System Sequence Diagrams

Recap

• When to create SSD?• How to identify classes/instances?• Use case descriptions• UML notations for SSD

Contents

• Interaction diagrams• UML notation• Examples

Interaction Diagrams

• Show how objects interact with one another

• UML supports two types of interaction diagrams Sequence diagramsCollaboration diagrams

Sequence Diagrams• AKA Interaction Diagrams – Semantically

equivalent to Collaboration Diagrams• Dynamic Model relating use cases and class

diagrams• Illustrates how objects interacts with each other• Shows time ordering of interactions• Generally a set of messages between

collaborating objects• Ordering of objects not significant

Sequence Diagrams• Show only one flow of control• Can model simple sequential flow,

branching, iteration, recursion and concurrency

• May need multiple diagrams– Primary– Variant– Exceptions

UML sequence diagrams

• sequence diagram: an "interaction diagram" that models a single scenario executing in the system– perhaps 2nd most used UML diagram (behind

class diagram)

• relation of UML diagrams to other exercises:– CRC cards -> class diagram– use cases -> sequence diagrams

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Key parts of a sequence diag.• participant: an object or entity that acts in the

sequence diagram– sequence diagram starts with an unattached "found

message" arrow

• message: communication between participant objects

• the axes in a sequence diagram:– horizontal: which object/participant is acting– vertical: time (down -> forward in time)

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Sequence Diagram (Basic)

Object : Class or Actor

Lifeline

message

name

X ObjectDestruction/Termination

<<create>><<destroy>>

Focus of Control/Activation

Sequence Diagram (Basic)

Student

aClass:Class

Register

:Scheduler

adjustRoom checkRooms

Sequence Diagrams (Advanced)

Seq# [Guard] *[Iteration] Return-List := Operation-Name (Argument-List)

recursion

*[Iteration Condition]

ConditionalLifeline {transient}

Sequence Diagram(make a phone call)

Caller Phone Recipient

Picks up

Dial tone

Dial

Ring notification Ring

Picks up

Hello

Sequence Diagrams – Object Life Spans

• Creation Create message Object life starts at that point

• Activation Symbolized by rectangular stripes Place on the lifeline where object is

activated. Rectangle also denotes when object

is deactivated.• Deletion

Placing an ‘X’ on lifeline Object’s life ends at that point

Activation bar

A

BCreate

XDeletion

Return

Lifeline

Sequence diag. from use case

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Lifetime of objects• creation: arrow with

'new' written above it– notice that an object

created after the start of the scenario appears lower than the others

• deletion: an X at bottom of object's lifeline– Java doesn't explicitly

delete objects; they fall out of scope and are garbage-collected

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Representing objects

• squares with object type, optionally preceded by object name and colon– write object's name if it clarifies the diagram– object's "life line" represented by dashed vert. line

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Messages between objects• message (method call) indicated by horizontal arrow

to other object– write message name and arguments above arrow

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Messages between objects– dashed arrow back indicates return– different arrowheads for normal / concurrent

(asynchronous) methods

Example

Example

Example

Summary

• SSD revision• Loops• Conditions in SSD