SE450: Horstmann Chapter 2

Cay S. Horstmann

Chapter 2

The Object-Oriented Design Process

• 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

• Analysis
• Design
• Implementation

• 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

• 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

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

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

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

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

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

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

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

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

• 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

• Minimize dependency: reduce coupling
• Example: Replace
  

  void print() // prints to System.out
  

  with
  

  String getText() // can print anywhere
  

• Removes dependence on System, PrintStream

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

• 1 : 1 or 1 : 0...1 relationship:
  

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

• 1 : n relationship:
  

  public class MessageQueue
  {
     . . .
     private ArrayList<Message> elements;
  }
  

• 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)

• 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

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

Variation #1

     You have typed an invalid mailbox number.

Variation #2

• 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

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

• 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

• 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

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

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

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

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

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

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

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

• Use for classes whose objects have interesting states

• 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
  

• 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. 4)

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
   

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

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.
   

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.

Variation #1

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

Variation #1: Hang up before confirmation

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

Variation #1: Hang up before confirmation

• Mailbox
• Message
• MailSystem

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

• 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

1. User dials extension. Telephone sends number to Connection
   (Add collaborator Telephone to Connection)
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 greeting. 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
   

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

• 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)

• 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
  

• 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

• 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
  

file:horstmann/ch02_mail/Message.java [source] [doc-public] [doc-private]

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 package horstmann.ch02_mail; /** A message left by the caller. */ public class Message { /** Construct a Message object. @param messageText the message text */ public Message(String messageText) { text = messageText; } /** Get the message text. @return message text */ public String getText() { return text; } private String text; }

file:horstmann/ch02_mail/MessageQueue.java [source] [doc-public] [doc-private]

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 package horstmann.ch02_mail; import java.util.ArrayList; /** A first-in, first-out collection of messages. This implementation is not very efficient. We will consider a more efficient implementation in chapter 3. */ public class MessageQueue { /** Constructs an empty message queue. */ public MessageQueue() { queue = new ArrayList<Message>(); } /** Remove message at head. @return message that has been removed from the queue */ public Message remove() { return queue.remove(0); } /** Append message at tail. @param newMessage the message to be appended */ public void add(Message newMessage) { queue.add(newMessage); } /** Get the total number of messages in the queue. @return the total number of messages in the queue */ public int size() { return queue.size(); } /** Get message at head. @return message that is at the head of the queue, or null if the queue is empty */ public Message peek() { if (queue.size() == 0) return null; else return queue.get(0); } private ArrayList<Message> queue; }

file:horstmann/ch02_mail/Mailbox.java [source] [doc-public] [doc-private]

001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 package horstmann.ch02_mail; /** A mailbox contains messages that can be listed, kept or discarded. */ public class Mailbox { /** Creates Mailbox object. @param aPasscode passcode number @param aGreeting greeting string */ public Mailbox(String aPasscode, String aGreeting) { passcode = aPasscode; greeting = aGreeting; newMessages = new MessageQueue(); keptMessages = new MessageQueue(); } /** Check if the passcode is correct. @param aPasscode a passcode to check @return true if the supplied passcode matches the mailbox passcode */ public boolean checkPasscode(String aPasscode) { return aPasscode.equals(passcode); } /** Add a message to the mailbox. @param aMessage the message to be added */ public void addMessage(Message aMessage) { newMessages.add(aMessage); } /** Get the current message. @return the current message */ public Message getCurrentMessage() { if (newMessages.size() > 0) return newMessages.peek(); else if (keptMessages.size() > 0) return keptMessages.peek(); else return null; } /** Remove the current message from the mailbox. @return the message that has just been removed */ public Message removeCurrentMessage() { if (newMessages.size() > 0) return newMessages.remove(); else if (keptMessages.size() > 0) return keptMessages.remove(); else return null; } /** Save the current message */ public void saveCurrentMessage() { Message m = removeCurrentMessage(); if (m != null) keptMessages.add(m); } /** Change mailbox's greeting. @param newGreeting the new greeting string */ public void setGreeting(String newGreeting) { greeting = newGreeting; } /** Change mailbox's passcode. @param newPasscode the new passcode */ public void setPasscode(String newPasscode) { passcode = newPasscode; } /** Get the mailbox's greeting. @return the greeting */ public String getGreeting() { return greeting; } private MessageQueue newMessages; private MessageQueue keptMessages; private String greeting; private String passcode; }

file:horstmann/ch02_mail/Connection.java [source] [doc-public] [doc-private]

001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 package horstmann.ch02_mail; /** Connects a phone to the mail system. The purpose of this class is to keep track of the state of a connection, since the phone itself is just a source of individual key presses. */ @SuppressWarnings("all") public class Connection { /** Construct a Connection object. @param s a MailSystem object @param p a Telephone object */ public Connection(MailSystem s, Telephone p) { system = s; phone = p; resetConnection(); } /** Respond to the user's pressing a key on the phone touchpad @param key the phone key pressed by the user */ public void dial(String key) { if (state == CONNECTED) connect(key); else if (state == RECORDING) login(key); else if (state == CHANGE_PASSCODE) changePasscode(key); else if (state == CHANGE_GREETING) changeGreeting(key); else if (state == MAILBOX_MENU) mailboxMenu(key); else if (state == MESSAGE_MENU) messageMenu(key); } /** Record voice. @param voice voice spoken by the user */ public void record(String voice) { if (state == RECORDING || state == CHANGE_GREETING) currentRecording += voice; } /** The user hangs up the phone. */ public void hangup() { if (state == RECORDING) currentMailbox.addMessage(new Message(currentRecording)); resetConnection(); } /** Reset the connection to the initial state and prompt for mailbox number */ private void resetConnection() { currentRecording = ""; accumulatedKeys = ""; state = CONNECTED; phone.speak(INITIAL_PROMPT); } /** Try to connect the user with the specified mailbox. @param key the phone key pressed by the user */ private void connect(String key) { if (key.equals("#")) { currentMailbox = system.findMailbox(accumulatedKeys); if (currentMailbox != null) { state = RECORDING; phone.speak(currentMailbox.getGreeting()); } else phone.speak("Incorrect mailbox number. Try again!"); accumulatedKeys = ""; } else accumulatedKeys += key; } /** Try to log in the user. @param key the phone key pressed by the user */ private void login(String key) { if (key.equals("#")) { if (currentMailbox.checkPasscode(accumulatedKeys)) { state = MAILBOX_MENU; phone.speak(MAILBOX_MENU_TEXT); } else phone.speak("Incorrect passcode. Try again!"); accumulatedKeys = ""; } else accumulatedKeys += key; } /** Change passcode. @param key the phone key pressed by the user */ private void changePasscode(String key) { if (key.equals("#")) { currentMailbox.setPasscode(accumulatedKeys); state = MAILBOX_MENU; phone.speak(MAILBOX_MENU_TEXT); accumulatedKeys = ""; } else accumulatedKeys += key; } /** Change greeting. @param key the phone key pressed by the user */ private void changeGreeting(String key) { if (key.equals("#")) { currentMailbox.setGreeting(currentRecording); currentRecording = ""; state = MAILBOX_MENU; phone.speak(MAILBOX_MENU_TEXT); } } /** Respond to the user's selection from mailbox menu. @param key the phone key pressed by the user */ private void mailboxMenu(String key) { if (key.equals("1")) { state = MESSAGE_MENU; phone.speak(MESSAGE_MENU_TEXT); } else if (key.equals("2")) { state = CHANGE_PASSCODE; phone.speak("Enter new passcode followed by the # key"); } else if (key.equals("3")) { state = CHANGE_GREETING; phone.speak("Record your greeting, then press the # key"); } } /** Respond to the user's selection from message menu. @param key the phone key pressed by the user */ private void messageMenu(String key) { if (key.equals("1")) { String output = ""; Message m = currentMailbox.getCurrentMessage(); if (m == null) output += "No messages." + "\n"; else output += m.getText() + "\n"; output += MESSAGE_MENU_TEXT; phone.speak(output); } else if (key.equals("2")) { currentMailbox.saveCurrentMessage(); phone.speak(MESSAGE_MENU_TEXT); } else if (key.equals("3")) { currentMailbox.removeCurrentMessage(); phone.speak(MESSAGE_MENU_TEXT); } else if (key.equals("4")) { state = MAILBOX_MENU; phone.speak(MAILBOX_MENU_TEXT); } } private MailSystem system; private Mailbox currentMailbox; private String currentRecording; private String accumulatedKeys; private Telephone phone; private int state; private static final int DISCONNECTED = 0; private static final int CONNECTED = 1; private static final int RECORDING = 2; private static final int MAILBOX_MENU = 3; private static final int MESSAGE_MENU = 4; private static final int CHANGE_PASSCODE = 5; private static final int CHANGE_GREETING = 6; private static final String INITIAL_PROMPT = "Enter mailbox number followed by #"; private static final String MAILBOX_MENU_TEXT = "Enter 1 to listen to your messages\n" + "Enter 2 to change your passcode\n" + "Enter 3 to change your greeting"; private static final String MESSAGE_MENU_TEXT = "Enter 1 to listen to the current message\n" + "Enter 2 to save the current message\n" + "Enter 3 to delete the current message\n" + "Enter 4 to return to the main menu"; }

file:horstmann/ch02_mail/MailSystem.java [source] [doc-public] [doc-private]

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 package horstmann.ch02_mail; import java.util.ArrayList; /** A system of voice mail boxes. */ public class MailSystem { /** Constructs a mail system with a given number of mailboxes @param mailboxCount the number of mailboxes */ public MailSystem(int mailboxCount) { mailboxes = new ArrayList<Mailbox>(); // Initialize mail boxes. for (int i = 0; i < mailboxCount; i++) { String passcode = "" + (i + 1); String greeting = "You have reached mailbox " + (i + 1) + ". \nPlease leave a message now."; mailboxes.add(new Mailbox(passcode, greeting)); } } /** Locate a mailbox. @param ext the extension number @return the mailbox or null if not found */ public Mailbox findMailbox(String ext) { int i = Integer.parseInt(ext); if (1 <= i && i <= mailboxes.size()) return mailboxes.get(i - 1); else return null; } private ArrayList<Mailbox> mailboxes; }

file:horstmann/ch02_mail/Telephone.java [source] [doc-public] [doc-private]

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 package horstmann.ch02_mail; import java.util.Scanner; /** A telephone that takes simulated keystrokes and voice input from the user and simulates spoken text. */ public class Telephone { /** Construct phone object. @param aScanner that reads text from a character-input stream */ public Telephone(Scanner aScanner) { scanner = aScanner; } /** Speak a message to System.out. @param output the text that will be "spoken" */ public void speak(String output) { System.out.println(output); } /** Loops reading user input and passes the input to the Connection object's methods dial, record or hangup. @param c the connection that connects this phone to the voice mail system */ public void run(Connection c) { boolean more = true; while (more) { String input = scanner.nextLine(); if (input == null) return; if (input.equalsIgnoreCase("H")) c.hangup(); else if (input.equalsIgnoreCase("Q")) more = false; else if (input.length() == 1 && "1234567890#".indexOf(input) >= 0) c.dial(input); else c.record(input); } } private Scanner scanner; }

file:horstmann/ch02_mail/MailSystemTester.java [source] [doc-public] [doc-private]

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 package horstmann.ch02_mail; import java.util.Scanner; /** This program tests the mail system. A single phone communicates with the program through System.in/System.out. */ public class MailSystemTester { public static void main(String[] args) { MailSystem system = new MailSystem(MAILBOX_COUNT); Scanner console = new Scanner(System.in); Telephone p = new Telephone(console); Connection c = new Connection(system, p); p.run(c); } private static final int MAILBOX_COUNT = 20; }

Revised: 2007/09/11 16:16