Object-Oriented Analysis1 Using CRC Cards and Scenarios to Get Started on a Good Object- Oriented...

Object-Oriented Analysis 1

Object-Oriented Analysis

Using CRC Cards and Scenarios to Get Started on a Good Object-

Oriented DesignSlides from Jochen Rick adapted by Dan Fleck


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


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


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.


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?


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

• Steps1. Brainstorm Candidate Classes2. Create Initial Class-Responsibility-Collaboration Cards3. Come up with scenarios of use in the domain4. 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)


OOA Step 1Brainstorming 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?


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.


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


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)


OOA Step 2CRC 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


CRC Cards: What Goes Where?


CRC Card Format (Front)

Class Name (OOA)

Purpose/Role (OOA):

Patterns (OOD):

Stereotypes(OOA/OOD):


CRC Card Format (Back)

Class NameSuperclass: Subclasses:

Responsibilities Collaborators


CRC Card Sample

DocumentPurpose: A Document acts as a container for

graphics and text.

Patterns: Composite-ComponentStereotypes: Structurer


CRC Card Sample

DocumentSuperclass: Subclasses:

Responsibilities CollaboratorsKnow Contents

Know Storage Location

Insert and Removes text,graphics and other elements


Stereotypes

• 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


OOA Step 3Scenarios 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


Scenario Guidelines

• Concrete:– 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


Scenario Guidelines

• Keep a record of scenarios played out

• Explore exception conditions last

• Separate role-play from analysis


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


OOA for a Clock

• Brainstorm (Step 1 – Candidate Classes)– Time– Counter– , Clock, Display– , – , TimeZones,


OOA for a Clock

• Brainstorm– Face for the clock, internal ticker, hours, minutes,

seconds, knob for setting the clock

• Filter for Core Objects


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


OOA for a Clock

• Brainstorm– 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


ALERT!!!!

• Bad OO Design in Progress

• What did we do wrong?

I’m still designing a program – what’s my data and what do I have to do?

I need to think about a community of cooperating objects!!!


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


OOA for a Clock: Second Try

• Brainstorm objects for Clock– Display, Time, Ticker/SecondTimer, Clock,

Formatter

• Filter:• CRC Cards

– Two scenarios1. When the ticker pulses the clock, the internal

representation of time must increment2. When a display is requested, the time must be fetched

and formatted


Scenario 1: The Ticker Ticks

• The SecondsTicker pulses the Clock




• The Clock updates Time





• Time updates itself





• Time updates itself

End of Scenario


Scenario 2: Clock Responds with the Time

• Display the time



• Display the time• Return hours,

minutes, & seconds




minutes, & seconds• Translate the time

into the display format




minutes, & seconds• Translate the time

into the display format

End of Scenario


OOA for a Clock: Second Try


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.


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 $100....(Design Constraints)...


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: …

Document

Purpose: …

Patterns:

Stereotypes: …


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.


OO Analysis Exercise:Checkers

• Things to keep in mind– http://www.jimloy.com/checkers/rules2.htm – 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.


Candidate Classes

• Brainstorming:


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.

Object-Oriented Analysis1 Using CRC Cards and Scenarios to Get Started on a Good Object- Oriented...

Documents

Transcript of Object-Oriented Analysis1 Using CRC Cards and Scenarios to Get Started on a Good Object- Oriented...