I must be missing the point but...

Question

I do not know how to ask this question without an example so please bear with me....
My example is an contact book, say I have many friends (If only more of them were Java programmers) but quite a few share the same telephone number and, pervesely, quite often move to another, often already shared, telephone number. Phew!
So, I have:
an Telephone class:
String number
a Friend class:
String name;
Telephone tel;
and myself:
Friend[] friends;
Suppose I write a GUI that wraps this hierarchy; it shows me a list of my friends and allows me to edit them - specifically to change their telephone numbers.
Now, here's the bit I don't understand:
Somewhere I have to have a list of telephone numbers so that when I pop up my 'friend editor' I can generate a drop down list of available telephone numbers (or even add a new one).
Does this list exist in 'myself', should I loop through all the friends to find out what're available or something else?
Also, surely I should 'encapsulate' the 'friend editor' in some way so that 'it just knows' where to get the list from (Ie popup a 'telephone number selector') and if so how would it achieve this?
This is not a GUI/Swing question - I'm pretty sure I can make a half decent GUI - but a question as to how the classes interract with each other whilst still maintaining encapsulation.
Lastly, I must be missing the point or, due to my C experience, expecting too little of the language but all the tutorials seem to miss this interraction - they seem either to build bare GUIs or basic/extended classes but never a real application showing this aspect.
Please can someone furnish an explanation/example ?
Thanks
Philip

Answer 1

oh goodie I love discussions like this... well, sort of. I developed a program to store contacts, each contact had a name, an address and various other fields, so what's the relavence? Well, you want a program to store phone numbers and what not, and you need different classes to accomplish running the program, well this is where I think I can help. In my program there were three methods in the main class, they did delete, insert, and edit, then there was save, and load. Inside of those methods is where things came to life. I hope the code below helps in your program somehow.// cm.java
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
// =========================================================
// Class: cm
// This class drives the contact manager.  It contains the
// main method which gets called as soon as this application
// begins to run.
// =========================================================
class cm extends Frame implements ActionListener
   // Container of contact objects (one object per business
   // contact).
   private Vector contacts = new Vector (100);
   // List of names component.  (Must specify java.awt in
   // front of List to distinguish the List class in the
   // java.awt package from the List class in the java.util
   // package.)
   private java.awt.List names = new java.awt.List ();
   // Delete and update button components.
   private Button delete;
   private Button update;
   // Default constructor.
   public cm ()
      // Assign Contact Manager to title bar of frame window.
      super ("Contact Manager");
      // Add a listener that responds to window closing
      // events.  When this event occurs (by clicking on the
      // close box in the title bar), save contacts and exit.
      addWindowListener (new WindowAdapter ()
                             public void windowClosing
                                           (WindowEvent e)
                                saveContacts ();
                                System.exit (0);
      // Place an empty label in the north part of the frame
      // window.  This is done to correct an AWT positioning
      // problem.  (One thing that you'll come to realize as
      // you work with the AWT is that there are lots of bugs.)
      Label l = new Label ();
      add ("North", l);
      // Place the names component in the center part of the
      // frame window.
      add ("Center", names);
      // Create a panel object to hold four buttons.
      Panel p = new Panel ();
      Button b;
      // Add an Insert button to the Panel object and register
      // the current cm object as a listener for button events.
      p.add (b = new Button ("Insert"));
      b.addActionListener (this);
      // Add a Delete button to the Panel object and register
      // the current cm object as a listener for button events.
      p.add (delete = new Button ("Delete"));
      delete.addActionListener (this);
      // The Delete button should be disabled until there is at
      // least one contact to delete.
      delete.setEnabled (false);
      // Add an Update button to the Panel object and register
      // the current cm object as a listener for button events.
      p.add (update = new Button ("Update"));
      update.addActionListener (this);
      // The Update button should be disabled until there is at
      // least one contact to update.
      update.setEnabled (false);
      // Add a Finish button to the Panel object and register
      // the current cm object as a listener for button events.
      p.add (b = new Button ("Finish"));
      b.addActionListener (this);
      // Add the panel object to the frame window container.
      add ("South", p);
      // Set the background of the frame window container to
      // lightGray (to give a pleasing effect).
      setBackground (Color.lightGray);
      // Set the size of the frame window container to 400
      // pixels horizontally by 200 pixels vertically.
      setSize (400, 200);
      // Do not allow the user to resize the frame window.
      setResizable (false);
      // Load all contacts.
      loadContacts ();
      // Make sure that the frame window is visible.
      setVisible (true);
   public void actionPerformed (ActionEvent e)
      if (e.getActionCommand ().equals ("Delete"))
          delete ();
      else
      if (e.getActionCommand ().equals ("Finish"))
          saveContacts ();
          System.exit (0);
      else
      if (e.getActionCommand ().equals ("Insert"))
          insert ();
      else
          update ();
   public Insets getInsets ()
      // Return an Insets object that describes the number of
      // pixels to reserve as a border around the edges of the
      // frame window.
      return new Insets (10, 10, 10, 10);
   public static void main (String [] args)
      // Create a new cm object and let it do its thing.
      new cm ();
   private void delete ()
      // Obtain index of selected contact item from the names
      // component.
      int index = names.getSelectedIndex ();
      // If no item was selected, index is -1.  We cannot update
      // a contact if no contact item in the names component was
      // selected - because we would have nothing to work with.
      if (index != -1)
          // Remove the contact item from the names component.
          names.remove (index);
          // Remove the Contact object from the contacts Vector
          // object.
          contacts.remove (index);
          // If there are no more contacts ...
          if (contacts.size () == 0)
              delete.setEnabled (false);
              update.setEnabled (false);
          else
          // Make sure that the first contact item in the names
          // list is highlighted.
          names.select (0);
   private void insert ()
      // Create an Insert data entry form to enter information
      // for a new contact.
      DataEntryForm def = new DataEntryForm (this, "Insert");
      // If the bOk Boolean flag is set, this indicates the user
      // exited the form by pressing the Ok button.
      if (def.bOk)
          // Create a Contact object and assign information from
          // the form to its fields.
          Contact temp = new Contact ();
          temp.fname = new String (def.fname.getText ());
          temp.lname = new String (def.lname.getText ());
          temp.phone = new String (def.phone.getText ());
          temp.fax = new String (def.fax.getText ());
          temp.email = new String (def.email.getText ());
          // Add a new contact item to the names component.
          names.add (temp.lname + ", " + temp.fname);
          // Add the Contact object to the contacts Vector
          // object.
          contacts.add (temp);
          // Make sure that the Delete and Update buttons are
          // enabled.
          delete.setEnabled (true);
          update.setEnabled (true);
      // Destroy the dialog box.
      def.dispose ();
      // Make sure that the first contact item in the names list
      // is highlighted.
      names.select (0);
   // ===========================================================
   // Load all contacts from contacts.dat into the contacts
   // Vector object.  Also, make sure that the last name/first
   // name from each contact is combined into a String object and
   // inserted into the names component - as a contact item.
   // ===========================================================
   private void loadContacts ()
      FileInputStream fis = null;
      try
          fis = new FileInputStream ("contacts.dat");
          DataInputStream dis = new DataInputStream (fis);
          int nContacts = dis.readInt ();
          for (int i = 0; i < nContacts; i++)
               Contact temp = new Contact ();
               temp.fname = dis.readUTF ();
               temp.lname = dis.readUTF ();
               temp.phone = dis.readUTF ();
               temp.fax = dis.readUTF ();
               temp.email = dis.readUTF ();
               names.add (temp.lname + ", " + temp.fname);
               contacts.add (temp);
          if (nContacts > 0)
              delete.setEnabled (true);
              update.setEnabled (true);
      catch (IOException e)
      finally
         if (fis != null)
             try
                 fis.close ();
             catch (IOException e) {}
      // Make sure that the first contact item in the names list
      // is highlighted.
      names.select (0);
   // ========================================================
   // Save all Contact objects from the contacts Vector object
   // to contacts.dat.  The number of contacts are saved as an
   // int to make it easy for loadContacts () to do its job.
   // ========================================================
   private void saveContacts ()
      FileOutputStream fos = null;
      try
          fos = new FileOutputStream ("contacts.dat");
          DataOutputStream dos = new DataOutputStream (fos);
          dos.writeInt (contacts.size ());
          for (int i = 0; i < contacts.size (); i++)
               Contact temp = (Contact) contacts.elementAt (i);
               dos.writeUTF (temp.fname);
               dos.writeUTF (temp.lname);
               dos.writeUTF (temp.phone);
               dos.writeUTF (temp.fax);
               dos.writeUTF (temp.email);
      catch (IOException e)
         MsgBox mb = new MsgBox (this, "CM Error",
                                 e.toString ());
         mb.dispose ();
      finally
         if (fos != null)
             try
                 fos.close ();
             catch (IOException e) {}
   private void update ()
      // Obtain index of selected contact item from the names
      // component.
      int index = names.getSelectedIndex ();
      // If no item was selected, index is -1.  We cannot update
      // a contact if no contact item in the names component was
      // selected - because we would have nothing to work with.
      if (index != -1)
          // Obtain a reference to the Contact object (from the
          // contacts Vector object) that is associated with the
          // index.
          Contact temp = (Contact) contacts.elementAt (index);
          // Create and display an update entry form.
          DataEntryForm def = new DataEntryForm (this, "Update",
                                                 temp.fname,
                                                 temp.lname,
                                                 temp.phone,
                                                 temp.fax,
                                                 temp.email);
          // If the user pressed Ok...
          if (def.bOk)
              // Update the contact information in the contacts
              // Vector object.
              temp.fname = new String (def.fname.getText ());
              temp.lname = new String (def.lname.getText ());
              temp.phone = new String (def.phone.getText ());
              temp.fax = new String (def.fax.getText ());
              temp.email = new String (def.email.getText ());
              // Make sure the screen reflects the update.
              names.replaceItem (temp.lname + ", " + temp.fname,
                                 index);
          // Destroy the dialog box.
          def.dispose ();
          // Make sure that the first contact item in the names
          // list is highlighted.
          names.select (0);
// ========================================================
// Class: Contact
// This class describes the contents of a business contact.
// ========================================================
class Contact
   public String fname;
   public String lname;
   public String phone;
   public String fax;
   public String email;
// ==========================================================
// Class: DataEntryForm
// This class provides a data entry form for entering contact
// information.
// ==========================================================
class DataEntryForm extends Dialog implements ActionListener
   // bOk is a boolean flag.  When true, it indicates that
   // the Ok button was pressed to terminate the dialog box
   // (as opposed to the Cancel button).
   public boolean bOk;
   // The following components hold the text that the user
   // entered into the visible text fields.
   public TextField fname;
   public TextField lname;
   public TextField phone;
   public TextField fax;
   public TextField email;
   public void actionPerformed (ActionEvent e)
      // If the user pressed the Ok button, indicate this
      // by assigning true to bOk.
      if (e.getActionCommand ().equals ("Ok"))
          bOk = true;
      // Destroy the dialog box and return to the point
      // just after the creation of the DataEntryForm object.
      dispose ();
   public DataEntryForm (Frame parent, String title)
      // Call the other constructor.  The current constructor
      // is used for insert operations.  The other constructor
      // is used for update operations.
      this (parent, title, "", "", "", "", "");     
   public DataEntryForm (Frame parent, String title,
                         String fname, String lname,
                         String phone, String fax,
                         String email)
      // Initialize the superclass layer.
      super (parent, title, true);
      // Choose a grid bag layout so that components can be more
      // accurately positioned.  (It looks nicer.)
      setLayout (new GridBagLayout ());
      // Add appropriate first name, last name, phone, fax, and
      // email components to the current DataEntryForm container.
      // (Remember, DataEntryForm is a subclass of Dialog.
      // Dialog is a container.  Therefore, DataEntryForm
      // inherits the ability to be a container.)
      addComponent (this, new Label ("First name: "), 0, 0, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.WEST);
      this.fname = new TextField (15);
      addComponent (this, this.fname, 1, 0, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      if (title.equals ("Update"))
          this.fname.setText (fname);
      addComponent (this, new Label ("Last name: "), 0, 1, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.WEST);
      this.lname = new TextField (15);
      addComponent (this, this.lname, 1, 1, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      if (title.equals ("Update"))
          this.lname.setText (lname);
      addComponent (this, new Label ("Phone number: "), 0, 2, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.WEST);
      this.phone = new TextField (15);
      addComponent (this, this.phone, 1, 2, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      if (title.equals ("Update"))
          this.phone.setText (phone);
      addComponent (this, new Label ("FAX number: "), 0, 3, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.WEST);
      this.fax = new TextField (15);
      addComponent (this, this.fax, 1, 3, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      if (title.equals ("Update"))
          this.fax.setText (fax);
      addComponent (this, new Label ("Email address: "), 0, 4, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.WEST);
      this.email = new TextField (15);
      addComponent (this, this.email, 1, 4, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      if (title.equals ("Update"))
          this.email.setText (email);
      addComponent (this, new Label (""), 0, 5, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      addComponent (this, new Label (""), 1, 5, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      Button b;
      // Add an Ok button to this container.
      addComponent (this, b = new Button ("Ok"), 0, 6, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      b.addActionListener (this);
      // Add a Cancel button to this container.
      addComponent (this, b = new Button ("Cancel"), 1, 6, 1, 1,
                    GridBagConstraints.NONE,
                    GridBagConstraints.CENTER);
      b.addActionListener (this);
      // Set the size of the dialog window to 250 pixels
      // horizontally by 200 pixels vertically.
      setSize (250, 200);
      // Do not allow users to resize the dialog window.
      setResizable (false);
      // Make sure that the dialog window is visible.
      setVisible (true);
   private void addComponent (Container con, Component com,
                              int gridx, int gridy,
                              int gridw, int gridh, int fill,
                              int anchor)
      // Get the current layout manager.  It is assumed to
      // be a GridBagLayout object.
      LayoutManager lm = con.getLayout ();
      // Create a GridBagConstraints object to make it
      // possible to customize component positioning.
      GridBagConstraints gbc = new GridBagConstraints ();
      // Assign the x and y grid positions.
      gbc.gridx = gridx;
      gbc.gridy = gridy;
      // Assign the number of grid blocks horizontally and
      // vertically that are occupied by the component.
      gbc.gridwidth = gridw;
      gbc.gridheight = gridh;
      // Specify the component's resize policy (fill) and
      // the direction in which the component is positioned
      // when its size is smaller than available space (anchor).
      gbc.fill = fill;
      gbc.anchor = anchor;
      // Set the new constraints that the grid bag layout
      // manager will use.
      ((GridBagLayout) lm).setConstraints (com, gbc);
      // Add the component to the container.
      con.add (com);
// ===========================================================
// Class: MsgBox
// This class displays a message box to the user.  The message
// is usually an error message.  The user must press the Ok
// button to terminate the message box.
// ===========================================================
class MsgBox extends Dialog implements ActionListener
   public void actionPerformed (ActionEvent e)
      // Terminate the dialog box in response to the user
      // pressing the Ok button.
      dispose ();
   public MsgBox (Frame parent, String title, String msg)
      // Initialize the superclass layer.
      super (parent, title, true);
      // Store the msg argument in a Label object and add
      // this object to the center part of the dialog window.
      Label l = new Label (msg);
      add ("Center", l);
      // Create a Button object and add it to the south part
      // of the dialog window.
      Button b = new Button ("Ok");
      add ("South", b);
      // Make the current object a listener to events that
      // occur as a result of the user pressing the Ok
      // button.
      b.addActionListener (this);
      // Make sure that the Ok button has the focus.
      b.requestFocus ();
      // Do not allow users to resize the dialog window.
      setResizable (false);
      // Allow the layout manager to choose an appropriate
      // size for the dialog window.
      pack ();
      // Make sure that the dialog window is visible.
      setVisible (true);
}