CRC Cards A tool and method for systems analysis and design Part of the OO development paradigm Highly interactive and human intensive Results in the definition of objects and classes Adapted from Zenebe Miao 2001 HISTORY Introduced at OOPSLA in 1989 by Kent Beck and Ward Cunningham as an approach for teaching object oriented design In 1995 CRC cards are used extensively in teaching and exploring early design ideas CRC cards have become increasingly popular in recent years As formal methods proliferate CRC cards have become for some projects the simple low risk alternative for doing object oriented development Adapted from Zenebe Miao 2001 What s a CRC Card CRC stands for Class Responsibility and Collaboration Class A set of objects that share common structure and common behavior Super class a class from which another class inherits Subclass a class that inherits from one or more classes Responsibility Some behavior for which an object is held accountable Collaboration process whereby several objects cooperate to provide some higher level behavior Adapted from Zenebe Miao 2001 What s a CRC CARD Cont An index card that is annotated in a group setting to represent a class of objects its behavior and its interactions An informal approach to OO modeling Created through scenarios based on the system requirements that model the behavior of the system Adapted from Zenebe Miao 2001 Sample CRC Card Front Back Adapted from Zenebe Miao 2001 REQUIREMENTS Cards should be physical cards not virtual cards CASE tools for support of CRC cards are useful but cannot replace the interaction that physical cards facilitate 3x5 or 4x6 inch are the perfect size But you can really use anything you want Napkins Envelopes Refreshments Optional Adapted from Zenebe Miao 2001 THE CRC CARD SESSION The session includes a physical simulation of the system and execution of scenarios Principles of successful session All ideas are potential good ideas Flexibility Group Dynamic Adapted from Zenebe Miao 2001 BEFORE THE SESSION Forming the Group The ideal size for the CRC card team 5 or 6 people The team should be composed of One or two domain experts two analysts an experienced OO designer one group s leader facilitator Adapted from Zenebe Miao 2001 The CRC Card Team Adapted from Zenebe Miao 2001 Source The CRC Card Book by Bellin et al 1997 DURING THE SESSION All the group members are responsible for holding moving and annotating one or more cards as messages fly around the system Group members create supplement stack and wave cards during the walk through of scenarios A session scribe writes the scenarios Adapted from Zenebe Miao 2001 PROCESS 1 Brainstorming One useful tool is to find all of the nouns and verbs in the problem statement 2 Class Identification The list of classes will grow and then shrink as the group filters out the good ones 3 Scenario execution Role play The heart of the CRC card session Adapted from Zenebe Miao 2001 STRENGTHS The cards and the exercise are non threatening informal Provide a good environment for working and learning Inexpensive portable flexible and readily available Allow the participants to experience first hand how the system will work Useful tool for teaching people the object oriented paradigm Adapted from Zenebe Miao 2001 LIMITATIONS Provide only limited help in the aspects of design Do not have enough notational power to document all the necessary components of a system Do not specify implementation specifics Can not provide view of the states through which objects transition during their life cycle Adapted from Zenebe Miao 2001 CRC GOOD PRACTICE Start with the simplest scenarios Take the time to select meaningful class names Take the time to write a description of the class If in doubt act it out Lay out the cards on the table to get an intuitive feel for system structure Be prepared to be flexible Adapted from Zenebe Miao 2001 Case Example A small technical library system for an R D organization Requirement Statement Participants Who Why Creating Classes The CRC Card Sessions scenario execution Adapted from Zenebe Miao 2001 Case example Finding Classes Suggested Classes Library Librarian User Borrower Article Material Item Due Date Fine Lendable Book Video and Journal Classes after filtering Librarian Lendable Book Video Journal Date Borrower and User Assigning Cards A CRC Card per Class put name description of the class Adapted from Zenebe Miao 2001 Scenario execution Scenario executions Role Plays For what Filter and test identified classes Identify additional classes Identify responsibilities and collaborators can be derived from the requirements use cases responsibilities that are obvious from the name of the class be cautious avoid extraneous responsibilities Filter and test responsibilities and collaborators Attributes only the primary ones Adapted from Zenebe Miao 2001 Finding Responsibilities Things that the class has knowledge about or things that the class can do with the knowledge it has Tips Indicators Verb phrases in the problem or use case Ask what the class knows What how the class does Ask what information must be stored about the class to make it unique Adapted from Zenebe Miao 2001 Finding Collaborators A class asks another class when it needs information that it does not have or needs to modify information that it does not have Client Server relationship Tips Indicators Ask what the class does not know and needs to know And who can provide that Adapted from Zenebe Miao 2001 Case example Scenario Execution Identify Scenarios By domain experts Main scenarios check out return and search Start with the simple ones The first one always takes the longest Domain experts have high level of contribution during the early scenarios Adapted from Zenebe Miao 2001 Case example Checkout Scenario Who should have the overall responsibilities for the task check out Librarian What does the task entail Shouldn t there be collaborations in the opposite direction Collaborations in CRC cards are one way relationships from the client to the server OO Who should do the checking out of the Book Librarian or Book itself Controversial Adapted from Zenebe Miao 2001 Case example Checkout Scenario Who should tell Borrower to update its knowledge about outstanding Book Librarian or Book Do we need a collaboration between Book and Borrower for the know set of books responsibility Collaborations are not usually needed for responsibilities that simply hold information only for