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CRC: Classes, Responsibilities and Collaborations | CS 4311, Papers of Computer Science

University of Texas - El PasoComputer Science

Material Type: Paper; Class: Software Eng: Design & Implmnt; Subject: Computer Science; University: University of Texas - El Paso; Term: Unknown 1989;

Typology: Papers

Pre 2010

Uploaded on 08/19/2009

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CRC: Classes, Responsibilities,
and Collaborations
CS 4311
Questions:
• What is “Object-Oriented Design”?
Question:
• What makes a good class “good”?
Characteristics of a Good Class
Objects Should Responsibilities
• Convey a sense of purpose of an object and
its place in the system
• Record services that a class provides to
fulfill roles within the system
• Record knowledge (maintenance) and
manipulation of information in the system
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CRC: Classes, Responsibilities,

and Collaborations

CS 4311

Questions:

  • What is “Object-Oriented Design”?

Question:

  • What makes a good class “good”?

Characteristics of a Good Class

Objects Should Responsibilities

  • Convey a sense of purpose of an object and its place in the system
  • Record services that a class provides to fulfill roles within the system
  • Record knowledge (maintenance) and manipulation of information in the system

Responsibilities

  • “Doing” responsibilities
  • “Knowing” responsibilities

Responsibilities of a class

  • Convey a sense of purpose of an object
  • Every instance of the class has these responsibilities

CRC

  • CRC is a design method.
  • Why CRC?
    • Intuitive.
    • Easy to teach.

CRC Process

  • Identify classes.
  • Identify responsibilities.
  • Assign responsibilities to classes.
  • Identify collaborations.
  • Collect responsibilities into contracts.
  • Define protocols for contracts.

CRC Card

  • One class per card
  • Write subclasses on lines below the name
  • List attributes and purpose on back of card
  • Assign responsibilities

Concrete Class: Numeric Keypad Superclass: Keypad Subclasses: None Responsibilities Collaborations

Abstract Classes

  • An instance of this is never created
  • Single location for behavior common to several related classes

Finding Classes

  • Look at requirements (SRS)
  • Model physical objects
  • Model conceptual entities
  • Choose meaningful word.

Finding Classes 2

  • Be wary of subjects outside the system. Does an object need to act on behalf of the user?
  • Model categories of classes
  • Model known interfaces to world (user interfaces, interfaces to programs).

Identifying Responsibilities

• SRS:

  • find all verbs that represent actions that must be performed
  • Find all information that must be maintained

Assigning Responsibilities

  • Assign every responsibility to some class
  • Evenly distribute the intelligence and work- load
  • State responsibilities as generally as possible
  • Keep behavior with related information
  • Collect information about 1 thing in 1 place

Relationships between classes

  • To identify more responsibilities
  • Types of relations:

Is-Kind-Of Relationship

  • Frequently indicates superclass/subclass
  • Try to identify common attributes or actions
  • Can the superclass capture responsibility of subclasses?
  • Example: LineCreationTool is a kind of CreationTool - One responsibility is to determine the location - This can be shared (Consider TextCreationTool ) - Think about code reuse

Is-Analogous-to Relationship

  • May indicate superclass/subclass.
  • Example LineCreationTool is analogous to TextCreationTool
  • Look for common or similar responsibilities.

Is-part-of

Relationship

  • Not superclass/subclass.
  • The whole need not act like the parts
  • Need to be careful about assigning responsibilities.
  • Object composed of other objects need to know about its parts.

Two Approaches on Distributing

Intelligence Assigning Responsibilities-