Object-Oriented Design Process

From Problem to Code

• Analysis
• Design
• Implementation

Analysis Phase

• Completely defines tasks to be solved
• Free from internal contradictions
• Readable both by domain experts and software developers
• Reviewable by diverse interested parties
• Testable against reality

Describe what needs to be done, not how it should be done.

Develop a set of use cases. A use case is a description of a sequence of actions that yields a benefit for the user of a system.

Design Phase

• Identify classes
• Identify behavior of classes
• Identify relationships among classes

• Textual description of classes and key methods
• Diagrams of class relationships
• Diagrams of important usage scenarios
• State diagrams for objects with rich state

Implementation Phase

• Implement and test classes
• Combine classes into program
• Avoid "big bang" integration
• Prototypes can be very useful

Object and Class Concepts

• Object: Three characteristic concepts
• State
• Behavior
• Identity
• Class: Collection of similar objects

Identifying Classes

• Mailbox
• Message
• User
• Passcode
• Extension
• Menu

Identifying Classes

• MessageQueue stores messages
• Don't worry yet how the queue is implemented
  

Categories of Classes

• Tangible Things
• Agents
• Events and Transactions
• Users and Roles
• Systems
• System interfaces and devices
• Foundational Classes

Identifying Responsibilities

• Add message to tail
• Remove message from head
• Test whether queue is empty

Responsibilities

• OO Principle: Every operation is the responsibility of a single class
• Example: Add message to mailbox
• Who is responsible: Message or Mailbox?

Class Relationships

• Dependency ("uses")
• Aggregation ("has")
• Inheritance ("is")

Dependency Relationship

• C  depends on D: Method of C manipulates objects of D
• Example: Mailbox depends on Message
• If C doesn't use D, then C can be developed without knowing about D

Coupling

• Minimize dependency: reduce coupling
• Example: Replace
  

  void print() // prints to System.out
  

  with
  

  String getText() // can print anywhere

• Removes dependence on System, PrintStream

Aggregation

• Object of a class contains objects of another class
• Example: MessageQueue aggregates Messages
• Example: Mailbox aggregates MessageQueue
• Implemented through instance fields

Multiplicities

• 1 : 1 or 1 : 0...1 relationship:
  

  public class Mailbox
  {
     . . .
     private Greeting myGreeting;
  }

• 1 : n relationship:
  

  public class MessageQueue
  {
     . . .
     private ArrayList elements;
  }

Inheritance

• More general class = superclass
• More specialized class = subclass
• Subclass supports all method interfaces of superclass (but implementations may differ)
• Subclass may have added methods, added state
• Subclass inherits from superclass
• Example: ForwardedMessage inherits from Message
• Example: Greeting does not inherit from Message (Can't store greetings in mailbox)

Use Cases

• Analysis technique
• Each use case focuses on a specific scenario
• Use case = sequence of actions
• Action = interaction between actor and computer system
  
• Each action yields a result
• Each result has a value to one of the actors
• Use variations for exceptional situations

Sample Use Case

Leave a Message

1. Caller dials main number of voice mail system
2. System speaks prompt
   

   Enter mailbox number followed by #

3. User types extension number
4. System speaks
   

   You have reached mailbox xxxx. Please leave a message now

5. Caller speaks message
6. Caller hangs up
7. System places message in mailbox

Sample Use Case -- Variations

Variation #1

     You have typed an invalid mailbox number.

Variation #2

CRC Cards

• CRC = Classes, Responsibilities, Collaborators
  
• Developed by Beck and Cunningham
• Use an index card for each class
• Class name on top of card
• Responsibilities on left
• Collaborators on right 

CRC Cards

CRC Cards

• Responsibilities should be high level
• 1 - 3 responsibilities per card
• Collaborators are for the class, not for each responsibility

Walkthroughs

• Use case: "Leave a message"
• Caller connects to voice mail system
• Caller dials extension number
• "Someone" must locate mailbox
• Neither Mailbox nor Message can do this
• New class: MailSystem
• Responsibility: manage mailboxes

Walkthroughs

UML Diagrams

• UML = Unified Modeling Language
• Unifies notations developed by the "3 Amigos" Booch, Rumbaugh, Jacobson
  
• Many diagram types
• We'll use three types:
• Class Diagrams
• Sequence Diagrams
• State Diagrams

Class Diagrams

• Rectangle with class name
• Optional compartments
• Attributes
• Methods
• Include only key attributes and methods

Class Diagrams

Class Relationships

Multiplicities

• any number (0 or more): *
• one or more: 1..*
• zero or one: 0..1
• exactly one: 1

Composition

• Special form of aggregation
• Contained objects don't exist outside container
• Example: message queues permanently contained in mail box

Association

• Some designers don't like aggregation
• More general association relationship
• Association can have roles

Association

• Some associations are bidirectional
  Can navigate from either class to the other
• Example: Course has set of students, student has set of courses
  
• Some associations are directed
  Navigation is unidirectional
• Example: Message doesn't know about message queue containing it
  

Interface Types

• Interface type describes a set of methods
• No implementation, no state
• Class implements interface if it implements its methods
  
• In UML, use stereotype «interface»

Tips

• Use UML to inform, not to impress
  
• Don't draw a single monster diagram
• Each diagram must have a specific purpose
• Omit inessential details
  

Sequence Diagrams

• Each diagram shows dynamics of scenario
• Object diagram: class name underlined

Self call

Object Construction

State Diagram

• Use for classes whose objects have interesting states

Design Documentation

• Recommendation: Use Javadoc comments
• Leave methods blank
  

  /**
     Adds a message to the end of the new messages.
     @param aMessage a message
  */
  public void addMessage(Message aMessage)
  {
  }

• Don't compile file, just run Javadoc
• Makes a good starting point for code later
  

Reference