/*
   Chapter 9:     UserLogonTracking Class
   Programmer:   Brad Shedd
   Date:      April 22, 2004
   Program Name:  UserLogonTracking.java
   Purpose:      To provide a reusable UserLogon class
*/

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

public class UserLogonTracking 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;
      UserLogonTracking frame = new UserLogonTracking();
      frame.pack();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(width, height);
      frame.centerOnScreen(width, height);
      frame.setVisible(true);
   }

   public UserLogonTracking()
   {
      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;
         }
      }
   }

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