1
Outline
Derive some user stories for next Project
Consider Responsibility Driven Design
Design a solution as a set of candidate objects
with well-defined responsibilities
Role play different scenarios to understand the
problem and help make design decisions
— Assign responsibilities, which is the most
important part of OO Design
Consider some design guidelines
2
User Stories
Much taken from
User Stories Applied: For Agile Software Development
Mike Cohn
Mountain Goat Software
3
User Stories
Agile Process such as Scrum and Extreme
programming (XP) introduced the practice of
expressing requirements in the form of user
stories
A user story is a short descriptions of
functionality–told from the perspective of a
user–that are valuable to either a user of the
software or the customer of the software
4
A few Example User Stories
The following are typical user stories for a job
posting and search site:
— A user can post her resume to the web site
— A user can search for jobs
— A company can post new job openings
— A user can limit who can see her résumé
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User Stories
A user story describes functionality that will be valuable to either a user or purchaser of the system
User stories are traditionally written on an index card when the team and customers are communicating
— They will be written now as a line of text
• in the slides that follow, and
• in the project specification
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Aspects of a User Story
A user story can provide three things:
— Written description of the story, used for planning
and as a reminder
— Placeholder for future conversations among the user, customer, and developer
• User: You, me, section leaders, maybe you can sell it?
• Customer: Rick
• Developer: You
— Tests that convey and document details that can be
used to determine when a story is complete
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The student affairs office want to put some newfound activity
fee funds toward a Jukebox in the student center. The Jukebox
must allow students to play a song. No money will be
required. Instead, a student will swipe a magnetic ID card
through a card reader, view the song collection and choose a
song. Students will each be allowed to play up to 1500
minutes worth of "free" Jukebox music in their academic
careers, but never more than two songs on any given date. No
song can be played more than five times a day*.
*What a drag it would be to hear "Dancing Queen" 14 times while eating lunch
(apologies to ABBA)
In team of 2, write three user
stories for the Cashless Jukeboxwe'll collate them in 5 minutes live
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Other Stories?
In the past, other user stories seemed valuable to
the students and the customer
— We will some, eliminate others intentionally small font:1. A user can select a Song from the collection of songs
2. Songs can be played up to 5 times per day
3. User can hear audio files play
4. Any user can play up to 2 songs per day
5. Jukebox can find a user given an ID
6. Notify Student the song is not selectable
7. The system should be able to queue songs on a FIFO basis
8. Show the play list (queue) to help users decide what to do
9. Have a nice GUI interface
10. User can swipe card
11. Students see their account status
12. Students can see how long all songs in the queue would play
13. Administrator can add and remove Students
14. Administrator can add and remove songs
15. Use this for "WebRadio"
16. The system should be able to play mp3s
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Responsibility Driven Design
Responsibility Driven Design, Rebecca Wirfs Brock, 1990
The Coffee Machine Design Problem, Alistair Cockburn, C/C++ User's
Journal, May and June 1998.
Introducing Object-Oriented Design with ActiveLearning, Rick Mercer , Consortium for Computing inSmall Colleges, 2000
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In Rebecca Wirfs Brocks' Words
Responsibility-Driven Design is a way to design that
emphasizes behavioral modeling using objects, responsibilities
and collaborations. In a responsibility-based model, objects
play specific roles and occupy well-known positions in the
application architecture. Each object is accountable for a
specific portion of the work. They collaborate in clearly
defined ways, contracting with each other to fulfill the larger
goals of the application. By creating a "community of objects",
assigning specific responsibilities to each, you build a
collaborative model of our application.
Responsible: able to answer for one's conduct and obligations—trustworthy, Merriam Webster
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Responsibility Driven Designin Rick's words
1. Identify candidate objects that model a system
as a sensible set of abstractions
2. Determine the responsibility of each object
— what an instance of the class must be able to do,
— and what each instance must know about itself
3. Understand the system through role play
— To help complete its responsibility, an object
often needs help from other objects
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OO Design Principle
The Single Responsibility Principle
Classes should have a single responsibilityhttp://en.wikipedia.org/wiki/Single_responsibility_principle
Why?
— Cohesion, when high, reduces complexity, makes the system more understandable
http://en.wikipedia.org/wiki/Cohesion_%28computer_science%29
— Maintenance: Fixing or changing a module should not break other parts of the system
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First Design a ModelNote: design is iterative
Find a set of objects (candidate classes) that
model a solution
Each will be a part of the bigger system
Each should have a single responsibility
What are these objects?
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Find the Objects
Candidate objects may come from
— An understanding of the problem domain
• knowledge of the system that the problem
specification may have missed or took for granted
— The words floating around the room Alistair
Cockburn
— The nouns in the problem statement
• Underline the noun phrases to look for the objects
that could model the system
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The student affairs office want to put some newfound activity
fee funds toward a Jukebox in the student center. The Jukebox
must allow students to play a song. No money will be
required. Instead, a student will swipe a magnetic ID card
through a card reader, view the song collection and choose a
song. Students will each be allowed to play up to 1500
minutes worth of "free" Jukebox music in their academic
careers, but never more than two songs on any given date. No
song can be played more than five times a day*.
*What a drag it would be to hear "Dancing Queen" 14 times while eating lunch
(apologies to ABBA)
The Problem Specification repeated
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A First Cut at the Candidate
Objects (may become classes)
What objects effectively model the system? What is
the responsibility, Example
Song: Know song title, artist, playtime, how often it's been played today
Others?
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Yesses
Jukebox: coordinates activitiesone instance to start things and keep them going
JukeboxAccount changed from Student: maintain
one account: model user who play songs
Song: one song that can be played
CardReader: reads the magnetic ID card
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A No
StudentIdCard: store user data
Object-Oriented Design Guideline
Eliminate classes that are outside the system
— The hallmark of such a class is one whose only
importance to the system is the data contained in it.
— Student identification number is of great importance
— The system should not care whether the ID number was
read from a swiped magnetic ID card, typed in at the
keyboard, or "if a squirrel arrived carrying it in his
mouth" Arthur Reil
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More Candidate Objects?
SongCollection: songs to choose from
What about storing a collection of accounts?
JukeBoxAccountCollection
What about a compact disk player?
Could have a software equivalent like
SongPlayer to play audio files?
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Date
Date: Can determine when a song is played and
the current date.
— Maybe
— Can we use use java.util.GregorianCalendar?
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Another No?
StereoSystem: Amplifies the music
— No, it's on the other side what we have to build
The next slide summarizes some needed
candidate objects
— It also sets the boundaries of the system
• There are model of the real world objects
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Candidate Objects and the system boundary
CardReaderGets student ID JukeboxAccountCollection
Stores all JukeboxAccount objects
JukeBoxCoordinates
activities
SongPlayerPlays a song
SongCollectionStores all Songs that can be played
JukeboxAccount
Song
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Role Play
Need 7 volunteers to play the roles of these
objects
— You must be willing to write down responsibilities
as they are discovered on the whiteboard
• These are potential methods
– Should be related to encourage high cohesion
– Should have meaningful names
— When done, form teams of 2 or 3 and complete a
class diagram
• We'll check it if you want, do not turn in these