Object-Oriented Software Engineering
Practical Software Development using UML and Java
Chapter 8,
Modelling Interactions and Behaviour
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 2
8.1 Interaction Diagrams
Interaction diagrams are used to model the dynamic
aspects of a software system
? They help you to visualize how the system runs.
? An interaction diagram is often built from a use case and
a class diagram,
— The objective is to show how a set of objects
accomplish the required interactions with an actor.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 3
Interactions and messages
? Interaction diagrams show how a set of actors and
objects communicate with each other to perform:
— The steps of a use case,or
— The steps of some other piece of functionality.
? The set of steps,taken together,is called an interaction.
? Interaction diagrams can show several different types of
communication.
— E.g,method calls,messages send over the network
— These are all referred to as messages.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 4
Elements found in interaction diagrams
? Instances of classes
— Shown as boxes with the class and object identifier
underlined
? Actors
— Use the stick-person symbol as in use case diagrams
? Messages
— Shown as arrows from actor to object,or from object
to object
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 5
Creating instances diagrams
You should develop a class diagram and a use case
model before starting to create an interaction diagram.
? There are two kinds of interaction diagrams,
— Sequence diagrams
— Collaboration diagrams
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 6
Sequence diagrams – an example
requestToRegister
addToSchedule
<<create>>
:Student
:Registration
:CourseSection
addToRegistrationList
* * *CourseSection
requestToRegister
RegistrationCourse
getPrerequisite
Student
addToSchedule
hasPassedCourseaddToRegistrationList
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 7
Sequence diagrams
A sequence diagram shows the sequence of messages exchanged by
the set of objects performing a certain task
? The objects are arranged horizontally across the diagram.
? An actor that initiates the interaction is often shown on the left.
? The vertical dimension represents time.
? A vertical line,called a lifeline,is attached to each object or actor.
? The lifeline becomes a broad box,called an activation box during
the live activation period.
? A message is represented as an arrow between activation boxes of
the sender and receiver.
— A message is labelled and can have an argument list and a
return value.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 8
Sequence diagrams –
same example,more details
requestToRegister
(aStudent)
addToSchedule
aStudent:
Student
:Registration
:CourseSectionGUI
requestToRegister
:Course
[hasPrerequisite]
<<create>>
prereq,=
getPrerequisitehasPrerequisite,=
hasPassedCourse(prereq)
addToRegistrationList
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 9
? An iteration over objects is indicated by an asterisk
preceding the message name
* *P u r c h a s e
q u a n t i t y
B i ll I t e m
p r i c e
* [ f o r a ll P u r c h a s e ] g e t S u b t o t a l
,I t e m,B i ll
g e t U n it P r i c e
c o m p u t e T o t a l
,P u r c h a s e
Sequence diagrams –
an example with replicated messages
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 10
? If an object’s life ends,this is shown with an X at the
end of the lifeline
Sequence diagrams –
an example with object deletion
c a n c e lB o o k i n g
d e le t e F ro m I t i n e ra ry
c a n c e l
,P a s s e n g e r R o le,B o o k i n g,S p e c i f i c F li g h t
d e le t e F ro m P a s s e n g e r L is t
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 11
Collaboration diagrams – an example
2,addToSchedule1,<<create>>
:Student:Registration:CourseSection
3,addToRegistrationList
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 12
Collaboration diagrams
Collaboration diagrams emphasise how the objects
collaborate in order to realize an interaction
? A collaboration diagram is a graph with the objects as
the vertices.
? Communication links are added between objects
? Messages are attached to these links.
— Shown as arrows labelled with the message name
? Time ordering is indicated by prefixing the message with
some numbering scheme.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 13
Collaboration diagrams –
same example,more details
1,requestToRegister(aStudent)
<<local>>
5,addToSchedule
<<parameter>>
aStudent:
Student
:Registration
:CourseSectionGUI,Course
4,[hasPrerequisite]
<<create>>
2,prereq,= getPrerequisite
3,hasPrerequisite,=
hasPassedCourse(prereq)
<<parameter>>
5,addToRegistrationList
<<parameter>>
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 14
Communication links
? A communication link can exist between two objects
whenever it is possible for one object to send a message
to the other one.
? Several situations can make this message exchange
possible:
1,The classes of the two objects have an association
between them,
- This is the most common case.
- If all messages are sent in the same direction,then probably
the association can be made unidirectional.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 15
Other communication links
2,The receiving object is stored in a local variable of
the sending method.
- This often happens when the object is created in the sending
method or when some computation returns an object,
- The stereotype to be used is ?local? or [L].
3,A reference to the receiving object has been received
as a parameter of the sending method,
- The stereotype is ?parameter? or [P].
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 16
Other communication links
4,The receiving object is global,
- This is the case when a reference to an object can be
obtained using a static method,
- The stereotype ?global?,or a [G] symbol is used in this
case.
5,The objects communicate over a network.
- We suggest to write ?network?.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 17
How to choose between using a sequence
or collaboration diagram
Sequence diagrams
? Make explicit the time ordering of the interaction.
— Use cases make time ordering explicit too
— So sequence diagrams are a natural choice when you
build an interaction model from a use case.
? Make it easy to add details to messages.
— Collaboration diagrams have less space for this
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 18
How to choose between using a sequence
or collaboration diagram
Collaboration diagrams
? Can be seen as a projection of the class diagram
— Might be preferred when you are deriving an
interaction diagram from a class diagram.
— Are also useful for validating class diagrams.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 19
Collaboration diagrams and patterns
A collaboration diagram can be used to represent aspects of
a design pattern
S t u d e n t
P e r s i s t e n t S t u d e n t
,C li e n t, P r o x y
1,r e qu e s t
2,[ i nf or m at io n ne e de d
an d no t lo ad ed ]
l oa d H ea v y W e ig ht
,H e a v y W e i g h t
P ro x y
C o u r s e S e c t io n
a)
b)
pr ox y he av y W ei gh t
c l i en t
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 20
8.2 State Diagrams
A state diagram describes the behaviour of a system,
some part of a system,or an individual object.
? At any given point in time,the system or object is in a
certain state.
— Being in a state means that it is will behave in a
specific way in response to any events that occur.
? Some events will cause the system to change state.
— In the new state,the system will behave in a
different way to events.
? A state diagram is a directed graph where the nodes are
states and the arcs are transitions.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 21
State diagrams – an example
? tic-tac-toe game
OTurn
XTurn
Tie
OWin
XWin
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 22
States
? At any given point in time,the system is in one state.
? It will remain in this state until an event occurs that
causes it to change state.
? A state is represented by a rounded rectangle containing
the name of the state.
? Special states:
— A black circle represents the start state
— A circle with a ring around it represents an end state
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 23
Transitions
? A transition represents a change of state in response to
an event.
— It is considered to occur instantaneously.
? The label on each transition is the event that causes the
change of state.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 24
after(30s)
after(5s)
after(25s)
RedLight
GreenLight
YellowLight
after(25s since exit from
state RedLight)
RedLight
GreenLightChangeTriggered
YellowLight
after(5s)
after(30s)
vehicleWaitingToTurn
GreenLightNoTrigger
State diagrams – an example of transitions
with time-outs and conditions
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 25
State diagrams – an example with
conditional transitions
requestToRegister
(aStudent)
/createRegistration
Closed
classSize >= maximum
cancel
openRegistration
Planned
OpenEnoughStudents
OpenNotEnoughStudents
classSize >= minimum
requestToRegister
(aStudent)
/createRegistration
closeRegistration
closeRegistration
cancel
Cancelled
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 26
Activities in state diagrams
? An activity is something that takes place while the
system is in a state,
— It takes a period of time,
— The system may take a transition out of the state in
response to completion of the activity,
— Some other outgoing transition may result in:
- The interruption of the activity,and
- An early exit from the state.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 27
State diagram – an example with activity
press button
do:
play chosen
selection
MusicPlayingProposeSelection
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 28
Actions in state diagrams
? An action is something that takes place effectively
instantaneously
— When a particular transition is taken,
— Upon entry into a particular state,or
— Upon exit from a particular state
? An action should consume no noticeable amount of time
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 29
State diagram – an example with actions
Enter /
run motor forwards
Enter /
run motor in reverse
Enter /
stop motor
Enter /
stop motor
openingCompleted
closingCompleted
pressButton
pressButton
pressButton
Closing Open
OpeningClosed
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 30
State diagrams – another example
E x i t / s t o p
e n d O f P r o g r a m
s t a r t O f T a p e / s t o p
e n d O f T a p e
W a i t
R e w i n d i n gR e c o r d i n g
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 31
Nested substates and guard conditions
A state diagram can be nested inside a state,
? The states of the inner diagram are called substates.
First Second Third
ReverseNeutral
selectDrive
reachThirdSpeed
[driveSelected]
dropBelowThirdSpeed
reachSecondSpeed
[driveSelected]
dropBelowSecondSpeed
[driveSelected]
selectFirst selectSecond selectNeutral
selectNeutral
selectReverse
selectFirst selectSecond
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 32
State diagram – an example with substates
requestToRegister
(aStudent)
/createRegistration
openRegistration
Planned
EnoughStudents
NotEnoughStudents
classSize >= minimum
Open
Closed
classSize >= maximum
cancel
closeRegistration
closeRegistration
Cancelled
do:
unregister
students
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 33
8.3 Activity Diagrams
? An activitydiagram is like a state diagram.
— Except most transitions are caused by internal events,such as
the completion of a computation.
? An activity diagram
— Can be used to understand the flow of work that an object or
component performs,
— Can also be used to visualize the interrelation and interaction
between different use cases.
— Is most often associated with several classes.
? One of the strengths of activity diagrams is the representation of
concurrent activities.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 34
Activity diagrams – an example
Verify
course not
full
Receive course
registration request
Check
prerequisites
Check
special
permission
Complete
registration
[ok]
[ok]
[ok][not ok]
[not ok]
[not ok]
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 35
Representing concurrency
? Concurrency is shown using forks,joins and rendezvous.
— A fork has one incoming transition and multiple
outgoing transitions.
- The execution splits into two concurrent threads.
— A rendezvous has multiple incoming and multiple
outgoing transitions.
- Once all the incoming transitions occur all the outgoing
transitions may occur.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 36
Representing concurrency
— A join has multiple incoming transitions and one
outgoing transition.
- The outgoing transition will be taken when all incoming
transitions have occurred.
- The incoming transitions must be triggered in separate
threads.
- If one incoming transition occurs,a wait condition occurs at
the join until the other transitions occur.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 37
Swimlanes
Activity diagrams are most often associated with several
classes,
? The partition of activities among the existing classes can
be explicitly shown using swimlanes.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 38
Activity diagrams – an example with
swimlanes
Verify
course not
full
Receive course
registration request
Check
prerequisites
Check
special
permission
Complete
registration
[ok]
[ok]
[ok][not ok]
[not ok]
[not ok]
Student CourseSection
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 39
8.4 Implementing Classes Based on
Interaction and State Diagrams
? You should use these diagrams for the parts of your
system that you find most complex.
— I.e,not for every class
? Interaction,activity and state diagrams help you create a
correct implementation.
? This is particularly true when behaviour is distributed
across several use cases.
— E.g,a state diagram is useful when different
conditions cause instances to respond differently to
the same event.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 40
Example,The CourseSectionclass
States,
? ‘Planned’:
—closedOrCancelled == false && open
== false
? ‘Cancelled’:
—closedOrCancelled == true &&
registrationList.size() == 0
? ‘Closed’ (course section is too full,or being taught):
—closedOrCancelled == true &&
registrationList.size() > 0
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 41
Example,The CourseSectionclass
States,
? ‘Open’ (accepting registrations):
—open == true
? ‘NotEnoughStudents’ (substate of ‘Open’):
—open == true &&
registrationList.size() <
course.getMinimum()
? ‘EnoughStudents’ (substate of ‘Open’):
—open == true &&
registrationList.size() >=
course.getMinimum()
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 42
Example,The CourseSectionclass
* * *CourseSection
requestToRegister
RegistrationCourse
getPrerequisite
Student
addToSchedule
hasPassedCourseaddToRegistrationList
Class diagram
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 43
Example,The CourseSectionclass
public class CourseSection
{
// The many-1 abstraction-occurence association
private Course course;
// The 1-many association to class Registration
private List registationList;
// The following are present only to determine
// the state
// The initial state is ‘Planned’
private boolean open = false;
private boolean closedOrCancelled = false;
...
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 44
Example,The CourseSectionclass
public CourseSection(Course course)
{
this.course = course;
RegistrationList = new LinkedList();
}
public void cancel()
{
// to ‘Cancelled’ state
open = false;
closedOrCancelled = true;
unregisterStudents();
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 45
Example,The CourseSectionclass
public void openRegistration()
{
if(!closedOrCancelled)
// must be in ‘Planned’ state
{
open = true;
// to 'OpenNotEnoughStudents' state
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 46
Example,The CourseSectionclass
public void closeRegistration()
{
// to 'Cancelled' or 'Closed' state
open = false;
closedOrCancelled = true;
if (registrationList.size() <
course.getMinimum())
{
unregisterStudents();
// to ‘Cancelled’ state
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 47
Example,The CourseSectionclass
public void requestToRegister(Student student)
{
if (open) // must be in one of the two 'Open' states
{
// The interaction specified in the sequence diagram
Course prereq = course.getPrerequisite();
if (student.hasPassedCourse(prereq))
{
// Indirectly calls addToRegistrationList
new Registration(this,student);
}
// Check for automatic transition to 'Closed' state
if (registrationList.size() >= course.getMaximum())
{
// to ‘Closed’ state
open = false;
closedOrCancelled = true;
}
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 48
Example,The CourseSectionclass
// Activity associated with ‘Cancelled’ state.
private void unregisterStudents()
{
Iterator it = registrationList.iterator();
while (it.hasNext())
{
Registration r = (Registration)it.next();
r.unregisterStudent();
it.remove();
}
}
// Called within this package only,by the
// constructor of Registration
void addToRegistrationList(
Registration newRegistration)
{
registrationList.add(newRegistration);
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 49
8.5 Difficulties and Risks in Modelling
Interactions and Behaviour
Dynamic modelling is a difficult skill
? In a large system there are a very large number of possible paths a
system can take.
? It is hard to choose the classes to which to allocate each behaviour:
— Ensure that skilled developers lead the process,and ensure that
all aspects of your models are properly reviewed,
— Work iteratively,
- Develop initial class diagrams,use cases,responsibilities,
interaction diagrams and state diagrams;
- Then go back and verify that all of these are consistent,modifying
them as necessary,
— Drawing different diagrams that capture related,but distinct,
information will often highlight problems,
Practical Software Development using UML and Java
Chapter 8,
Modelling Interactions and Behaviour
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 2
8.1 Interaction Diagrams
Interaction diagrams are used to model the dynamic
aspects of a software system
? They help you to visualize how the system runs.
? An interaction diagram is often built from a use case and
a class diagram,
— The objective is to show how a set of objects
accomplish the required interactions with an actor.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 3
Interactions and messages
? Interaction diagrams show how a set of actors and
objects communicate with each other to perform:
— The steps of a use case,or
— The steps of some other piece of functionality.
? The set of steps,taken together,is called an interaction.
? Interaction diagrams can show several different types of
communication.
— E.g,method calls,messages send over the network
— These are all referred to as messages.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 4
Elements found in interaction diagrams
? Instances of classes
— Shown as boxes with the class and object identifier
underlined
? Actors
— Use the stick-person symbol as in use case diagrams
? Messages
— Shown as arrows from actor to object,or from object
to object
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 5
Creating instances diagrams
You should develop a class diagram and a use case
model before starting to create an interaction diagram.
? There are two kinds of interaction diagrams,
— Sequence diagrams
— Collaboration diagrams
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 6
Sequence diagrams – an example
requestToRegister
addToSchedule
<<create>>
:Student
:Registration
:CourseSection
addToRegistrationList
* * *CourseSection
requestToRegister
RegistrationCourse
getPrerequisite
Student
addToSchedule
hasPassedCourseaddToRegistrationList
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 7
Sequence diagrams
A sequence diagram shows the sequence of messages exchanged by
the set of objects performing a certain task
? The objects are arranged horizontally across the diagram.
? An actor that initiates the interaction is often shown on the left.
? The vertical dimension represents time.
? A vertical line,called a lifeline,is attached to each object or actor.
? The lifeline becomes a broad box,called an activation box during
the live activation period.
? A message is represented as an arrow between activation boxes of
the sender and receiver.
— A message is labelled and can have an argument list and a
return value.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 8
Sequence diagrams –
same example,more details
requestToRegister
(aStudent)
addToSchedule
aStudent:
Student
:Registration
:CourseSectionGUI
requestToRegister
:Course
[hasPrerequisite]
<<create>>
prereq,=
getPrerequisitehasPrerequisite,=
hasPassedCourse(prereq)
addToRegistrationList
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 9
? An iteration over objects is indicated by an asterisk
preceding the message name
* *P u r c h a s e
q u a n t i t y
B i ll I t e m
p r i c e
* [ f o r a ll P u r c h a s e ] g e t S u b t o t a l
,I t e m,B i ll
g e t U n it P r i c e
c o m p u t e T o t a l
,P u r c h a s e
Sequence diagrams –
an example with replicated messages
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 10
? If an object’s life ends,this is shown with an X at the
end of the lifeline
Sequence diagrams –
an example with object deletion
c a n c e lB o o k i n g
d e le t e F ro m I t i n e ra ry
c a n c e l
,P a s s e n g e r R o le,B o o k i n g,S p e c i f i c F li g h t
d e le t e F ro m P a s s e n g e r L is t
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 11
Collaboration diagrams – an example
2,addToSchedule1,<<create>>
:Student:Registration:CourseSection
3,addToRegistrationList
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 12
Collaboration diagrams
Collaboration diagrams emphasise how the objects
collaborate in order to realize an interaction
? A collaboration diagram is a graph with the objects as
the vertices.
? Communication links are added between objects
? Messages are attached to these links.
— Shown as arrows labelled with the message name
? Time ordering is indicated by prefixing the message with
some numbering scheme.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 13
Collaboration diagrams –
same example,more details
1,requestToRegister(aStudent)
<<local>>
5,addToSchedule
<<parameter>>
aStudent:
Student
:Registration
:CourseSectionGUI,Course
4,[hasPrerequisite]
<<create>>
2,prereq,= getPrerequisite
3,hasPrerequisite,=
hasPassedCourse(prereq)
<<parameter>>
5,addToRegistrationList
<<parameter>>
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 14
Communication links
? A communication link can exist between two objects
whenever it is possible for one object to send a message
to the other one.
? Several situations can make this message exchange
possible:
1,The classes of the two objects have an association
between them,
- This is the most common case.
- If all messages are sent in the same direction,then probably
the association can be made unidirectional.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 15
Other communication links
2,The receiving object is stored in a local variable of
the sending method.
- This often happens when the object is created in the sending
method or when some computation returns an object,
- The stereotype to be used is ?local? or [L].
3,A reference to the receiving object has been received
as a parameter of the sending method,
- The stereotype is ?parameter? or [P].
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 16
Other communication links
4,The receiving object is global,
- This is the case when a reference to an object can be
obtained using a static method,
- The stereotype ?global?,or a [G] symbol is used in this
case.
5,The objects communicate over a network.
- We suggest to write ?network?.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 17
How to choose between using a sequence
or collaboration diagram
Sequence diagrams
? Make explicit the time ordering of the interaction.
— Use cases make time ordering explicit too
— So sequence diagrams are a natural choice when you
build an interaction model from a use case.
? Make it easy to add details to messages.
— Collaboration diagrams have less space for this
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 18
How to choose between using a sequence
or collaboration diagram
Collaboration diagrams
? Can be seen as a projection of the class diagram
— Might be preferred when you are deriving an
interaction diagram from a class diagram.
— Are also useful for validating class diagrams.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 19
Collaboration diagrams and patterns
A collaboration diagram can be used to represent aspects of
a design pattern
S t u d e n t
P e r s i s t e n t S t u d e n t
,C li e n t, P r o x y
1,r e qu e s t
2,[ i nf or m at io n ne e de d
an d no t lo ad ed ]
l oa d H ea v y W e ig ht
,H e a v y W e i g h t
P ro x y
C o u r s e S e c t io n
a)
b)
pr ox y he av y W ei gh t
c l i en t
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 20
8.2 State Diagrams
A state diagram describes the behaviour of a system,
some part of a system,or an individual object.
? At any given point in time,the system or object is in a
certain state.
— Being in a state means that it is will behave in a
specific way in response to any events that occur.
? Some events will cause the system to change state.
— In the new state,the system will behave in a
different way to events.
? A state diagram is a directed graph where the nodes are
states and the arcs are transitions.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 21
State diagrams – an example
? tic-tac-toe game
OTurn
XTurn
Tie
OWin
XWin
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 22
States
? At any given point in time,the system is in one state.
? It will remain in this state until an event occurs that
causes it to change state.
? A state is represented by a rounded rectangle containing
the name of the state.
? Special states:
— A black circle represents the start state
— A circle with a ring around it represents an end state
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 23
Transitions
? A transition represents a change of state in response to
an event.
— It is considered to occur instantaneously.
? The label on each transition is the event that causes the
change of state.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 24
after(30s)
after(5s)
after(25s)
RedLight
GreenLight
YellowLight
after(25s since exit from
state RedLight)
RedLight
GreenLightChangeTriggered
YellowLight
after(5s)
after(30s)
vehicleWaitingToTurn
GreenLightNoTrigger
State diagrams – an example of transitions
with time-outs and conditions
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 25
State diagrams – an example with
conditional transitions
requestToRegister
(aStudent)
/createRegistration
Closed
classSize >= maximum
cancel
openRegistration
Planned
OpenEnoughStudents
OpenNotEnoughStudents
classSize >= minimum
requestToRegister
(aStudent)
/createRegistration
closeRegistration
closeRegistration
cancel
Cancelled
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 26
Activities in state diagrams
? An activity is something that takes place while the
system is in a state,
— It takes a period of time,
— The system may take a transition out of the state in
response to completion of the activity,
— Some other outgoing transition may result in:
- The interruption of the activity,and
- An early exit from the state.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 27
State diagram – an example with activity
press button
do:
play chosen
selection
MusicPlayingProposeSelection
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 28
Actions in state diagrams
? An action is something that takes place effectively
instantaneously
— When a particular transition is taken,
— Upon entry into a particular state,or
— Upon exit from a particular state
? An action should consume no noticeable amount of time
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 29
State diagram – an example with actions
Enter /
run motor forwards
Enter /
run motor in reverse
Enter /
stop motor
Enter /
stop motor
openingCompleted
closingCompleted
pressButton
pressButton
pressButton
Closing Open
OpeningClosed
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 30
State diagrams – another example
E x i t / s t o p
e n d O f P r o g r a m
s t a r t O f T a p e / s t o p
e n d O f T a p e
W a i t
R e w i n d i n gR e c o r d i n g
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 31
Nested substates and guard conditions
A state diagram can be nested inside a state,
? The states of the inner diagram are called substates.
First Second Third
ReverseNeutral
selectDrive
reachThirdSpeed
[driveSelected]
dropBelowThirdSpeed
reachSecondSpeed
[driveSelected]
dropBelowSecondSpeed
[driveSelected]
selectFirst selectSecond selectNeutral
selectNeutral
selectReverse
selectFirst selectSecond
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 32
State diagram – an example with substates
requestToRegister
(aStudent)
/createRegistration
openRegistration
Planned
EnoughStudents
NotEnoughStudents
classSize >= minimum
Open
Closed
classSize >= maximum
cancel
closeRegistration
closeRegistration
Cancelled
do:
unregister
students
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 33
8.3 Activity Diagrams
? An activitydiagram is like a state diagram.
— Except most transitions are caused by internal events,such as
the completion of a computation.
? An activity diagram
— Can be used to understand the flow of work that an object or
component performs,
— Can also be used to visualize the interrelation and interaction
between different use cases.
— Is most often associated with several classes.
? One of the strengths of activity diagrams is the representation of
concurrent activities.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 34
Activity diagrams – an example
Verify
course not
full
Receive course
registration request
Check
prerequisites
Check
special
permission
Complete
registration
[ok]
[ok]
[ok][not ok]
[not ok]
[not ok]
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 35
Representing concurrency
? Concurrency is shown using forks,joins and rendezvous.
— A fork has one incoming transition and multiple
outgoing transitions.
- The execution splits into two concurrent threads.
— A rendezvous has multiple incoming and multiple
outgoing transitions.
- Once all the incoming transitions occur all the outgoing
transitions may occur.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 36
Representing concurrency
— A join has multiple incoming transitions and one
outgoing transition.
- The outgoing transition will be taken when all incoming
transitions have occurred.
- The incoming transitions must be triggered in separate
threads.
- If one incoming transition occurs,a wait condition occurs at
the join until the other transitions occur.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 37
Swimlanes
Activity diagrams are most often associated with several
classes,
? The partition of activities among the existing classes can
be explicitly shown using swimlanes.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 38
Activity diagrams – an example with
swimlanes
Verify
course not
full
Receive course
registration request
Check
prerequisites
Check
special
permission
Complete
registration
[ok]
[ok]
[ok][not ok]
[not ok]
[not ok]
Student CourseSection
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 39
8.4 Implementing Classes Based on
Interaction and State Diagrams
? You should use these diagrams for the parts of your
system that you find most complex.
— I.e,not for every class
? Interaction,activity and state diagrams help you create a
correct implementation.
? This is particularly true when behaviour is distributed
across several use cases.
— E.g,a state diagram is useful when different
conditions cause instances to respond differently to
the same event.
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 40
Example,The CourseSectionclass
States,
? ‘Planned’:
—closedOrCancelled == false && open
== false
? ‘Cancelled’:
—closedOrCancelled == true &&
registrationList.size() == 0
? ‘Closed’ (course section is too full,or being taught):
—closedOrCancelled == true &&
registrationList.size() > 0
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 41
Example,The CourseSectionclass
States,
? ‘Open’ (accepting registrations):
—open == true
? ‘NotEnoughStudents’ (substate of ‘Open’):
—open == true &&
registrationList.size() <
course.getMinimum()
? ‘EnoughStudents’ (substate of ‘Open’):
—open == true &&
registrationList.size() >=
course.getMinimum()
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 42
Example,The CourseSectionclass
* * *CourseSection
requestToRegister
RegistrationCourse
getPrerequisite
Student
addToSchedule
hasPassedCourseaddToRegistrationList
Class diagram
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 43
Example,The CourseSectionclass
public class CourseSection
{
// The many-1 abstraction-occurence association
private Course course;
// The 1-many association to class Registration
private List registationList;
// The following are present only to determine
// the state
// The initial state is ‘Planned’
private boolean open = false;
private boolean closedOrCancelled = false;
...
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 44
Example,The CourseSectionclass
public CourseSection(Course course)
{
this.course = course;
RegistrationList = new LinkedList();
}
public void cancel()
{
// to ‘Cancelled’ state
open = false;
closedOrCancelled = true;
unregisterStudents();
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 45
Example,The CourseSectionclass
public void openRegistration()
{
if(!closedOrCancelled)
// must be in ‘Planned’ state
{
open = true;
// to 'OpenNotEnoughStudents' state
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 46
Example,The CourseSectionclass
public void closeRegistration()
{
// to 'Cancelled' or 'Closed' state
open = false;
closedOrCancelled = true;
if (registrationList.size() <
course.getMinimum())
{
unregisterStudents();
// to ‘Cancelled’ state
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 47
Example,The CourseSectionclass
public void requestToRegister(Student student)
{
if (open) // must be in one of the two 'Open' states
{
// The interaction specified in the sequence diagram
Course prereq = course.getPrerequisite();
if (student.hasPassedCourse(prereq))
{
// Indirectly calls addToRegistrationList
new Registration(this,student);
}
// Check for automatic transition to 'Closed' state
if (registrationList.size() >= course.getMaximum())
{
// to ‘Closed’ state
open = false;
closedOrCancelled = true;
}
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 48
Example,The CourseSectionclass
// Activity associated with ‘Cancelled’ state.
private void unregisterStudents()
{
Iterator it = registrationList.iterator();
while (it.hasNext())
{
Registration r = (Registration)it.next();
r.unregisterStudent();
it.remove();
}
}
// Called within this package only,by the
// constructor of Registration
void addToRegistrationList(
Registration newRegistration)
{
registrationList.add(newRegistration);
}
}
? Lethbridge/Laganière 2001 Chapter 8,Modelling Interactions and Behaviour 49
8.5 Difficulties and Risks in Modelling
Interactions and Behaviour
Dynamic modelling is a difficult skill
? In a large system there are a very large number of possible paths a
system can take.
? It is hard to choose the classes to which to allocate each behaviour:
— Ensure that skilled developers lead the process,and ensure that
all aspects of your models are properly reviewed,
— Work iteratively,
- Develop initial class diagrams,use cases,responsibilities,
interaction diagrams and state diagrams;
- Then go back and verify that all of these are consistent,modifying
them as necessary,
— Drawing different diagrams that capture related,but distinct,
information will often highlight problems,
