1. Object Oriented
Analysis and Design
System Events
&
Contracts

2. Object Oriented Analysis and Design
Software
Requirements
Specification
What are the business needs ?
(long text)
Use Cases
What are the domain process ?
(text + visual notation)
Conceptual
Model
What are the concepts, terms ?
(visual)
System
Sequence
Diagrams
What are the system events, operations ?
(visual)
Collaboration / sequence
Diagrams
How software objects interact to fulfill task ?
(visual)
Contracts
What do the system operations do ?
(text)
Design
Class
Diagrams
What is the specification for the software classes ?
(visual)

3. System Sequence Diagrams
Corresponds to:
Buy Item Use Case
Cashier
:System
enterItem(UPC,quantity)
endSale()
makePayment(amount)
Repeat until no more items

4. System Sequence Diagrams
System Response:
3. Determines item price and adds item info to the running sales transaction
Description and price of the current item is presented
5. Calculates and presents sale total
Actor Actions:
This use case begins when a Customer arrives at a POST checkout system with items to purchase
2. Cashier records the identifier for each item. If there is more than one of the same item, Cashier enters the quantity
4. On completion of item entry, Cashier indicates to POST that item entry is complete
6. Cashier tells Customer the total
Cashier
:System
enterItem(UPC,quantity)
endSale()
makePayment(amount)

5. System Sequence Diagrams elements
Corresponds to:
Buy Item Use Case
System BLACK BOX
Actor
Cross reference to use case
Cashier
:System
enterItem(UPC,quantity)
endSale()
makePayment(amount)
System event
Repeat until no more items
Text to clarify logic, control, iterations, etc..

6. System Sequence Diagrams elements

7. System Sequence Diagrams Identifying system events
It is necessary to be clear about the system boundary
Actors to be considered are those who trigger the system
Costumer is not an actor !
From use case typical course of events, identify system (external) events

8. System Sequence Diagrams Naming System Events
Express at the level of intent rather than in terms of physical input medium
Preferably, start the name with a verb
endSale() ESCPressed()
makePayment(amount) enterAmountTendered(amount)
enterPayment(amount)
Better
Worse

9. Object Oriented Analysis and Design
Software
Requirements
Specification
What are the business needs ?
(long text)
Use Cases
What are the domain process ?
(text + visual notation)
Conceptual
Model
What are the concepts, terms ?
(visual)
System
Sequence
Diagrams
What are the system events, operations ?
(visual)
Collaboration / sequence
Diagrams
How software objects interact to fulfill task ?
(visual)
Contracts
What do the system operations do ?
(text)
Design
Class
Diagrams
What is the specification for the software classes ?
(visual)

10. Contracts Describes what an operation commits to achieve Depends on
What will happen
rather than
how it will be achieved
Depends on
conceptual model
system sequence diagrams
identification of system operations

11. Contracts example: enterItem
Name: enterItem(upc:number, quantity:integer)
Responsibility: Enter (record) sale of an item and add it to sale. Display item description and price.
Type: System
Cross References: System Functions: R1.1, R1.3, R1.9.
Use Cases: Buy Items
Notes: Use fast dbase access
Exceptions: If UPC is invalid, indicate an error
Pre-Conditions: UPC is known to system

12. Contracts example: enterItem
Post-Conditions:
If new sale, a Sale was created (instance creation)
If new sale, new Sale was associated with POST (association formed)
A SalesLineItem was created (instance created)
SalesLineItem was associated with Sale (association formed)
SaleLineItem.quantity was set to quantity (attribute modification)
SaleLineItem was associated with ProductSpecification based on UPC match (association formed)

13. Contracts Contract Sections
Name: Name of operation, and parameters
Responsibilities: An informal description of responsibilities this operation must fulfill
Type: Name of type (concept, software class, interface)
Cross References: System function reference numbers, use cases, etc.
Notes: Design notes, algorithms, etc.
Exceptions: Exceptional cases

14. Contracts Contract Sections
Output: Non-UI outputs such as messages or records that are sent outside of system
Pre-Conditions: Assumptions about the state of system before execution of operation
Post-Conditions:
The state of system after completion of operation

15. Contracts How to make Contracts
Identify system operations from system sequence diagrams
For each system operation, construct a contract
Start by writing responsibilities section
Then complete post conditions section
Then move to pre-conditions section

16. Contracts How to make Contracts
Writing post conditions
It should describe the state changes that occur to objects in the conceptual model (rather than actions)
Possible state changes:
Instance creation or deletion
Attribute modification
Association formed or broken

17. Contracts How to make Contracts
Writing post conditions
Use past passive tense form (was..)
Establish memory between existing objects:
The most common mistake:
not forming associations
Association to several objects is very likely
Think of the system and its object like a stage in a theatre
Craig Larman

18. Contracts More on Contracts
Generating complete and accurate set of post-conditions is not likely and not necessary
Following pre-conditions worth capturing:
Things that are important to test in software
Things that affect the success of operation
Use notes section to capture known design statements
Use exceptions section to discuss reaction to exceptional situations

19. From Use Cases to Contracts
Buy Item …
Typical Course of
Events:
Use Case
Contracts
Operation:
enterItem …
Post-conditions:
1. …
endSale
Cashier System
enterItem
endSale
makePayment
System
Sequence
Diagram
System
enterItem()
endSale()
makePayment()
Operations

20. More example (from larman’s book)
Contracts
More example (from larman’s book)

24. More example login use case  contract
Contracts
More example login use case  contract

25. Use Case Description

28. Object Oriented Analysis and Design
Software
Requirements
Specification
What are the business needs ?
(long text)
Collaboration / sequence
Diagrams
How software objects interact to fulfill task ?
(visual)
Use Cases
What are the domain process ?
(text + visual notation)
Conceptual
Model
What are the concepts, terms ?
(visual)
System
Sequence
Diagrams
What are the system events, operations ?
(visual)
Contracts
What do the system operations do ?
(text)
Design
Class
Diagrams
What is the specification for the software classes ?
(visual)