Transcript CS 4311 Software Design and Implementation

CRC: Classes, Responsibilities,
and Collaborations
CS 4311
12/09
OO Design
• Identify or define collections of data and
operations on the data
• Define classes
• Determine how classes interact
Objects Should
Characteristics of a Good Class
Responsibilities
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responsibilities
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responsibilities
Aside: Objects are not just “data
and methods”
• At this point in design, you must consider
objects at the conceptual level.
• An object is something that has
responsibilities.
CRC
• CRC is a design method.
• Why CRC?
– Intuitive.
– Easy to teach.
CRC Process
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Identify classes.
Identify responsibilities.
Assign responsibilities to classes.
Identify collaborations.
Collect responsibilities into contracts.
Define protocols for contracts.
CRC Card
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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
Collaboration
• A collaboration occurs when one class
requires the services of another class in
order to fulfill its responsibilities.
• Indicates classes are coupled.
Contracts
• A contract is a set of related responsibilities.
• A contract specifies a service provided by
one class to other classes
Protocols
• A protocol contains
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method name and type
method parameters and types
preconditions
postconditions
• A protocol is a formalization of a contract
Finding Classes
• A class is a collection of objects.
• Objects = “things”
• Look for nouns in documents such as the
requirements (SRS)
Finding Classes: Advice
• Be careful of nouns outside the system.
– Does an object need to act on behalf of the
user?
• Model known interfaces to world (user
interfaces, interfaces to programs).
Identifying Responsibilities
• Responsibilities are doing and knowing
• This suggests verbs in English
• Look for verbs in documents
– Actions that must be performed
– Modes of maintaining information
Assigning Responsibilities
• Assign every responsibility to some class
• Evenly distribute the intelligence and workload
• State responsibilities as generally as
possible
• Keep behavior with related information
• Collect information about 1 thing in 1 place
Example
There is a voting system with several types of voting machines.
These include touch screen machines, voice activated
machines (for the sight impaired), and a mouse/keyboard
machine (prototyping an interface for web-based voting).
There are several different ballots for the machines, one for
the city, one for the county, and one for the state. A voter
approaches the machine, enters the keycode given to her or
him by the election clerk, and the system presents the ballots,
one at a time. The voter makes selections, and then casts each
of the ballots. Once cast, a vote cannot be modified.
Example: Nouns
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system
Voting machine
Machine
Touch Screen Machine
Voice Activated Machine
Mouse/Keyboard
Machine
Interface
Ballot
One
Keycode
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City
County
State
Her/Him
Election Clerk
Time
Voter
Selection
Each
Vote
Example: Classes
Likely
• Machine
– Touch Screen
– Voice Activated
– Mouse Keyboard
• Ballot
– State
– County
– City
• Voter
• Vote
Questionable
• keycode
• selection
Example: Responsibilities
• Machine
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Knows which ballots are in the election
Can present ballots to a voter
Can validate a keycode
Knows what selections a voter has made
Can cast a ballot
• Vote
– Knows what ballot is being voted on
– Knows a voters selection for items on the ballot
• Ballot
– Knows the type of item (issue or candidate)
– Knows the seats and candidates
– Knows the issues and the choices
Example: Collaborations
• Machine and Vote are clients of Ballot
• Machine is a client of Vote
Problem #1 Missing Classes
• If you can’t easily assign a responsibility,
you may be missing a class.
• Try to reduce complexity
• Example: grouping elements in a drawing.
– The element need not know it’s part of a group.
– The drawing need not know about groups.
– But a group class can handle the
responsibilities. The group class knows about
its members and propagates actions to them.
Problem #2 Arbitrary
Assignment
• If you don’t know what class to assign a
responsibility to, consider
– What are the possibilities (what classes can you
assign to)?
– What are the implications?
• Which is most obvious?
• Which is most efficient?
Two Approaches on Distributing
Intelligence
Assigning Responsibilities-1
• Heuristic: Evenly distribute intelligence
– Intelligence is what the system knows, actions
that it can perform, and impact it has on other
systems and users with which it interacts
– A class gets intelligence based on how much it
knows or can do, and how many other objects it
can affect.
Assigning Responsibilities-2
• Heuristic: State responsibilities as generally as
possible.
– Can find common responsibilities that can be shared
• Heuristic: Keep behavior with related
information.
– If information changes, no update messages must be
sent between objects
– The object that needs to know that something has
changed will know it.
Assigning Responsibilities-3
• Heuristic: Keep information about one thing in
one place
– Do not share responsibility for maintaining specific
information!!
– Minimize duplication
Advice
• Try to express responsibilities in general
terms. You are still exploring.
• Try to keep all of a class’s responsibilities
at the same conceptual level (same level of
abstraction).
Walk-through
• What is happening when the machine is
idle?
• What happens when a card is inserted?
• What happens when the user chooses
“withdrawal” from main menu?
• What happens when the user presses the
cancel key?
Finding abstract classes
• Re-examine list of candidate classes and
group related classes
– Identify those that share attributes
– Identify those that share behavior
• Goal is to identify as many abstract classes
as possible