and draw numbered arrows

I think they are harder to understand but easier to write

Detailed:Initialize total to zeroInitialize counter to zeroInput the first gradewhile user has not as yet entered the sentinel add this grade into the running total

add one to the grade counter input the next grade

(possibly the sentinel)if the counter is not equal to zero

set the average to total /counterprint the average

elseprint 'no grades were entered'

Pseudocode can be used at any level (code w/out syntax)Higher Level: (doEmerTrans() of Model)

aDoc=new DocInfo(…)

aTrans= freetranslator()

checkout aDoc to aTrans

aDoc.setCompleted()

That tells me that I need to look further at freetranslator() (easy)and at checkout() …Decide to use setCheckedout(aTrans) in DocInfo … here is pseudocode

translator=aTranstranslator.doEmer()

Now need to elaborate doEmer() in Translator

Here is a vending machine

State diagrams look at the internals of an object – more next year

Flow charts don’t work as well with objects but can be useful for detailed logic

There are tools for this

• Tools to help produce them

• Tools to produce code from them

• Tools to take code and produce diagrams

What do I honestly use to design?

• Start usually with a rough use case diagram• then a very messy object diagram• then a rough class diagram• then I pick a use case and using my pen move

around the object diagram being sure I can do that use case (bit like interaction diagram

• usually that involves jotting down some pseudocode, which like on previous gives me more methods for the class diagram and more things to pseudocode – quit when it gets easy

But design is also about communication

• What I said on last slide is fine to get started• But ….

– You may be working with others– You may not be the one who is maintaining the system– You may forget what you did and why

So readable design documentation is needed

Worksheet four

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