CSC/ECE 517 Fall 2010/ch1 S6 CC

Usually most of the real world problems are large, complex and less well defined. Therefore the analysis and refinement of design would be a longer and complex process. As our problem becomes large and complicated, it is unlikely that our initial designs are perfect. Therefore it is a good practice to repeatedly check these initial designs and make necessary changes before we can convert these into final ones. One way of refining our initial designs is by using CRC cards.

Introduction

CRC stands for Class Responsibility Collaborator.CRC card modeling is one of the prominent Object Oriented Design Technique. This technique is used in determining the various classes, their functionalities and their collaborations in the software system.Class-Responsibility-Collaboration cards have been first introduced by Kent Beck and Ward Cunningham in their paper "A Laboratory for teaching Object-Oriented Thinking". CRC cards are used to document all the classes and their collaboration with the other classes in the system. This helps us in the process of designing a flawless system. This is also a cheap way of designing a system. Not just one but multiple designers can work together in designing a system using CRC cards modeling technique.

Structure of a CRC card

CRC card consists of three parts.

Class Name

The class represents a collection of similar objects. Objects are things of interest in the system being designed. They can be a person, place, thing or any other important concept information to the system at hand. The class name appears across the top of the card.

Class Responsibilities

A Responsibility is anything that the class knows or does. These responsibilities are the things that the class has knowledge about itself, or the things the class can do with the knowledge it has.

For example a person class might have knowledge for its name, address and phone number. In other example an automobile class might have knowledge of its size, the number of doors or it might be able to do things like stop and go. The responsibilities of the class appear on the left side of the card.

Collaborators

A Collaborator is another class that is used to get information from or perform actions for the class at hand. It often works with a particular class to complete a step in a scenario. All the collaborator classes appear on the right side of the card.

A CRC card might also contain additional information such as Super and Sub classes, name of the author of the class etc. The back side of the card is generally used to give a detailed explanation of the class the card represents.

CRC Session

The CRC session is conducted by different types of people involved in the project. The overall size of the CRC team should be limited to six. Because as the team size increases the number of conversations increase exponentially. Generally four types of people participate in the crc session. They are

1 . Domain Users

The Domain Users are the users of the system being designed.

2 . OO Design Analyst

Object oriented Analysts are the ones who are familiar with the OO methodologies and techniques.

3 . Facilitator

Facilitator is the one who is responsible to keep the crc session progress forward

4 . Scribe

Scribes are the members who are responsible for documenting any business logic and discussion that is not captured on the CRC cards.

Discovery of Class names

The initial work that is carried on in the CRC session is the discovery of classes and their responsibilities. For the sake of designing a better system, the problem statement has to be clearly stated. Discovery of classes is a non-trivial process. The names of the classes can be extracted from the nouns present in the problem summary statement. This may not be the final list. Some of the class names stated may not be required and other class names might not be listed.

Discovery of Class responsibilities

Responsibilities are related to actions. They are generally identified by selecting the verbs from the summary of requirements. Although verbs relate to responsibilities, verbs are not the only way of discovering responsibilities. Multiple verbs can be combined for discovering new responsibilities. If two or more responsibilities are shared by multiple classes then each class will have the responsibility. As this is an iterative process, the summary statement and the set of responsibilities have to be revised regularly. The final set of responsibilities may not contain all the responsibilities in the initial draft.

Discovery of Class Collaborations

To identify the Collaborations, it is necessary to study the responsibilities and the classes the object interacts with. We can also discover collaborations by making a note of classes the present object should interact with in order to complete the assigned set of responsibilities. At the time of collaborations we might even discover new classes that might be required for completing the set of responsibilities. In order to discover a collaboration, use cases are required. An use case is a sequence of events performed in response to the request or event.

Example

Problem Statement :

Our aim is to design an airline self check in system.

• A self check-in Kiosk at the air port consists of passport scanner, display console, baggage check in controller, boarding pass dispenser and card swiper.
• Kiosk should be able to start a new session when the booking reference number is typed in or passport is scanned. It should provide access to component parts for each session
• The passport scanner should be able to scan the bar-code on the passport and check if any record exist corresponding to the scanned passport by connecting to the database.
• The Display console should provide a touch based console to accept the entered airline pnr. It should be able to pass the query onto the reservations database and display the result. Based on the result it should also be able to provide access to the baggage check in system.
• Baggage check in controller should provide a count for the maximum number of check in bags allowed based on the data retrieved by scanning the passport and should also be able to weigh the bag and check if the weight of the bag is in the allowed limit or not. It should also provide an option for the user to check in extra bags if he/she wants to. Based on the number of extra bags entered by the user, it should calculate the required amount.
• Boarding pass dispenser should be able to print the boarding passes and should pass the message to the Kiosk to start a new session

Now that we are given a problem statement, we should carefully go through the whole statement and using the nouns present in it we should be able to prepare the first set of nouns. As we go through the problem statement, we come across the following nouns -

• Kiosk
• Passport scanner
• Display Console
• Baggage check in controller
• Boarding pass dispenser
• Card swipe

If we rigidly list all the nouns in the summary statement, then the above list would have been longer. But by applying heuristics we should be able to extract an essential list of classes. Domain analysts also play an important role in selecting a set of essential classes.

Our next step is to identify the responsibilities of each class listed above.

For every class we should be able to identify a set of responsibilities from the given summary statement. The verbs present in the summary statement helps us in identifying the set of responsibilities for each class.

• Kiosk : should be able to start a new session and provide access to component parts.
• Passport scanner : scan the bar code on the passport and should be able to query the reservations database
• Display console : should be able to accept the booking reference no from the customer, query the reservations database, display the result.
• Baggage check in controller : count the maximum no of allowed bags, weigh each bag, beep if a bag exceeds limit, pass the bags to personnel, facilitate the customer to check in additional baggage
• Boarding pass dispenser : should dispense boarding passes corresponding to the passport scanned, should pass the information to Kiosk that the session has ended after all the boarding pass have been dispensed
• Card swipe : should scan the magnetic strip on the card, authenticate it and should pass the information to the database such as amount to be transferred for the extra baggage.

Our next step is to discover all the collaborations that exist in the system. In order to discover the relations between different classes, we need to use various use cases.