Object-Oriented Design & Patterns

Cay S. Horstmann

Chapter 2

The Object-Oriented Design Process

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Chapter Topics

• From Problem to Code
• The Object and Class Concepts
• Identifying Classes
• Identifying Responsibilities
• Relationships Between Classes
  
• Use Cases
  
• CRC Cards
• UML Class Diagrams
• Sequence Diagrams
• State Diagrams
  
• Using javadoc for Design Documentation
  
• Case Study: A Voice Mail System

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

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<Message> 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
  

Case Study: Voice Mail System

• Use text for voice, phone keys, hangup
  
• 1 2 ... 0 # on a single line means key
  
• H on a single line means "hang up"
• All other inputs mean voice
• In GUI program, will use buttons for keys (see ch. 5)

Use Case: Reach an Extension

1. User dials main number of 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
   

Use Case: Leave a Message

1. Caller carries out Reach an Extension
2. Caller speaks message
3. Caller hangs up
4. System places message in mailbox
   

Use Case: Log in

1. Mailbox owner carries out Reach an Extension
2. Mailbox owner types password and #
(Default password = mailbox number. To change, see Change the Passcode)
3. System plays mailbox menu:
   

   Enter 1 to retrieve your messages.
   Enter 2 to change your passcode.
   Enter 3 to change your greeting.
   

Use Case: Retrieve Messages

1. Mailbox owner carries out Log in
2. Mailbox owner selects "retrieve messages" menu option
3. System plays message menu:
   

   Press 1 to listen to the current message
   Press 2 to delete the current message
   Press 3 to save the current message
   Press 4 to return to the mailbox menu
   

4. Mailbox owner selects "listen to current message"
   
5. System plays current new message, or, if no more new messages, current old message.
   Note: Message is played, not removed from queue
6. System plays message menu
7. User selects "delete current message". Message is removed.
8. Continue with step 3.

Use Case: Retrieve Messages

Variation #1

Use Case: Change the Greeting

1. Mailbox owner carries out Log in
2. Mailbox owner selects "change greeting" menu option
3. Mailbox owner speaks new greeting
4. Mailbox owner presses #
5. System sets new greeting

Use Case: Change the Greeting

Variation #1: Hang up before confirmation

Use Case: Change the Passcode

1. Mailbox owner carries out Log in
2. Mailbox owner selects "change passcode" menu option
3. Mailbox owner dials new passcode
4. Mailbox owner presses #
5. System sets new passcode

Use Case: Change the Passcode

Variation #1: Hang up before confirmation

CRC Cards for Voice Mail System

• Mailbox
• Message
• MailSystem

Initial CRC Cards: Mailbox

Initial CRC Cards: MessageQueue

Initial CRC Cards: MailSystem

Telephone

• Who interacts with user?
  
• Telephone takes button presses, voice input
• Telephone speaks output to user

Telephone

Connection

• With whom does Telephone communicate
• With MailSystem?
• What if there are multiple telephones?
• Each connection can be in different state
  (dialing, recording, retrieving messages,...)
  
• Should mail system keep track of all connection states?
• Better to give this responsibility to a new class

Connection

Analyze Use Case: Leave a message

1. User dials extension. Telephone sends number to Connection
   (Add collaborator Connection to Telephone)
2. Connection asks MailSystem to find matching Mailbox
3. Connection asks Mailbox for greeting
   (Add responsibility "manage greeting" to Mailbox,
   add collaborator Mailbox to Connection)
4. Connection asks Telephone to play greeting
5. User speaks message. Telephone asks Connection to record it.
   (Add responsibility "record voice input" to Connection)
6. User hangs up. Telephone notifies Connection.
7. Connection constructs Message
(Add card for Message class,
   add collaborator  Message to Connection)
8. Connection adds Message to Mailbox
   

Result of Use Case Analysis

Result of Use Case Analysis

Result of Use Case Analysis

Result of Use Case Analysis

Analyse Use Case: Retrieve messages

1. User types in passcode. Telephone notifies Connection
2. Connection asks Mailbox to check passcode.
   (Add responsibility "manage passcode" to Mailbox)
3. Connection sets current mailbox and asks Telephone to speak menu
4. User selects "retrieve messages". Telephone passes key to Connection
5. Connection asks Telephone to speak menu
6. User selects "listen to current message". Telephone passes key to Connection
7. Connection gets first message from current mailbox.
   (Add "retrieve messages" to responsibility of Mailbox).
   Connection asks Telephone to speak message
8. Connection asks Telephone to speak menu
9. User selects "save current message". Telephone passes key to Connection
10. Connection tells Mailbox to save message
    (Modify responsibility of Mailbox to "retrieve,save,delete messages")
11. Connection asks Telephone to speak menu

Result of Use Case Analysis

CRC Summary

• One card per class
• Responsibilities at high level
• Use scenario walkthroughs to fill in cards
• Usually, the first design isn't perfect.
  (You just saw the author's third design of the mail system)

UML Class Diagram for Mail System

• CRC collaborators yield dependencies
• Mailbox depends on MessageQueue
• Message doesn't depends on Mailbox
• Connection depends on Telephone, MailSystem, Message, Mailbox
• Telephone depends on Connection
  

Dependency Relationships

Aggregation Relationships

• A mail system has mailboxes
• A mailbox has two message queues
• A message queue has some number of messages
• A connection has a current mailbox.
  
• A connection has references to a mailsystem and a telephone

UML Class Diagram for Voice Mail System

Sequence Diagram for Use Case: Leave  a message

Interpreting a Sequence Diagram

• Each key press results in separate call to dial, but only one is shown
  
• Connection wants to get greeting to play
• Each mailbox knows its greeting
  
• Connection must find mailbox object:
  Call findMailbox on MailSystem object
• Parameters are not displayed (e.g. mailbox number)
• Return values are not displayed (e.g. found mailbox)
• Note that connection holds on to that mailbox over multiple calls
  

Sequence Diagram for Use Case: Retrieve messages

Connection State Diagram

Java Implementation

• ch02/mail/Message.java
• ch02/mail/MessageQueue.java
• ch02/mail/Mailbox.java
• ch02/mail/Connection.java
• ch02/mail/MailSystem.java
• ch02/mail/Telephone.java
• ch02/mail/MailSystemTester.java