CRC%20

1
CRC What is it?

• Class
• Responsibilities
• Collaborations

2
CRC What is it?

• Classes - a group of objects with similar
  attributes and methods - DOGS
• Object - an instance of a class - SPOT
• Attributes - facts belong to an object - SPOT is
  brown, short hair
• Relationships - an association between objects -
  SPOT to OWNER

3
CRC What is it?

• Collaboration Request a service from another
  object to fulfill a responsibility

Contract
client
server
4
CRC Phase 1

• Find the Objects
• Model the domain
• What must the system accomplish?
• What behavior is clearly outside the system?
• For each goal, what kinds of objects are needed
  to accomplish it?

5
CRC Phase 2

• Determine their Responsibilities
• What does each object have to know in order to
  accomplish each goal it is involved with?
• What steps towards accomplishing each goal is it
  responsible for?

6
CRC Phase 3

• Determine Collaborations
• With whom will each object collaborate in order
  to accomplish each of its responsibilities?
• What other objects in the system hold knowledge
  it needs, or know how to perform some operation
  it requires?
• What is the exact nature of the collaborations
  between the objects?

7
CRC Summary

• Objects become clients and servers within a
  system
• Responsibilities become contracts between them
• Collaborations are a way of determining which
  clients and which servers are bound by which
  contracts

8
CRC Walk-through

• Goal
• For a given network, print different kinds of
  files (Text File, Formatted File, Graphics File)
  to different kinds of printers (Dot Matrix
  Printer, Laser Printer)

9
CRC Walk-through
print
1printRequest(printer)
Print Server
File
user
4print(file)
5getContents()
2add(file)
3printer availablegetNextFile()
Printer
Queue
6printContents()
10
CRC Walk-through

• Classes
• File and subclasses Text File, Formatted File,
  and Graphics File
• Printer and subclasses Dot Matrix Printer and
  Laser Printer
• Print Server
• Queue

11
CRC Walk-through

• Responsibilities
• File
• know how to print itself
• know how to send contents to Printer
• Print Server
• know how to respond to requests to print a file
• know the printers on the network
• Printer
• know how to print the contents of a File
• know its printing status
• know its printing capabilities
• Queue
• know how to respond to request to add and remove
  files
• know how many files it contains

12
CRC Walk-through

• Collaborations
• To fulfill print itself, File is a client of
  Print Server
• To fulfill print request, Print Server is a
  client of Queue and a client of Printer
• To fulfill print contents, Printer is a client
  of File

13
CRC Finding Classes

• Use Requirements Specification
• No requirement specification?
• Write down goals of design
• Look for noun phrases
• Obvious classes/objects
• Obvious nonsense (Discard)
• Dont know???

14
CRC Finding Classes - Guidelines

• Model physical objects
• Disks, printers
• Model conceptual entities
• Windows, Files
• If more than one word is used for the same
  concept, choose the one that is most meaningful
  in terms of the rest of the system.
• Be wary of the use of adjectives. If the use of
  the adjective in conjunction with the noun
  implies different object behavior, then make a
  new class.

15
CRC Finding Classes - Guidelines

• Be wary of sentences in the passive voice, or
  those whose subjects are not part of the system.
• Model categories of classes. (Could be abstract
  superclasses.)
• Model known interfaces to the outside world.
• User Interface
• Model the values of attributes of objects, not
  the attributes. (Float, Wheel, etc.)

16
Drawing Editor Example

• Hand out Drawing Editor Requirements
  Specification

17
Drawing Editor Example

• Hand out Drawing Editor Requirements
  Specification
• Find Preliminary Classes
• Determine Noun Phrases

18
Drawing Editor Example
List of Preliminary Candidates for
Classes drawing editor cursor stop
point interactive graphics editor element line
creation tool user current selection end
point drawing control point rectangle
creation tool line text creation
tool rectangle diagonally opposite corner shape
of the cursor associated corner ellipse I-beam
ellipse creation tool text position point
tool character major radius mode of
operation mouse button width of the
rectangle editor text element minor
radius time corner height of the
rectangle selection tool region bounding
rectangle creation tool crosshair drawing
element start point
19
Drawing Editor Example

• Model physical objects
• Mouse button Probably an attribute of a mouse
  or an event

20
Drawing Editor Example
List of Preliminary Candidates for
Classes drawing editor cursor stop
point interactive graphics editor element line
creation tool user current selection end
point drawing control point rectangle
creation tool line text creation
tool rectangle diagonally opposite corner shape
of the cursor associated corner ellipse I-beam
ellipse creation tool text position point
tool character major radius mode of
operation mouse button width of the
rectangle editor text element minor
radius time corner height of the
rectangle selection tool region bounding
rectangle creation tool crosshair drawing
element start point
21
Drawing Editor Example

• Model conceptual entities
• Useful entities line, rectangle, ellipse,
  text, character, drawing element, drawing
• Tool composed of selection tool creation
  tool (four kinds of creation tool)
• Current selection user manipulates it
• Point, Start point, end point, corner (different
  kinds), major and minor radius, height width of
  rectangle, bounding rectangle?????

22
Drawing Editor Example

• One word for One concept
• Drawing editor, interactive graphics editor, or
  just editor?
• Drawing element or element?
• Text or Text Element?
• Append Element to line, rectangle, ellipse.

23
Drawing Editor Example
List of Preliminary Candidates for
Classes drawing editor cursor stop
point interactive graphics editor element line
creation tool user current selection end
point drawing control point rectangle
creation tool line element text creation
tool rectangle element diagonally opposite
corner shape of the cursor associated
corner ellipse element I-beam ellipse
creation tool text position point tool c
haracter major radius mode of operation mouse
button width of the rectangle editor text
element minor radius time corner height of
the rectangle selection tool region bounding
rectangle creation tool crosshair drawing
element start point
24
Drawing Editor Example

• Watch out for adjectives
• Do selection tool and the various creation tools
  behave differently?
• Rectangle, Bounding Rectangle, Region?
• Point, stop point, end point, control point?
• Corner, diagonally opposite corner, associated
  corner?

25
Drawing Editor Example
List of Preliminary Candidates for
Classes drawing editor cursor stop
point interactive graphics editor element line
creation tool user current selection end
point drawing control point rectangle
creation tool line element text creation
tool rectangle element diagonally opposite
corner shape of the cursor associated
corner ellipse element I-beam ellipse
creation tool text position point tool c
haracter major radius mode of operation mouse
button width of the rectangle editor text
element minor radius time corner height of
the rectangle selection tool region bounding
rectangle creation tool crosshair rectangle
drawing element start point
26
Drawing Editor Example

• Watch out for sentences with no clear subject
• The current selection is indicated visually by
  displaying the control points for the element
• Who is displaying the control points for the
  current selection?
• the user can create an element of the selected
  kind.
• Creation tool classes can be regarded as
  stand-ins for the user

27
Drawing Editor Example

• Model Categories
• Tool and Creation Tools
• Drawing Element

28
Drawing Editor Example

• Model Interfaces
• User?
• User Interface? Yes-part of design phase
• Cursor?
• Time?

29
Drawing Editor Example
List of Preliminary Candidates for
Classes drawing editor cursor stop
point interactive graphics editor element line
creation tool user current selection end
point drawing control point rectangle
creation tool line element text creation
tool rectangle element diagonally opposite
corner shape of the cursor associated
corner ellipse element I-beam ellipse
creation tool text position point tool c
haracter major radius mode of operation mouse
button width of the rectangle editor text
element minor radius time corner height of
the rectangle selection tool region bounding
rectangle creation tool crosshair rectangle dr
awing element start point
30
Drawing Editor Example

• Model values of attributes, not attributes
  themselves
• Height of Rectangle, Width of Rectangle?
• Major radius, minor radius?
• Position?
• Mode of Operation?
• Shape of the cursor?
• Corner?

31
Drawing Editor Example
List of Preliminary Candidates for
Classes drawing editor cursor stop
point interactive graphics editor element line
creation tool user current selection end
point drawing control point rectangle
creation tool line element text creation
tool rectangle element diagonally opposite
corner shape of the cursor associated
corner ellipse element I-beam ellipse
creation tool text position point tool c
haracter major radius mode of operation mouse
button width of the rectangle editor text
element minor radius time corner height of
the rectangle selection tool region bounding
rectangle creation tool crosshair rectangle dr
awing element start point
32
Drawing Editor Example
Summary of Remaining Candidate Classes Character
Ellipse Creation Tool Rectangle Element Control
Point Ellipse Element Selection Tool Creation
Tool Line Creation Tool Text Creation
Tool Current Selection Line Element Text
Element Drawing Point Tool Drawing
Editor Rectangle Drawing Element Rectangle
Creation Tool
33
Drawing Editor Example

• Record candidate classes on CRC card

Class Drawing
34
Drawing Editor Example

• Finding Abstract Classes
• Determine shared behavior
• Look for common attributes
• Group related classes

35
Drawing Editor Example
Tool
Creation Tool
Selection Tool
Rectangle Creation Tool
Line Creation Tool
Ellipse Creation Tool
Text Creation Tool
36
Drawing Editor Example

• Record superclasses and subclasses

Class Creation Tool
Tool
Ellipse Tool, Line Tool, Rectangle Tool, Text Tool