Software EngineeringSoftware Engineering

Object Oriented Analysis

ObjectivesObjectives

To explain Class-Responsibility-Collaborator Modelling.

To provide an example of CRC modelling in action

Analysis = Process + ModelsAnalysis = Process + Models

Process 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

Class-Responsibility-Collaborator Class-Responsibility-Collaborator ModellingModelling

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

CRC BenefitsCRC 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.

CRC CardsCRC Cards

class name:

class type: (e.g., device, property, role, event, ...)

class characteristics: (e.g., tangible, atomic, concurrent, ...)

responsibilities: collaborators:

[1] Identifying Classes[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”

Accepting ClassesAccepting Classes

1. Objects are accepted if they satisfy all (or almost all) of the following requirements:

2. Retained Information: the system needs to remember data about the object

3. Needed Services: the object must have an identifiable set of operations

4. Multiple Attributes: during analysis we focus on major information only. Objects should have many attributes

5. Common Attributes: A set of attributes apply to all occurrences of the object

6. Common Operations: A set of operations apply to all occurrences of the object

7. Essential Requirements: external entities that produce or consume essential information

Class TypesClass Types

1. Accepted classes are assigned a type:

2. External entities - other systems, devices, people

3. Things - reports, displays, signals

4. Occurrences or events - property transfer, completion of a series of robot movements

5. Roles - manager, engineer, sales person

6. Organizational Units - division, group, team

7. Places - manufacturing floor, loading dock

8. Devices - four-wheeled vehicles, computers

9. Property - of the problem, e.g. credit rating

10. Interaction - model interaction that occur among other objects, e.g. a purchase or a license

Class CharacteristicsClass Characteristics

1. Accepted classes are assigned a set of characteristics:

2. Tangibility: does the class represent a tangible (physical) or abstract (information) entity?

3. Inclusiveness: is the class atomic (includes no other classes) or aggregate (has at least one nested object)?

4. Sequentiality: is the class concurrent (has its own thread of control) or sequential (scheduled by outside resources)?

5. 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)?

6. Integrity: is the class corruptible (does not protect its resources from outside influence) or guarded (the class enforces access control)?

Example: Identifying ClassesExample: 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, …

Example: Accepting ClassesExample: Accepting Classes

Potential Object/Class Class requirements

Homeowner Rejected: 1, 2 (retained information, needed service) fail

Sensor Accepted: all apply

Control Panel Accepted: all apply

Installation rejected

System Accepted: all apply

Number and Type Rejected: 3 (multiple attributes) fails, attribute of sensor

Master Password Rejected: 3 fails

Telephone Number Rejected: 3 fails

Sensor Event Accepted: all apply

Example: CRC HeaderExample: CRC Header

class name: Sensor

class type: external entity

class characteristics: tangible, atomic, concurrent, guarded

responsibilities: collaborators:

[2] Identifying Responsibilities[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

Guidelines for Allocating Guidelines for Allocating Responsibilities to ClassesResponsibilities to Classes

1. System intelligence should be evenly distributed.

2. Each responsibility should be stated as generally as possible.

3. Information and the behavior that is related to it should reside within the same class.

4. Information about one thing should be localized with a single class, not distributed across multiple classes.

5. Responsibilities should be shared among related classes, when appropriate.

Example: Identifying ResponsibilitiesExample: 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)

Example: CRC ResponsibilitiesExample: 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 informationassign sensor informationsignal sensor event

[3] Identifying Collaborators[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)

[4] Reviewing the CRC Model[4] Reviewing the CRC Model

1. All participants in the review (of the CRC model) are given a subset of the CRC model index cards.

2. All use-case scenarios (and corresponding use-case diagrams) should be organized into categories.

3. 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.

4. 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.

5. If the responsibilities and collaborations noted on the index cards cannot accommodate the use-case, modifications are made to the cards.

Example: CRC ReviewExample: 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.

CRC TipsCRC 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.