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

User Stories Much taken from User Stories Applied: For Agile Software Development Mike Cohn Mountain Goat Software

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

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

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

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

In team of 2, write three user stories for the Cashless Jukebox we'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)

User stories for the Cashless Jukebox One example Any song can be 5 times per day at most

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

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 Active Learning, Rick Mercer , Consortium for Computing in Small Colleges, 2000

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

Responsibility Driven Design in 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

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_science%29 Maintenance: Fixing or changing a module should not break other parts of the system

First Design a Model Note: 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?

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

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)

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?

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

A No Object-Oriented Design Guideline 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

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?

Date Date: Can determine when a song is played and the current date. Maybe Can we use use java.util.GregorianCalendar? Could we use class Usage?

Another No? The next slide summarizes some needed candidate objects 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

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

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