Outline

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

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User Stories
Much taken from User Stories Applied For Agile
Software Development Mike CohnMountain Goat
Software
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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
  functionalitytold from the perspective of a
  userthat are valuable to either a user of the
  software or the customer of the software

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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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In team of 2, write three user stories for the
Cashless Jukeboxwe'll collate them in 5 minutes
live

• 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)

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User stories for the Cashless Jukebox

• One example
• Any song can be 5 times per day at most

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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
• A user can select a Song from the collection of
  songs
• Songs can be played up to 5 times per day
• User can hear audio files play
• Any user can play up to 2 songs per day
• Jukebox can find a user given an ID
• Notify Student the song is not selectable
• The system should be able to queue songs on a
  FIFO basis
• Show the play list (queue) to help users decide
  what to do
• Have a nice GUI interface
• User can swipe card
• Students see their account status
• Students can see how long all songs in the queue
  would play
• Administrator can add and remove Students
• Administrator can add and remove songs
• Use this for "WebRadio"
• The system should be able to play mp3s

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Responsibility Driven Design
Responsibility Driven Design, Rebecca Wirfs
Brock, 1990The Coffee Machine Design Problem,
Alistair Cockburn, C/C User's Journal, May and
June 1998. Introducing Object-Oriented Design
with Active Learning, Rick Mercer , Consortium
for Computing in Small 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.Responsib
  le able to answer for one's conduct and
  obligationstrustworthy, 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 responsibility
• http//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_
  science29
• 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 Problem Specification repeated

• 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)

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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 activities
• one 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
CardReader Gets student ID
JukeboxAccountCollection Stores all
JukeboxAccount objects
JukeboxAccount
JukeBox Coordinates activities
SongCollection Stores all Songs that can be played
Song
SongPlayer Plays a 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