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Object-Oriented Analysis

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. Motivation.

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Object-Oriented Analysis

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  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 Object-Oriented Analysis

  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 Object-Oriented Analysis

  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 Object-Oriented Analysis

  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. Object-Oriented Analysis

  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? 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) Object-Oriented Analysis

  7. 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? Object-Oriented Analysis

  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. Object-Oriented Analysis

  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 Object-Oriented Analysis

  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) Object-Oriented Analysis

  11. 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 Object-Oriented Analysis

  12. CRC Cards: What Goes Where? Object-Oriented Analysis

  13. CRC Card Format (Front) Class Name (OOA) Purpose/Role (OOA): Patterns (OOD): Stereotypes(OOA/OOD): Object-Oriented Analysis

  14. Class Name Superclass: Subclasses: Responsibilities Collaborators CRC Card Format (Back) Object-Oriented Analysis

  15. CRC Card Sample Document Purpose: A Document acts as a container for graphics and text. Patterns: Composite-Component Stereotypes: Structurer Object-Oriented Analysis

  16. Document Superclass: Subclasses: Responsibilities Collaborators CRC Card Sample Know Contents Know Storage Location Insert and Removes text, graphics and other elements Object-Oriented Analysis

  17. 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 Object-Oriented Analysis

  18. 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 Object-Oriented Analysis

  19. 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 Object-Oriented Analysis

  20. Scenario Guidelines • Keep a record of scenarios played out • Explore exception conditions last • Separate role-play from analysis Object-Oriented Analysis

  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 Object-Oriented Analysis

  22. OOA for a Clock • Brainstorm (Step 1 – Candidate Classes) • Time • Counter • , Clock, Display • , • , TimeZones, Object-Oriented Analysis

  23. OOA for a Clock • Brainstorm • Face for the clock, internal ticker, hours, minutes, seconds, knob for setting the clock • Filter for Core Objects Object-Oriented Analysis

  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 Object-Oriented Analysis

  25. 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 Object-Oriented Analysis

  26. 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!!! Object-Oriented Analysis

  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 Object-Oriented Analysis

  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 Object-Oriented Analysis

  29. Scenario 1: The Ticker Ticks • The SecondsTicker pulses the Clock Object-Oriented Analysis

  30. Scenario 1: The Ticker Ticks • The SecondsTicker pulses the Clock • The Clock updates Time Object-Oriented Analysis

  31. Scenario 1: The Ticker Ticks • The SecondsTicker pulses the Clock • The Clock updates Time • Time updates itself Object-Oriented Analysis

  32. Scenario 1: The Ticker Ticks • The SecondsTicker pulses the Clock • The Clock updates Time • Time updates itself End of Scenario Object-Oriented Analysis

  33. Scenario 2: Clock Responds with the Time • Display the time Object-Oriented Analysis

  34. Scenario 2: Clock Responds with the Time • Display the time • Return hours, minutes, & seconds Object-Oriented Analysis

  35. Scenario 2: Clock Responds with the Time • Display the time • Return hours, minutes, & seconds • Translate the time into the display format Object-Oriented Analysis

  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 Object-Oriented Analysis

  37. OOA for a Clock: Second Try Object-Oriented Analysis

  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. Object-Oriented Analysis

  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 $100....(Design Constraints)... Object-Oriented Analysis

  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: … Object-Oriented Analysis

  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. Object-Oriented Analysis

  42. 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. Object-Oriented Analysis

  43. Candidate Classes • Brainstorming: Object-Oriented Analysis

  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. Object-Oriented Analysis

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