1. Object-Oriented Analysis
Using CRC Cards and Scenarios to Get Started on a Good Object-Oriented Design
Slides from Jochen Rick adapted by Dan Fleck
CRC = Class Responsibility Collaboration
Coming up: Motivation

2. Motivation
It’s all very well understanding what an object is, but it’s much harder to decide what kinds of object you should design in order to implement an application in Smalltalk. Indeed given a set of requirements for an application and a development system like Smalltalk, ‘finding the objects’ is easily the most difficult task an experienced OO developer has to face. —Simon Lewis, The Art and Science of Smalltalk
Coming up: Good Object-Oriented Style

3. Good Object-Oriented Style
You can hack in any language, but how do you get reusable, maintainable code?
Just using Objects doesn’t ensure a good design
Many C++ programs have only have a single class
That’s not good object-oriented style
No process can guarantee good results
A good process just makes them more likely
Coming up: Design is a Process, not a Waterfall

4. Design is a Process, not a Waterfall
Design is an iterative activity
Start with Object-Oriented Analysis and Design
Move on to OO Programming
Return to OOA/OOD when necessary
When creating new functionality
To solve problems with the code
OO Analysis is just another perspective
Good designers (in any field) shift perspective frequently to create a better design
The boundary between OOA and OOD is fuzzy.
Actually, in any problem-solving task, switching perspectives is often useful for solving the problem.
Coming up: OOA vs. OOD

5. OOA vs. OOD
In analysis, we are mostly concerned with the DOMAIN MODEL. What are the objects in the domain and how do they collaborate.
In design, we need to integrate an APPLICATION MODEL. What objects do I need to add to get this thing to run on a computer and to be realized in some programming language?
Coming up: Object-Oriented Analysis

6. Object-Oriented Analysis
CRC Cards by Ward Cunningham
Goal: Understand the Domain as Objects
Object-Oriented Analysis is Language Independent
Force Developers to “think” in objects
Steps
Brainstorm Candidate Classes
Create Initial Class-Responsibility-Collaboration Cards
Come up with scenarios of use in the domain
Use Scenarios and role playing to refine CRC Cards
When do we do it?
Before we begin coding (to get a good start)
To add new functionality (update CRC cards and scenarios regularly)
Design is an iterative process. So, even when you are already coding, it may help you to think at the design level again. In particular, what is already out there for you to use may make you reconsider your design. If so, go back and work with the CRC cards. Not only is it a good practice, it is also excellent at keeping your team on the same page. For larger projects, that’s important.
Coming up: OOA Step 1 Brainstorming Candidate Classes

7. OOA Step 1 Brainstorming Candidate Classes
Write down all the objects that relate
Domain Analysis
Focus on the nouns (objects are nouns)
Good objects will have attributes and services
Now, filter and refine the candidates
Deal with the interface later (Not part of the domain, that’s the application model)
Are some candidates attributes of others?
Are some subclasses of others?
Are some instances of others?
Coming up: Brainstorming Tips

8. Brainstorming Tips Two’s only company, Ten’s a crowd
Don’t forget the user
All ideas are potentially good
Analyze – make connections
Think fast – ponder later
Every voice gets a turn
Relax – humor is OK.
Coming up: Candidate Class Review

9. Candidate Class Review
Has clear unambiguous name, recognized by domain experts
Has name that fits in with other systems developed by your organization
Uses singular noun for name
Begins with Uppercase Letter
Has responsibilities (what NOT how)
Remembers (knowledge)
Is needed (collaborates)
Actively participates
from CRC Card Book
Coming up: Filter Candidate Classes

10. Filter Candidate Classes
Core Classes (pretty sure these are in Analysis model)
Undecided Classes (probably not classes – might be attributes)
Eliminated Classes – (outside scope of system, Application model classes like UI components tied to implementation)
Coming up: OOA Step 2 CRC Cards

11. OOA Step 2 CRC Cards
For each core candidate class, create one CRC card
What’s a CRC Card?
Class-Responsibility-Collaboration
It’s just a 4x6in index card
Analysis vs. Design Information
Coming up: CRC Cards: What Goes Where?

12. CRC Cards: What Goes Where?
Superclass is only put in if necessary.
The collaborators (other classes) should match up with the responsibilities that they are needed for.
Coming up: CRC Card Format (Front)

13. CRC Card Format (Front)
Class Name (OOA)
Purpose/Role (OOA):
Patterns (OOD):
Stereotypes(OOA/OOD):
Coming up: CRC Card Format (Back)

14. CRC Card Format (Back) Class Name Superclass: Subclasses:
Responsibilities
Collaborators
Coming up: CRC Card Sample

15. CRC Card Sample Front Document
Purpose: A Document acts as a container for
graphics and text.
Patterns: Composite-Component
Stereotypes: Structurer
Coming up: CRC Card Sample

16. CRC Card Sample Back Document Superclass: Subclasses: Responsibilities
Collaborators
Know Contents
Know Storage Location
Insert and Removes text,
graphics and other
elements
Coming up: Stereotypes

17. Stereotypes Information Holder Structurer Service Provider Coordinator
knows and provides information
Structurer
Maintains relationships between objects and information about those relationships
Service Provider
Performs work for other objects
Coordinator
Reacts to events by delegating tasks to others
Controller
Makes decisions and closely directs others’ actions
Interfacer
Transforms information and requests between different parts of our system
Coming up: OOA Step 3 Scenarios with CRC Cards

18. OOA Step 3 Scenarios with CRC Cards
Invent Scenarios
What should these objects do?
What if…?
Play the Cards
Assign Roles
Go Through Scenario
Write down new responsibility
Add collaborator objects to help with that responsibility
Team members hold up cards as they participate
Coming up: Scenario Guidelines

19. Scenario Guidelines Concrete: Focus on “must do” items first
Bob tries to Login to the system with an incorrect password.
Sally creates a new Sorceress character and chooses auto-configuration.
Focus on “must do” items first
Start easy and move to complex
Coming up: Scenario Guidelines

20. Scenario Guidelines Keep a record of scenarios played out
Explore exception conditions last
Separate role-play from analysis
Coming up: OOA for a Clock

21. OOA for a Clock We want to make a clock. The clock should:
Have a way to set the current time
Display the time in hours, minutes, and seconds in different formats
Update the time to keep it current
Coming up: OOA for a Clock

22. OOA for a Clock Brainstorm (Step 1 – Candidate Classes)
Coming up: OOA for a Clock

23. OOA for a Clock Brainstorm Filter for Core Objects
Face for the clock, internal ticker, hours, minutes, seconds, knob for setting the clock
Filter for Core Objects
Coming up: OOA for a Clock

24. OOA for a Clock Brainstorm Filter Define the Clock class
Attributes: hours, mins, secs, timezone, state (low bat, etc…),
Services: update, setTime, alarm, setAlarm, display/time, displayDate, setAmPm, getAmPm
Coming up: OOA for a Clock

25. OOA for a Clock Brainstorm Filter Define the Clock class
Face for the clock, internal ticker, hours, minutes, seconds, knob for setting the clock
Filter
Ticker, hours, minutes, seconds
Define the Clock class
Attributes: seconds, minutes, hours, displayFormat
Services: get/set, nextSecond, display, setFormat
Coming up: ALERT!!!!

26. ALERT!!!! Bad OO Design in Progress What did we do wrong?
I’m still designing a program thinking – what’s my data and what do I have to do?
I need to think about a community of cooperating objects!!!
Coming up: Errors in this Design

27. Errors in this Design We assumed there was only one class: Clock
What’s reusable in that?
We started with data and not with what the object should be responsible for
We were way too focused on the programming from the start
Thinking in get/set mode (be aware of your idioms!)
Java/Python does that, but typically not Smalltalk
Coming up: OOA for a Clock: Second Try

28. OOA for a Clock: Second Try
Brainstorm objects for Clock
Display, Time, Ticker/SecondTimer, Clock, Formatter
Filter:
CRC Cards
Two scenarios
When the ticker pulses the clock, the internal representation of time must increment
When a display is requested, the time must be fetched and formatted
Coming up: Scenario 1: The Ticker Ticks

29. Scenario 1: The Ticker Ticks
The SecondsTicker pulses the Clock
Coming up: Scenario 1: The Ticker Ticks

30. Scenario 1: The Ticker Ticks
The SecondsTicker pulses the Clock
The Clock updates Time
Coming up: Scenario 1: The Ticker Ticks

31. Scenario 1: The Ticker Ticks
The SecondsTicker pulses the Clock
The Clock updates Time
Time updates itself
Coming up: Scenario 1: The Ticker Ticks

32. Scenario 1: The Ticker Ticks
The SecondsTicker pulses the Clock
The Clock updates Time
Time updates itself
End of Scenario
Coming up: Scenario 2: Clock Responds with the Time

33. Scenario 2: Clock Responds with the Time
Display the time
Coming up: Scenario 2: Clock Responds with the Time

34. Scenario 2: Clock Responds with the Time
Display the time
Return hours, minutes, & seconds
Coming up: Scenario 2: Clock Responds with the Time

35. Scenario 2: Clock Responds with the Time
Display the time
Return hours, minutes, & seconds
Translate the time into the display format
Coming up: Scenario 2: Clock Responds with the Time

36. Scenario 2: Clock Responds with the Time
Display the time
Return hours, minutes, & seconds
Translate the time into the display format
End of Scenario
Coming up: OOA for a Clock: Second Try

37. OOA for a Clock: Second Try
Coming up: Why CRC Cards?

38. Why CRC Cards? Help you identify objects and their responsibilities
Help you understand how the objects interact
Cards form a useful record of design activity
Cards work well in group situations and are understandable by non-technical stakeholders.
Coming up: In-class Exercise

39. In-class Exercise
This application will support the operations of a technical library for an R&D organization. This includes the searching for and lending of technical library materials, including books, videos, and technical journals. Users will enter their company ids in order to use the system; and they will enter material ID numbers when checking out and returning items.Each borrower can be lent up to five items. Each type of library item can be lent for a different period of time (books 4 weeks, journals 2 weeks, videos 1 week). If returned after their due date, the library user's organization will be charged a fine, based on the type of item( books $1/day, journals $3/day, videos $5/day).Materials will be lent to employees with no overdue lendables, fewer than five articles out, and total fines less than $ (Design Constraints)...
Coming up: In-class Exercise: Handout

40. In-class Exercise: Handout
Brainstorm Candidate Classes - Whole Class
Candidate Class Review - Whole class
Filter Classes - Individual Groups
Create the cards - Individual Groups
Information Holder - knows and provides information
Structurer - Maintains relationships between objects and information about those relationships
Service Provider - Performs work for other objects
Coordinator - Reacts to events by delegating tasks to others
Controller - Makes decisions and closely directs others’ actions
Interfacer - Transforms information and requests between different parts of our system
Play the cards - Individual Groups
Scenarios -
What happens when Johnny Codewarrior returns the book Document, Your job depends on it two days late? He has no other item checked out and no acquired fines.
What happens when Ivar Jacobson uses the search feature to look for the book Object-Oriented Software Engineering of which there are 2 copies available?
What happens when Grady Booch uses the search feature to look for the book Object-Oriented Analysis and Design with Application of which there is 1 copy available and the database is down?
Present Final Card Set - Individual Groups
Document
Purpose: …
Patterns:
Stereotypes: …
This slide is just a handout
Coming up: OO Analysis Exercise: Checkers

41. OO Analysis Exercise: Checkers
Checkers is played by two people, one with light and one with dark pieces.
Pieces move diagonally and opponents are captured by jumping over them.
Coming up: OO Analysis Exercise: Checkers

42. OO Analysis Exercise: Checkers
Things to keep in mind
A Piece can only move and capture forward.
When a piece makes it to the other side of the board, it becomes a king and can move diagonally in any direction as far as it wants.
Capturing is mandatory. A piece (or king) that is captured is removed from the board.
The player who has no pieces left or cannot move anymore has lost the game.
Coming up: Candidate Classes

43. Candidate Classes
Brainstorming:
Coming up: Scenarios

44. Scenarios
Bob moves first in the new game and moves a checker with a legal move.
Late in the game, Sally moves a checker into Bob’s home row, changing it to a King.
Sally takes Bob’s last checker, ending the game.
End of presentation