Modeling interactions and
behavior
Lecturer Dr. Mai Fadel
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Introduction
• An interaction model shows a set of actors
and objects interacting by exchanging
messages.
• A behavior model shows how an object or
system changes state in reaction to series
of events.
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8.1 Interaction diagrams
• Interaction diagrams are used to model the dynamic
aspects of a software system – they help to visualize
how the system runs.
• They show how a set of actors and objects communicate
with each other to perform the steps of a use case or
some other piece of functionality.
• Interaction diagrams can show several types of
communication. e.g. messages exchanged over a
network, simple procedure calls, and commands issued
by an actor through the user interface. Collectively these
are referred to as messages.
• The following elements can be found in an interaction
diagram:
– Instances of classes or actors: use same symbols for objects
and actors
– Messages: shown as arrows. One of the objectives of interaction
diagrams is to better understand the sequence of messages.
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Sequence diagrams
• A sequence diagram shows the sequence of messages
exchanged by the set of objects (and optionally an actor)
performing a certain task.
• Object are arranged from left to right – an actor that
initiates the interaction is often shown on the left.
• Vertical dimension represents time.
• Lifeline: is attached to each object or actor. It becomes
an activation box during the period of time that the object
is performing computations.
• The message is represented as an arrow between
activation boxes of the sender and receiver. (label,
argument list and response). Syntax:
Response:=message(arg,…)
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Example: the process of the registration of a
student in a course
1
*
1
Course
+getPrerequisite()
*
CourseSection
1
Registration
Student
+addToSchedule()
+hasPassedCourse()
+requestToRegister()
+addToRegistrationList()
Top Package::Actor1
*
:CourseSection
:Student
requestToRegister
create
:Registration
addToSchedule
addToRegistrationList
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Example: the process of the registration of a
student in a course – detailed level
:GUI
aStudent:Student
:CourseSection
:Course
requestToRegister
RequestToRegister(a student)
prereq := getPreRequisite
hasPrerequisite := hasPassedCourse(prereq)
opt
[hasPrerequisite] create
:Registration
addToSchedule
addToRegistrationList
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Notes about the example
• The use of combined fragment marked opt.
– Combined fragment is a subsequent of interaction
that is special in some way, and is shown in a box.
– The ‘opt’ label means that it may or may not occur.
– A boolean condition, written in square brackets,
describes the circumstances when it will occur.
– In this example, the condition is placed over the
CourseSection lifline, and indicates that the
subsequence in the combined fragment will only
occur if the hasPrerequisite variable is true.
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Representing loops in a combined fragment
Bill
1
*
Purchase
Item
*
1
:Bill
loop
0..numPurchases
:Purchase
:Item
getSubtotal
getUnitPrice
ComputeTotal
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Destroy symbol in sequence diagrams
:SpecificFlight
:Booking
:PassengerRole
cancelBooking
cancel
deleteFromItinerary
deleteFromPassengerList
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Communication Diagrams
• A communication diagram shows several objects
working together.
• It is very much like an object diagram except that
it shows communication links instead of links of
associations.
• It has much in common with sequence diagrams
• Message: arrow with label, and a prefix number
showing the order of the message
• Communication links can exist between two
objects whenever it is possible for one object to
send a message to the other one.
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1: create
2: addToSchedule
:CourseSection
:Student
:Registration
3: addToRegistrationList
1:requestToRegister(aStudent)
<<local>>
:GUI
2:prereq := getPrerequisite
:Course
:CourseSection
3:hasPrerequisite :=
hasPassedCourse(prereq)
<<parameter>>
4:create
aStudent:Student
6:addToRegistrationList
<<parameter>>
:Registration
5:addToSchedule
<<parameter>>
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How to choose between using a sequence
or a communication diagram
• Sequence diagrams are often the better choice for the
following situations:
– You want the reader to be able to easily see the order in which
the messages occur
– You want to build an interaction model from use case. (they
already have a sequence of steps, SD shows the objects
involved)
– You need to show the details of messages, such as parameters,
and return values. (CD too much clutter)
– You need to show loops, optional sequences and other things
that can only be properly expressed using combined fragments.
• Prefer CD when deriving interaction diagram from class
diagrams. (CD are very similar to object diagrams)
• CD can be used to validate class diagrams: suggest
adding an association in order to make an interaction
possible.
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State diagrams
• Also known as state machine diagram
• It is another way of expressing dynamic information about
a system
• At any given point in time, the system or object is said to
be in a certain state. It remains in this state until an event
occurs that causes it to change state.
• In each state, the system behaves in a different way.
• A transition represents a change of state in response to
an event, and is considered to occur instantaneously –
that is it takes no time.
• Symbols in state diagrams: rounded rectangles, arrows,
labels on arrows, black circle, black circle with a ring
around.
• A start state: when a system starts running, it immediately
takes a transition from the start state to a regular state.
Only one start state and one unlabelled transition from it.
• End state: when the system finishes its work when such a
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state is reached. More than one end state.
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Elapsed-time transition
The event that trigger a transition can be a certain amount
of elapsed time.
after (30s)
• No end state
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Transitions triggered by a condition
becoming true
Cancelled
Closed
State diagram of a CourseSection class
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Activities and actions in state diagrams
• You can present two kinds of computations
using state diagrams: activities and actions.
• Activity is something that occurs over a period of
time while the system is in a state.
– The system may take a transition out of the state in
response to completion of the activity.
– If some other transition is triggered first, then the
system has to terminate the activity as it leaves the
state
– It is shown textually by the word do/, followed by the
description of what is to be done.
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Activities and actions in state diagrams
• An action is something that takes place effectively
instantaneously in any of the following situations:
– When the system takes a particular transition.
– Upon entry to a particular state, no matter which transition
causes entry.
– Upon exit from a particular state no matter which transition is
being taken
• It should take place with no consumption of time =>
simple, such as sending a message, starting a hardware
device, or setting a variable.
• Syntax: event/action, enter/action, exit/action.
Press button
ProposeSelection
MusicPlaying
do/ play chosen
selection
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State diagram for a garage door opener
Partial state diagram for a tape recorder
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