1. Chapter 11 Following the Trail; Examining Execution Sequences

2. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
CRC Cards
Identifying Responsibilities
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams

3. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
CRC Cards
Identifying Responsibilities
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams

4. Responsibility-Driven Design (RDD)
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
Responsibility-Driven Design (RDD)
Takes a real-world view of objects and classes.
The purpose of having an object with data
is so that it can do something for us.
So it has a Responsibility to do what we need,
that is, to provide a Service
to us or some other object,
possibly a User object.

5. to provide that service when requested
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
Wherever a service exists,
we can say there is a
Responsibility
to provide that service when requested
to those who need it.

6. “Responsibilities include two key items:
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
“Responsibilities include two key items:
The knowledge that an object maintains
(i.e., the data it carries),
And the actions an object can perform
(i.e., its behaviors.)”
Rebecca Wirfs-Brock et al. (1990)

7. The knowledge (data) implies:
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
The knowledge (data) implies:
a group of responsibilities
to provide the attribute values
to an Actor, i.e.,
to any (qualified) person who asks.
Some Actors will simply request values,
others will update values,
or create or delete Instances.

8. The actions (behaviors) each imply
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
The actions (behaviors) each imply
a Responsibility to do something, e.g.:
An Invoice has a responsibility to print itself,
And to update its Total attribute,
Calculate and print its Tax and Grand Total.
A Line Item has a responsibility to calculate and return its Extended Price,
And to print itself.

9. The Invoice has a responsibility to
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
The Invoice has a responsibility to
ensure that the Account Balance is updated.
This responsibility it then delegates to the Account object.
The Account object has a responsibility to update its Balance attribute.

10. between the two classes of objects.
Chapter 11: Responsibilities and Collaborations Objects and Responsibilities.
In this scenario,
The invoice has no responsibility to advise the Customer when it has been printed,
But it must notify the User if it can not complete its assigned task.
In this sense we can say that there is an
Agreement,
or a
Contract
between the two classes of objects.

11. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Collaborations
11.4. CRC Cards
Identifying Responsibilities
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams
Contracts

12. Chapter 11: Responsibilities and Collaborations 11.2. Contracts.
The “agreement” between two classes
is such that:
When, say, a Customer object sends a message,
The chosen Invoice object will provide a service by printing itself.

13. Chapter 11: Responsibilities and Collaborations 11.2. Contracts.
Webster defines a contract as:
“A binding agreement between two or more persons or parties.”
Wirfs-Brock et al (1990) describe a contract as:
“ A list of services an instance of one class can request from an instance of another.”

14. Chapter 11: Responsibilities and Collaborations 11.2. Contracts.
Note this is a Client-Server relationship,
Where the client requests a service,
From the Server object,
and expects that it will be done.
But sometimes the server needs help to do its task . . .

15. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
11.4. CRC Cards
Identifying Responsibilities
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams
Collaborations

16. Chapter 11: Responsibilities and Collaborations 11.3. Collaborations.
In the last scenario,
The Invoice was unable to do the job alone,
And had to ask for help
From the Line Item and Account classes.
It delegated the updating to the Account class,
And multiplying Quantity by Unit Price to the Line Item class.

17. Chapter 11: Responsibilities and Collaborations 11.3. Collaborations.
Often a service needed by a client
Requires data that the server doesn’t have,
Or tasks that the server doesn’t know how to do,
So the server calls on other classes for help.
Object #1 #2
Request for service
Response
Client
Server
It starts like this:

18. Chapter 11: Responsibilities and Collaborations 11.3. Collaborations.
Then the server asks for help:
Object #1 #2
Request for service
Response
Client
Server #3
Request for assistance
Help response
Note that for this request,
the server is now a client.

19. Chapter 11: Responsibilities and Collaborations 11.3. Collaborations.
This we call a collaboration,
A request from a client to a server,
for a service
that will help the client fulfil
one of its own responsibilities.

20. Chapter 11: Responsibilities and Collaborations 11.3. Collaborations.
Wirfs-Brock has endorsed and promoted a handy tool for finding Responsibilities and Collaborations . . .

21. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
Identifying Responsibilities
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams
11.4. CRC Cards

22. There is one card per class. Write the class details at the top.
Chapter 11: Responsibilities and Collaborations Class Cards (CRC Cards).
There is one card per class.
Write the class details at the top.
The left column is for the responsibilities,
The right column is for the collaborating classes
(or the method in the collaborating class.)

23. Here is a CRC Card for a Library Book:
Chapter 11: Responsibilities and Collaborations Class Cards (CRC Cards).
Class:
Identifier
Book Book No.
Responsibilities
Collaborations
Know Book No.
Know Title
Know Author
Know Date Published
Know Date Purchased
Know Purchase Price
Know Depreciated Value
Know Date Last Out
Know Date Last In
Know Date last Shelved
Here is a CRC Card
for a Library Book:
SignBookOut
Subscriber
Sign-Out
CheckBookIn
Sign-Out
ShelveBook

24. Chapter 11: Responsibilities and Collaborations 11. 4
Chapter 11: Responsibilities and Collaborations Class Cards (CRC Cards).
Know Invoice no.
Know Customer No.
Class:
Identifier
Responsibilities
Collaborations
Invoice
Invoice No.
Know Invoice No.
Know Customer No.
Print Invoice
Calculate Invoice Total
Line Item
Class:
Identifier
Line Item Invoice No. + Line No.
Responsibilities
Collaborations
Know Invoice No.
Know Line No.
Know Product Code
Know Quantity Sold
Know Unit of Measure
Know Actual Unit Price
CalculateExtendedPrice
PrintLineItem
Product
Tax Rate
The Invoice
Example

25. Chapter 11: Responsibilities and Collaborations 11. 4
Chapter 11: Responsibilities and Collaborations Class Cards (CRC Cards).
Class:
Identifier
Invoice
Invoice No.
Receivables Account Account No.
Responsibilities
Collaborations
Know Invoice no.
Know Customer No.
Know Account No.
Know Customer No.
Know Description
Know Balance
Know Balance 30 Days
Know Balance 60 Days
Know Balance 90 Days
Identifier
Responsibilities
Collaborations
Class:
Receivables Transaction Transaction No.
Know Account No.
Know Transaction No.
Know Date
Know Cash Amount
PostToAccount
PrintStatementLine
Print Statement
AddTransactionToBalance
Receivables Transaction
The Invoice
Example

26. So for now we are . . . Finding names for the behaviors.
Chapter 11: Responsibilities and Collaborations Class Cards (CRC Cards).
To fill in the cards, there are a number
of ideas Wirfs-Brock recommends
for finding and naming Responsibilities
Remember, at Analysis that’s our goal:
Finding names for the behaviors.
Later, at design time, we will spec them out in detail.
So for now we are . . .

27. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
11.4. CRC Cards
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams
Identifying Responsibilities

28. First, enter all the “Know” responsibilities.
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
First, enter all the “Know” responsibilities.
Basically, each attribute must have:
An Update responsibility, and
A Provide-the-value-to-those-who-need-it
For these, we may wish to group attributes in related clusters.
Then enter any other behaviors that we’re already aware of.

29. Use Case Scenarios Walk-through the system, Using the Use Cases.
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
Use Case Scenarios
Walk-through the system, Using the Use Cases.
As Actors (users) walk through each Use Case, Note which class is being queried or updated at each step of the procedure.
Then check if an appropriate responsibility has already been identified, And if not then add it.

30. Chapter 11: Responsibilities and Collaborations 11. 5
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
Input/Output
Data transfers require at least Update and Provide responsibilities, and often Validate as well, and Report Errors.
Output:
Provide Data and Format,
Calculations,
Graphics,
etc.

31. Chapter 11: Responsibilities and Collaborations 11. 5
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
User Interactions
To get even more value from the Use cases, Bring in additional users to do more walk-thrus.
Watch especially for “what-ifs” that may not have been noticed earlier.

32. Prompts and Error Messages
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
Prompts and Error Messages
We may already be aware of some from our earlier work.
Each prompt represents input, So some class has a Receive responsibility.
Each Error message represents a responsibility to test for some condition, And to report on it.

33. Instance Creation and Deletion
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
Instance Creation and Deletion
Every class has a responsibility to crate instances of itself, And may need more than one way to do it (i.e., several «create» methods).
Every class has a responsibility to delete or archive unwanted instances.
Others may go into a “suspense” state while awaiting final archiving or deletion

34. all the responsibilities, there is a check to do
Chapter 11: Responsibilities and Collaborations Identifying Responsibilities.
Once we find
all the responsibilities,
there is a check to do
to ensure that each one is
in the appropriate class . . .

35. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
11.4. CRC Cards
Identifying Responsibilities
Identifying Collaborations
Sequence Diagrams
Assigning Responsibilities

36. For most responsibilities it’s obvious
Chapter 11: Responsibilities and Collaborations Assigning Responsibilities.
For most responsibilities it’s obvious
which class they belong in.
Normalization requires data be grouped by the class it describes, and
encapsulation requires that the code to manipulate it be in the same class.

37. Wirfs-Brock’s Power Principle
Chapter 11: Responsibilities and Collaborations Assigning Responsibilities.
Wirfs-Brock’s Power Principle
If all the “power” and “intelligence” in a system
Is in just a few classes, They become unwieldy, And re-useability is reduced.
So we may need to:
Split a class into two or more, or
Re-assign some responsibilities to other classes.

38. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
11.4. CRC Cards
Identifying Responsibilities
Assigning Responsibilities
Sequence Diagrams
Identifying Collaborations

39. These questions must be asked about each
Chapter 11: Responsibilities and Collaborations Identifying Collaborations.
These questions must be asked about each
responsibility we have discovered:
Does this class have everything it needs to do this?
If not, what is missing?
What data is missing?
What additional tasks are needed
before,
during, or
after the main task?

40. What classes in the model can supply these?
Chapter 11: Responsibilities and Collaborations Identifying Collaborations.
What classes in the model can supply these?
Can any of them?
If so, record the collaboration.
If not, then
Add an attribute or responsibility, or
Define a new class, or
Browse available class libraries.

41. Follow the chain to its end right now while the trail is still warm
Chapter 11: Responsibilities and Collaborations Identifying Collaborations.
Sometimes a client responsibility Requires help from a contracting server,
Who needs a “subcontractor”
Who needs another one, and so on.
When this happens,
Follow the chain to its end right now
while the trail is still warm

42. Then we approach it from the other view:
Chapter 11: Responsibilities and Collaborations Identifying Collaborations.
Then we approach it from the other view:
What significant data does this class carry?
What important things can it do?
Who needs it?
What other classes might
need the data
require the services of this class.

43. Chapter 11: Responsibilities and Collaborations
Objects and Responsibilities
Contracts
Collaborations
11.4. CRC Cards
Identifying Responsibilities
Assigning Responsibilities
Identifying Collaborations
Sequence Diagrams

44. from the point of view of
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
A CRC Card:
Documents the interactions among objects
i.e., the method calls
and data transfers
from the point of view of
one class at a time

45. Shows the complete pattern of method calls and collaborations
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
A CRC Card:
Shows the complete pattern
of method calls and collaborations
for that class

46. Society of Objects A Sequence Diagram:
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
On the other hand,
A Sequence Diagram:
Documents those same interactions,
showing the pattern of calls among a
Society of Objects
(i.e., a bunch of objects that regularly hold conversations)

47. Begin by following a use case
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
Begin by following a use case
Follow our CRC cards
Update cards where needed
At the same time map the steps on a sequence diagram . . .

48. Objects are shown along the top.
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
:Window :Position
Steve:Manager
Old:Occupancy
Joe:
Objects are shown along the top.
Use a class name
Or an instance name
Or both.
The colon ( : ) comes after the instance name and before the class name

49. t The vertical axis is Time Shown as dashed lines, and reading down
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
:Window :Position
Old:Occupancy
Steve:Manager
Joe: t
The vertical axis is Time
Shown as dashed lines, and reading down
Each vertical Time axis is called a Lifeline

50. Chapter 11: Responsibilities and Collaborations 11. 8
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
:Window :Position
Old:Occupancy
Steve:Manager
Joe: t
We place a skinny box on each lifeline
This is called a focus of control
or an activation line
It shows what part of its life the object is active

51. A dashed arrow indicates the response
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
:Window
:Position
Old:Occupancy
Steve:Manager
Joe:
displayPosName(posNo)
archiveOldOccupancy(date)
To begin handling a “Promotion”, a screen object queries a Position instance.
A dashed arrow indicates the response
Then the screen object sends a message to archive the old position occupancy data, with today’s date as parameter

52. Chapter 11: Responsibilities and Collaborations 11. 8
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
:Window
:Position
Old:Occupancy
Steve:Manager
Joe:
displayPosName(posNo)
New:
Occupancy
archiveOldOccupancy(date)
« create »
success()
advise()
Then we send a message to « create » a new Occupancy instance
Then a pair of unanswered messages to advise the Manager and Employee instances
These ones are asynchronous Signals.

53. Summary: So we work through our use case Using our CRC cards
Chapter 11: Responsibilities and Collaborations Sequence Diagrams
Summary:
So we work through our use case
Using our CRC cards
Drawing a Sequence Diagram as we go
And adding to the CRC cards as needed.

54. End of Chapter 11