/**
   Connect a phone to the mail system.
*/
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;
      thePhone = 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)
   {
      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;
      thePhone.speak(initialPrompt);
   }

   /**
      Try to connect the user with the specified mail box.
      @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;
            thePhone.speak(currentMailbox.getGreeting());
         }
         else
            thePhone.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;
            thePhone.speak(mailboxMenu);
         }
         else
            thePhone.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;
         thePhone.speak(mailboxMenu);
         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;
         thePhone.speak(mailboxMenu);
      }
   }

   /**
      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;
         thePhone.speak(messageMenu);
      }
      else if (key.equals("2"))
      {
         state = CHANGE_PASSCODE;
         thePhone.speak("Enter new passcode followed by the # key");
      }
      else if (key.equals("3"))
      {
         state = CHANGE_GREETING;
         thePhone.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 += messageMenu;
         thePhone.speak(output);
      }
      else if (key.equals("2"))
      {
         currentMailbox.saveCurrentMessage();
         thePhone.speak(messageMenu);
      }
      else if (key.equals("3"))
      {
         currentMailbox.removeCurrentMessage();
         thePhone.speak(messageMenu);
      }
      else if (key.equals("4"))
      {
         state = MAILBOX_MENU;
         thePhone.speak(mailboxMenu);
      }
   }

   private MailSystem system;
   private Mailbox currentMailbox;
   private String currentRecording;
   private String accumulatedKeys;
   private Telephone thePhone;
   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 initialPrompt = 
      "Please enter mailbox number followed by #";      
   private static final String mailboxMenu = 
      "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 messageMenu = 
      "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";
}

Mailbox.java

/**
   A mailbox contains messages that can be listed, kept or discarded.
*/
public class Mailbox
{
   /**
      Create 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 entered supplied 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
   */
   Message getCurrentMessage()
   {
      if (newMessages.getLength() > 0)
         return newMessages.getHead();
      else if (keptMessages.getLength() > 0)
         return keptMessages.getHead();
      else
         return null;
   }

   /**
      Remove the current message from the mailbox.
      @return the message that has just been removed
   */
   public Message removeCurrentMessage()
   {
      if (newMessages.getLength() > 0)
         return newMessages.remove();
      else if (keptMessages.getLength() > 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
   */
   String getGreeting()
   {
      return greeting;
   }

   private MessageQueue newMessages;
   private MessageQueue keptMessages;
   private String greeting;
   private String passcode;
}

Message.java

/**
   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;
}

Telephone.java

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
   Presents a phone GUI for the voice mail system.
*/
public class Telephone
{
   /**
      Constructs a telephone with a speaker, keypad,
      and microphone.
   */
   public Telephone()
   {

      JPanel speakerPanel = new JPanel();
      speakerPanel.setLayout(new BorderLayout());
      speakerPanel.add(new JLabel("Speaker:"),
         BorderLayout.NORTH);
      speakerField = new JTextArea(10, 25);
      speakerPanel.add(speakerField,
         BorderLayout.CENTER);

      String keyLabels = "123456789*0#";
      JPanel keyPanel = new JPanel();
      keyPanel.setLayout(new GridLayout(4, 3));
      for (int i = 0; i < keyLabels.length(); i++)
      {
         final String label = keyLabels.substring(i, i + 1);
         JButton keyButton = new JButton(label);
         keyPanel.add(keyButton);
         keyButton.addActionListener(new
            ActionListener()
            {
               public void actionPerformed(ActionEvent event)
               {
                  connect.dial(label);
               }
            });
      }

      final JTextArea microphoneField = new JTextArea(10, 25);

      JButton speechButton = new JButton("Send speech");
      speechButton.addActionListener(new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               connect.record(microphoneField.getText());
               microphoneField.setText("");
            }
         });

      JButton hangupButton = new JButton("Hangup");
      hangupButton.addActionListener(new
         ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               connect.hangup();
            }
         });

      JPanel buttonPanel = new JPanel();
      buttonPanel.add(speechButton);
      buttonPanel.add(hangupButton);
      
      JPanel microphonePanel = new JPanel();
      microphonePanel.setLayout(new BorderLayout());
      microphonePanel.add(new JLabel("Microphone:"),
         BorderLayout.NORTH);
      microphonePanel.add(microphoneField,
         BorderLayout.CENTER);
      microphonePanel.add(buttonPanel,
         BorderLayout.SOUTH);      

      JFrame frame = new JFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      Container contentPane = frame.getContentPane();
      contentPane.add(speakerPanel,
         BorderLayout.NORTH);
      contentPane.add(keyPanel, 
         BorderLayout.CENTER);
      contentPane.add(microphonePanel,
         BorderLayout.SOUTH);
      
      frame.pack();
      frame.setVisible(true);
   }

   /**
      Give instructions to the mail system user.
   */
   public void speak(String output)
   {
      speakerField.setText(output);
   }

   public void run(Connection c)
   {
      connect = c;
   }

   private JTextArea speakerField;
   private Connection connect;
}

MailSystemTest.java

public class MailSystemTest
{
   public static void main(String[] args)
   {
      MailSystem system = new MailSystem(MAILBOX_COUNT);

      Telephone p = new Telephone();
      Connection c = new Connection(system, p);
      p.run(c);
   }

   private static int MAILBOX_COUNT = 20;
}

Results