State and Sequence Diagrams
Modelling dynamic information
 So far we have seen:
Use Case Diagrams – requirements capture,
interface modelling
Use Case Scripts – more detailed and with
alternate, less common cases noted
Class Diagrams – description of the structure of a
class and possibly its relationship with other
classes
 These are static descriptions of software
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State and Sequence Diagrams
Dynamic Modelling
In order to model the dynamic (run-time)
relationships we need different tools
The UML provides us with State Diagrams and
Object Sequence Diagrams (OSD)
There are others...
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State and Sequence Diagrams
State Diagrams
What do state diagrams describe?
When things happen
The life-cycle of an instance (object)
How useful are they?
Often but not always used
Logically links a Class Diagram to a State Diagram –
events are operations on classes in the Class
Diagram
Clarifies the internal state and external behaviour of
an object
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State and Sequence Diagrams
State Diagram Elements
start point
states
end point(s)
transitions (labeled with events)
create
invoice
Unpaid
pay
Paid
destroy
invoice
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State and Sequence Diagrams
State Diagram examples
mate
Black wins
White's turn
stalemate
move
move
Black's turn
Draw
stalemate
mate
White wins
A chess game class
White takes the first turn, then turns alternate
After any move “mate” or “stalemate” may occur
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State and Sequence Diagrams
State Change Events
A state describes a particular condition of an
object for a period of time – it waits for an event
to change state or to “fire”
Events can be of the following types:
Change – occurs when a condition becomes true i.e.
a boolean
Call/Signal – occurs when an object receives a call
for one of its operations (from another object or from
itself)
Time trigger
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State and Sequence Diagrams
Guards
Known as a guard condition
It guards the transition i.e. the transition can
only take place if the guard condition
evaluates to true
Guards are constraints on behaviour
Example in Visio
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State and Sequence Diagrams
Event Types
Kinds of events
 receipt of a call on an operation - a message
 receipt of an explicit signal from another object - a
message
 a condition becoming true
 passage of a designated period of time
Basic semantics about event labelled
transitions
 events are triggers that activate state transitions
 if an event occurs and guard is false then the event is
ignored (events are not stored)
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State and Sequence Diagrams
Visio Support – State Chart
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State and Sequence Diagrams
Inside States
 Three kinds of internal content for a state
 These represent the internal activities or state
activities that are executed in that state
 Entry actions
 Exit actions
 Internal transitions - do not cause a state change
 All the above represent the operations for the class
that is being modelled
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State and Sequence Diagrams
How to find States
 A state chart captures all the possible responses
of a single object to all the Use Cases in which it
is involved
 So a cross reference of which of the Use Cases a
class is involved in will give you possible states
for an object of that class
 The state chart can be seen as a description of
all the possible life-cycles for an object
Provides a dynamic view of a class
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State and Sequence Diagrams
Object Sequence Diagrams
OSDs depict a Use Case Script visually but
also show the classes (objects) used in the
script
They show the sequence of object interaction
Read the Visio tutorials for detailed information
on the Sequence Chart
The next slides show example Sequence
Charts
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State and Sequence Diagrams
Simple Example
M
ary: Hotel
Guest
1: leaving
M
aryX: Guest
324: Room
fred: Cleaner
hilton: Hotel
FredX: CleanerX
2: I amvacating
3: cleanm
e
4: cleanroom324
5: donerom
m324
6: youareclean
7: I amavailable
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State and Sequence Diagrams
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State and Sequence Diagrams
Sequence Diagrams
Purpose
Captures the
behaviour of a single
use case - how
messages are sent
within a use case
Shows how objects
interact with each
other - show how they
send and receive
messages
Focuses on time
Syntax
Object
Name : Class
Time
message
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State and Sequence Diagrams
Sequence Diagram
Caller : Person

Example
Nokia : Telephone
System
Receiver : Person
1: pick up
2: dial tone
3: dial
5: ring notification
4: ring
6: pick up
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State and Sequence Diagrams
Sequence Diagram
Types of messages
simple
 flat flow of control - control is passed without describing
any details
synchronous
 nested flow of control - operation call
 handling of the message is completed before the caller
resumes execution
asynchronous
 there is no explicit return to the caller
 the sender continues to execute after sending the message
without waiting for it to be handled
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State and Sequence Diagrams
Sequence Diagram
Activation
:A
 Time during which object
performs an action either
directly or through another
object (subcontractor)
Return
:B
1:
:A
:B
Implicit
Explicit
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State and Sequence Diagrams
Drawing an OSD
Select a Use Case for which you have a Script
Make a list of classes which appear on that
script (you may have done this already if you
have created a Class Diagram)
Use Visio to create a new Sequence Diagram
Do the simple things first and build it up
Place the objects (classes) on the diagram
Think about the order that you put them in
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State and Sequence Diagrams
Drawing an OSD
When creating messages, for the first draft just
use the simple message call shape
Refine to synchronous or asynchronous at the 2nd
draft
Finally, put the rectangle duration box
(operation life) overlay onto the timeline for
each object
Then review it and do a 2nd draft
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State and Sequence Diagrams
Self-paced tutorial
Create a State Diagram in Visio for the third version of
the example Soup Menu application
Hints: states include ingredient choice (three types) and
action choice
What kind of transitions are there? - write down a list
Hints: remember that each transition is a flow from one
state to another
Finally, what events/actions are there for the transitions
in your list?
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