1. Object Oriented Analysis
Software Engineering
Object Oriented Analysis

2. Objectives To explain Class-Responsibility-Collaborator Modelling.
To provide an example of CRC modelling in action

3. Analysis = Process + Models
Model Output
1. Elicit customer requirements and identify use-cases
Use-Case Diagrams
2. Extract candidate classes, Identify attributes and methods, Define a class hierarchy
Class Responsibility Collaborator (CRC) Cards
3. Build an object-relationship model
Class Diagram
4. Build an object-behaviour model
Interaction Diagram

4. Class-Responsibility-Collaborator Modelling
A simple means of identifying and organizing classes
Not an official part of UML
A CRC model is a collection of index cards that represent classes
Each CRC card contains:
Class name, type, characteristic
Responsibilities
Collaborations
Index cards because they can only hold a limited amount of information  enforces high-level analysis
Steps in CRC modelling: Identify [1] Classes, [2] Responsibilities, [3] Collaborators, [4] Review the Model

5. CRC Benefits
They are portable. No computers are required so they can be used anywhere. Even away from the office.
They allow the participants to experience first hand how the system will work.

6. CRC Cards
class name:
class type: (e.g., device, property, role, event, ...)
class characteristics: (e.g., tangible, atomic, concurrent, ...)
responsibilities:
collaborators:

7. [1] Identifying Classes
Classes and Objects are extracted from the Use-Cases by doing a grammatical parse
Grammatical Parse:
Underline nouns or noun clauses (these describe candidate objects)
Enter the candidate objects (and their associated classes) into a table
Remove synonyms
Solution Space: objects required to implement the solution
Problem Space: objects required to describe the problem
“The really hard problem [in OO] is discovering what are the ‘right’ objects in the first place”

8. Accepting Classes
Objects are accepted if they satisfy all (or almost all) of the following requirements:
Retained Information: the system needs to remember data about the object
Needed Services: the object must have an identifiable set of operations
Multiple Attributes: during analysis we focus on major information only. Objects should have many attributes
Common Attributes: A set of attributes apply to all occurrences of the object
Common Operations: A set of operations apply to all occurrences of the object
Essential Requirements: external entities that produce or consume essential information

9. Class Types Accepted classes are assigned a type:
External entities - other systems, devices, people
Things - reports, displays, signals
Occurrences or events - property transfer, completion of a series of robot movements
Roles - manager, engineer, sales person
Organizational Units - division, group, team
Places - manufacturing floor, loading dock
Devices - four-wheeled vehicles, computers
Property - of the problem, e.g. credit rating
Interaction - model interaction that occur among other objects, e.g. a purchase or a license

10. Class Characteristics
Accepted classes are assigned a set of characteristics:
Tangibility: does the class represent a tangible (physical) or abstract (information) entity?
Inclusiveness: is the class atomic (includes no other classes) or aggregate (has at least one nested object)?
Sequentiality: is the class concurrent (has its own thread of control) or sequential (scheduled by outside resources)?
Persistence: is the class transient (created and removed during program operation), temporary (created during program operation and removed at termination) or permanent (stored in a database)?
Integrity: is the class corruptible (does not protect its resources from outside influence) or guarded (the class enforces access control)?

11. Example: Identifying Classes
Narrative:
SafeHome software enables the homeowner to configure the security system when it is installed, monitors all sensors connected to the security system, and interacts with the homeowner through a keypad and function keys contained in the SafeHome control panel.
During installation, the SafeHome control panel is used to “program” and configure the system. Each sensor is assigned a number and type, a master password is programmed for arming and disarming the system, and telephone number(s) are input for dialing when a sensor event occurs. …
Potential Objects/Classes
Homeowner, sensor, control panel, installation, system (alias security system), number, type, master password, telephone number, sensor event, …

12. Example: Accepting Classes
Potential Object/Class
Class requirements
Homeowner
Rejected: 1, 2 (retained information, needed service) fail
Sensor
Accepted: all apply
Control Panel
Installation
rejected
System
Number and Type
Rejected: 3 (multiple attributes) fails, attribute of sensor
Master Password
Rejected: 3 fails
Telephone Number
Sensor Event

13. Example: CRC Header class name: Sensor class type: external entity
class characteristics: tangible, atomic, concurrent, guarded
responsibilities:
collaborators:

14. [2] Identifying Responsibilities
Responsibilities (attributes and methods) are extracted from the Use-Cases descriptions
Attributes:
Describe the object
Select those things that reasonably belong to an object
Question: What data items fully define this object in the context of the particular use-case?
Methods (Operations):
Define the behaviour of the object and alter the object’s attributes
Types of operations – data manipulation, computation, event monitoring
Do a grammatical parse of the Use-Case description and isolate verbs

15. Guidelines for Allocating Responsibilities to Classes
System intelligence should be evenly distributed.
Each responsibility should be stated as generally as possible.
Information and the behavior that is related to it should reside within the same class.
Information about one thing should be localized with a single class, not distributed across multiple classes.
Responsibilities should be shared among related classes, when appropriate.

16. Example: Identifying Responsibilities
Narrative:
SafeHome software enables the homeowner to configure the security system when it is installed, monitors all sensors connected to the security system, and interacts with the homeowner through a keypad and function keys contained in the SafeHome control panel.
During installation, the SafeHome control panel is used to “program” and configure the system. Each sensor is assigned a number and type, a master password is programmed for arming and disarming the system, and telephone number(s) are input for dialing when a sensor event occurs.
Example Operations:
Assign (belongs to Sensor)
Program (belongs to System)
Arm/Disarm (belong to System)

17. Example: CRC Responsibilities
Example Attributes:
sensor information = sensor number + sensor type + alarm threshold
class name: Sensor
class type: external entity
class characteristics: tangible, atomic, concurrent, guarded
responsibilities:
collaborators:
keep sensor information
assign sensor information
signal sensor event

18. [3] Identifying Collaborators
Collaborations represent requests from a client to a server in fulfillment of a client responsibility
One object collaborates with another if it needs to send a message
Relationships:
is-part-of (classes that are contained within an aggregate class as attributes)
has-knowledge-of (one class must acquire information from another)
depends-upon (dependency not covered by part-of or knowledge-of)

19. [4] Reviewing the CRC Model
All participants in the review (of the CRC model) are given a subset of the CRC model index cards.
All use-case scenarios (and corresponding use-case diagrams) should be organized into categories.
The review leader reads the use-case deliberately. As the review leader comes to a named object, she passes the token to the person holding the corresponding class index card.
When the token is passed, the holder of the class card is asked to describe the responsibilities noted on the card. The group determines whether one (or more) of the responsibilities satisfies the use-case requirement.
If the responsibilities and collaborations noted on the index cards cannot accommodate the use-case, modifications are made to the cards.

20. Example: CRC Review Use-Case Narrative:
The homeowner observes the control panel to determine if the system is ready for input. If the system is not ready, the homeowner must physically close window/doors so that the ready indicator is present [a not ready indicator implies that a sensor is open].
When review leader comes to “control panel” token is passed to the person holding the control panel CRC card.
Phrase “implies that a sensor is open” means a responsibility must validate this.
The control panel CRC card has sensor as a collaborator
The token is next passed to the sensor CRC card.

21. CRC Tips
Don’t generate long lists of responsibilities. This is missing the point. The responsibilities should easily fit on a card.
The review stage is crucial. Spend a lot of time here.