## Object-Oriented Design & Patterns

#### Cay S. Horstmann

### Chapter 2

### The Object-Oriented Design Process

## 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

Three Phases:

- Analysis
- Design
- Implementation

Case Study: Voice Mail System

## Analysis Phase

Functional Specification

- 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

Goals

- Identify classes
- Identify behavior of classes
- Identify relationships among classes

Artifacts

- 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

Rule of thumb: Look for nouns in problem description

- `Mailbox`
- `Message`
- `User`
- `Passcode`
- `Extension`
- `Menu`

## Identifying Classes

Focus on concepts, not implementation

- `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

Rule of thumb: Look for verbs in problem description
Behavior of `MessageQueue`:

- 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

1.1. In step 3, user enters invalid extension number
1.2. Voice mail system speaks

         You have typed an invalid mailbox number.

1.3. Continue with step 2.

### Variation #2

2.1. After step 4, caller hangs up instead of speaking message
2.3. Voice mail system discards empty message

## 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

- 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

1.1. Start at Step 6
1.2. User selects "save current message".
Message is removed from new queue and appended to old queue
1.3. Continue with step 3.

## 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

1.1. Start at step 3.
1.2. Mailbox owner hangs up.
1.3. System keeps old greeting.

## 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

1.1. Start at step 3.
1.2. Mailbox owner hangs up.
1.3. System keeps old passcode.

## CRC Cards for Voice Mail System

Some obvious classes

- `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

## 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

- Ch2/mail/Message.java
- Ch2/mail/MessageQueue.java
- Ch2/mail/Mailbox.java
- Ch2/mail/Connection.java
- Ch2/mail/MailSystem.java
- Ch2/mail/Telephone.java
- Ch2/mail/MailSystemTester.java