1. Object Oriented System Design COS 50-3
Marc Conrad
D104 (Park Square Building)
Acknowledgement: slides on this week’s lecture mostly provided by Dayou Li.
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Marc Conrad - University of Luton

2. Marc Conrad - University of Luton
Sequence Diagram
A sequence diagram shows how blocks (objects) within a use case cooperate with each other. It shows the occurrence of a sequence of events as the responses to stimuli, which is called stimuli-responses relationship or the internal logic of a system.
A stimulus comes from a block and goes to another block. It triggers a sequence of events in that block.
The receiver may also send “results” (feedback) back to the sender.
Events must be known before constructing a sequence diagram.
07/11/ :01:08
Marc Conrad - University of Luton

3. Sequence Diagram (Cont’d)
Events
07/11/ :01:08
Marc Conrad - University of Luton

4. Marc Conrad - University of Luton
Sequence Diagram
Sequence diagram shows how blocks (objects) within a used case cooperate with each other. It shows the occurrence of a sequence of events as the responses to stimuli, which is called stimuli-responses relationship or the internal logic of a system.
A stimulus comes from a block and goes to another block. It triggers a sequence of events in that block.
The receiver may also send “results” (feedback) back to the sender.
Events must be known before constructing a sequence diagram.
07/11/ :01:08
Marc Conrad - University of Luton

5. Sequence Diagram (Cont’d)
Syntax and Semantics
signal (interprocess)
Block’s life line (showing life cycle)
message (intra-process)
Distinguishing a system and the outside world
(RationalRose: Use Actors )
Events occurring within a block
feedback
signal
07/11/ :01:08
Marc Conrad - University of Luton

6. Sequence Diagram (Cont’d)
Pragmatics:
List all blocks right-hand side of the system border,
Draw a life line for each block,
List all events on the left-hand side of the system border,
Draw vertical bars on the corresponding life lines to represent the events.
Identify and draw signals,
Identify and draw messages,
Identify and draw feedback signals.
07/11/ :01:08
Marc Conrad - University of Luton

7. Sequence Diagram (Cont’d)
Example: (focusing on Returning item use case)
Blocks:
-- Customer Panel,
-- Deposit item receiver,
-- Receipt basis,
-- Deposit item,
-- Receipt printer.
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Marc Conrad - University of Luton

8. Sequence Diagram (Cont’d)
Events (when a customer insert an item):
If (customer is new) {
Create a new account; } Do
If (returned item is acceptable == true)
Classify;
Increment items;
Increment values;
else
reject;
07/11/ :01:08
Marc Conrad - University of Luton

9. Sequence Diagram (Cont’d)
Events (when the customer presses receipt button):
Print Logo and date;
for (index = 0; index < 3; index ++) {
Find name and number for a type of item;
Find deposit value for a type of item;
Sum;
Print sum; }
Ready for the next customer;
07/11/ :01:08
Marc Conrad - University of Luton

10. Sequence Diagram (Cont’d)
System border
Customer panel
Dep item receiver
Receipt
basis
Dep item
Rec. printer
start
create
A new customer press start button
ready
new item Do {
check
classify
Increment items, Increment values, }
While (! receipt button pressed);
Item
items
inc item/value
07/11/ :01:08
Marc Conrad - University of Luton

11. Sequence Diagram (Cont’d)
System border
Customer panel
Dep item receiver
Receipt
basis
Dep item
Rec.t printer
receipt
The customer presses the receipt button
printReceipt
print Logo, date
Print Logo and date,
Find name and number for a type of item,
Find deposit value for a type of item, Sum,
Print sum
get value
value
get name
name
print
07/11/ :01:08
Marc Conrad - University of Luton

12. Sequence Diagram (Cont’d)
System border
Customer panel
Dep item receiver
Receipt
basis
Dep item
Rec. printer
receipt
The customer presses the receipt button
printReceipt
print Logo, date
Print Logo and date,
Find name and number for a type of item,
Find deposit value for a type of item, Sum,
Print sum
sum
get value
value
get name
name
printSum
07/11/ :01:08
Marc Conrad - University of Luton

13. Rational Rose – Sequence Diagram
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Marc Conrad - University of Luton

14. Rational Rose – Collaboration Diagram
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Marc Conrad - University of Luton

15. (Vehicle Hire Company)
Case Study
(Vehicle Hire Company)
EuroStar is a vehicle hire company. It provides a variety of vehicles for its customers. Each vehicle has a record in the company’s database, including Register Number, Maker, Model, Colour, Engine Size, etc.
The company has a manager and a number of Register Staff. The register staff takes a customer’s reservation and searches for a vehicle that matches the order by sending a query to the company’s computerised database. When the requested type of vehicle is available, the staff confirms the reservation. If it is not available, the staff gives suggestions of alternative vehicle.
07/11/ :01:08
Marc Conrad - University of Luton

16. (Vehicle Hire Company)
Case Study
(Vehicle Hire Company)
Customers are required to return vehicles to the company before a Return Time. They are liable to a fine if they return vehicles late.
EuroStar now wants to have a Web site in order that customers can make their reservation through the Internet. Suppose that you are working in a software house and EuroStar is your client. You are asked to develop such an on-line booking system for the company.
07/11/ :01:08
Marc Conrad - University of Luton

17. Requirement Model (use case)
-- customers
-- making a reservation
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Marc Conrad - University of Luton

18. Analysis Model (3-type objects)
Console -- interface between the system and the user, an interface object.
Database -- vehicle records and availability, an entity object.
Register -- Control of data flow, a control object.
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Marc Conrad - University of Luton

19. Design Model (sequence diagram)
Console
Register
Database
Events:
read customer’s request,
form a query,
search for a vehicle,
find alternative if the request vehicle is unavailable,
display alternative vehicle,
set return time,
display return time.
07/11/ :01:08
Marc Conrad - University of Luton

20. Design Model (sequence diagram)
Console
Register
Database Do
read customer’s request,
form a query,
search for a vehicle,
if unavailable
find alternative
display alternative,
else
set return time,
display return time;
While
submit a request
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Marc Conrad - University of Luton

21. Object Oriented Design Model II (Week 5)
COS50-3 OOSD
Object Oriented Design Model II
(Week 5)
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Marc Conrad - University of Luton

22. Marc Conrad - University of Luton
Selection in SD
If… else…
Fork and condition repression.
Branch of control in the receiving block’s lifeline.
07/11/ :01:08
Marc Conrad - University of Luton

23. Selection in SD (Cont’d)
A soda machine example:
Customers buy soda drinks from a soda machine.
Register
Front
Dispenser
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Marc Conrad - University of Luton

24. Selection in SD (Cont’d)
When input (inserted coins ) = price
System border
Front
Register
Dispenser
Insert
Select
Send
Send
Delivery
07/11/ :01:08
Marc Conrad - University of Luton

25. Selection in SD (Cont’d)
When input (inserted coins ) > price
System border
Front
Register
Dispenser
Insert
Select
Send
Delivery
Send (input = price)
Check for change (input > price)
Return change
Send (input > price)
Delivery
07/11/ :01:08
Marc Conrad - University of Luton

26. Selection in SD (Cont’d)
Selection requires a second life line. One condition leads to the original life line and the other condition leads to the second one.
Front
Register
Dispenser
Original life line
Second life line
Delivery
Send (input = price)
Check for change (input > price)
Return change
Send (input > price)
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Marc Conrad - University of Luton

27. Marc Conrad - University of Luton
Control in SD
Synchronous and asynchronous
A synchronous message/signal is a control which has to wait for an answer before continuing. Hence, the sender passes the control to the receiver and cannot do anything until the receiver sends the control back.
An asynchronous message is a control which does not need to wait before continuing. Hence, the sender actually does not pass the control to the receiver. The sender and the receiver carry on their work concurrently.
07/11/ :01:08
Marc Conrad - University of Luton

28. Marc Conrad - University of Luton
Control in SD (Cont’d)
Syntax:
Example: the “return item” use case in the recycling machine.
-- Concentrate on the rounded rectangle and answer the question: “Should getName and getValue be synchronous or asynchronous?
synchronous
asynchronous
07/11/ :01:08
Marc Conrad - University of Luton

29. Marc Conrad - University of Luton
Control in SD (Cont’d)
System border
Customer panel
Dep item receiver
Receipt
basis
Dep item
Receipt printer
receipt
The customer presses the receipt button
printReceipt
print Logo, date
Print Logo and date,
Find name and number for a type of item,
Find deposit value for a type of item, Sum,
Print sum
get value
value
get name
name
print
07/11/ :01:08
Marc Conrad - University of Luton

30. Marc Conrad - University of Luton
Control in SD (Cont’d)
On one hand, getValue signal and getName signal can be asynchronous as the system does not need to wait for values before requesting name.
On the other hand, “print” message could be sent out to the printer if getValue and getName were asynchronous.
For this reason, getValue signal and getName signal must be synchronous.
07/11/ :01:08
Marc Conrad - University of Luton

31. Marc Conrad - University of Luton
Control in SD (Cont’d)
System border
Customer panel
Dep item receiver
Receipt
basis
Dep item
Receipt printer
receipt
The customer presses the receipt button
printReceipt
print Logo, date
Print Logo and date,
Find name and number for a type of item,
Find deposit value for a type of item, Sum,
Print sum
get value
value
get name
name
print
07/11/ :01:08
Marc Conrad - University of Luton

32. Marc Conrad - University of Luton
Control in SD (Cont’d)
Concentrate on other signals and messages.
-- receipt signal: it does not even need a feedback. Therefore, it can asynchronous.
-- printReceipt signal: it can be asynchronous, too.
-- printLogo, date message: it can also be asynchronous, as no feedback is required.
07/11/ :01:08
Marc Conrad - University of Luton

33. Marc Conrad - University of Luton
Control in SD (Cont’d)
System border
Customer panel
Dep item receiver
Receipt
basis
Dep item
Receipt printer
receipt
The customer presses the receipt button
printReceipt
print Logo, date
Print Logo and date,
Find name and number for a type of item,
Find deposit value for a type of item, Sum,
Print sum
get value
value
get name
name
print
07/11/ :01:08
Marc Conrad - University of Luton

34. Marc Conrad - University of Luton
Recursion in SD
Recursion or self-call -- an object/block may have an operation that invokes itself.
Example (personal loan):
Operation that invokes itself
Year(0): loan(0);
Year(1): loan(1) = loan(0)  (1 + interestRate);
Year(2): loan(2) = loan(1)  (1 + interestRate)
= [loan(0)  (1 + interestRate)]  (1 + interestRate);
Year(3): loan(3) = loan(2)  (1 + interestRate)
= [loan(1)  (1 + interestRate)]  (1 + interestRate);
={[loan(0)  (1 + interestRate)]  (1 + interestRate)}
 (1 + interestRate);
First called for lump sum
Second called for new capital
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Marc Conrad - University of Luton

35. Marc Conrad - University of Luton
Recursion in SD
Syntax:
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Marc Conrad - University of Luton

36. Marc Conrad - University of Luton
Structure of SD
Centralised structure -- Fork: Everything is handled and controlled by the left-most block.
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Marc Conrad - University of Luton

37. Structure of SD (Cont’d)
Decentralised structure -- Stair: There is no central control block.
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Marc Conrad - University of Luton