/*
      Chapter 9:  The UserTracking Class
      Programmer: Brad Shedd
      Date:    April 28, 2004
      Filename:   UserTracking.java
      Purpose: To provide a reusable UserTracking class
 */

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

public class UserTrackingDemo extends JFrame implements ActionListener
{
   private JTextField jtfUser, jtfNumUsers;
   private JButton jbtLogon, jbtLogoff;
   private ArrayList userList;
   private int numUsers = 0;

    public static void main(String[] args)
    {
      int width = 200;
      int height = 130;
        UserTrackingDemo frame  = new UserTrackingDemo();
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(width, height);
     frame.centerOnScreen(width, height);
      frame.setVisible(true);
    }

    public UserTrackingDemo()
    {

       userList = new ArrayList(20); //sets up for 20 entries

        setTitle("User Tracking");

        JPanel p = new JPanel();
        p.setLayout(new FlowLayout());
        p.add(new JLabel("User:   "));
       p.add(jtfUser = new JTextField(10));

       p.add(jbtLogon = new JButton("Log On"));
        p.add(jbtLogoff = new JButton("Log Off"));


        p.add(new JLabel("Number Of User:  "));
       p.add(jtfNumUsers = new JTextField(5));
       jtfNumUsers.setEditable(false);


        jbtLogon.addActionListener(this);
        jbtLogoff.addActionListener(this);

        getContentPane().setLayout(new BorderLayout());
        getContentPane().add(p,BorderLayout.CENTER);
    }

   public void centerOnScreen(int width, int height)
   {
     int top, left, x, y;

    // Get the screen dimension
     Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();

     // Determine the location for the top left corner of the frame
     x = (screenSize.width - width)/2;
     y = (screenSize.height - height)/2;
     top = (x < 0) ? 0 : x;
     left = (y < 0) ? 0 : y;

     // Set the frame to the specified location
     this.setLocation(top, left);
   }

  public void actionPerformed(ActionEvent e)
    {
      String userEntered = jtfUser.getText();
      if(userEntered.length() > 0)
      {
         if(e.getSource() == jbtLogon)    // user clicked Use user
         {
         try
            {
                  logonUser(userEntered);
                     jtfNumUsers.setText(String.valueOf(numUsers));
                   jtfUser.setText("");
                   jtfUser.requestFocus();
         }
         catch (Exception ex)
            {
                   JOptionPane.showMessageDialog(this, ex.getMessage(), "Logon failed.", JOptionPane.ERROR_MESSAGE);

            }

        }
           else if  (e.getSource() == jbtLogoff)      // user clicked Change User
           {
            try
            {
                     logoffUser(userEntered);
                     jtfNumUsers.setText(String.valueOf(numUsers));
                   jtfUser.setText("");
                   jtfUser.requestFocus();
        }
         catch (Exception ex)
            {
                   JOptionPane.showMessageDialog(this, ex.getMessage(), "Logoff failed.", JOptionPane.ERROR_MESSAGE);

            }

            }
          }

            else  // can never happen
            {
              JOptionPane.showMessageDialog(this, "Please enter a valid user name.", "Invalid entry.Try again.", JOptionPane.ERROR_MESSAGE);
              jtfUser.requestFocus();
         }

      }// end of try

        private void logonUser(String user) throws Exception
        {
         if(!userList.contains(user))
         {
            user.trim();
            userList.add(user);
            ++numUsers;
         }
         else
            throw new Exception("User already logged on.");
        }

        private void logoffUser(String user)throws Exception
        {
         int index = 0;
         index = userList.indexOf(user.trim());
         if(index < 0)
            throw new Exception("User not logged on.");
        else
        {
         userList.remove(index);
         --numUsers;
        }
    }
}

Homepage