[warship]

hello.
i am trying to log on to the video library system that i just developed. when i type "Administrator" in the username text field and 0 in the password field and press OK i get a lot of NullPointerException problems (the appropriate pieces of code are shown below). i had to press End on the jGRASP console in order to end the program. how can this problem be solved?

Code:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
 
public class HomeEntertainment extends JPanel implements ActionListener{
   private JTabbedPane jtp = new JTabbedPane();
   private JPanel cP1 = new JPanel();
   private JPanel cP2 = new JPanel();
   private JPanel cP3 = new JPanel();
   private JPanel cP4 = new JPanel();
   private JPanel bP1 = new JPanel();
   private JPanel bP2 = new JPanel();
   private JPanel bP3 = new JPanel();
   private JPanel bP4 = new JPanel();
   private JButton jbAdministratorLogOn = new JButton("Log On");
   private JButton jbNewUserRegister = new JButton("Register");
   private JButton jbExistingUserLogOn = new JButton("Log On");
   private JButton jbExitTheSystem = new JButton("Exit the system");
	
   public HomeEntertainment(){
      cP1.setLayout(new BorderLayout());
      cP2.setLayout(new BorderLayout());
      cP3.setLayout(new BorderLayout());
      cP4.setLayout(new BorderLayout());
      bP1.setBorder(new TitledBorder("Make a choice"));
      bP2.setBorder(new TitledBorder("Make a choice"));
      bP3.setBorder(new TitledBorder("Make a choice"));
      bP4.setBorder(new TitledBorder("Make a choice"));
      bP1.add(jbAdministratorLogOn);
      bP2.add(jbNewUserRegister);
      bP3.add(jbExistingUserLogOn);
      bP4.add(jbExitTheSystem);
      cP1.add(bP1, BorderLayout.SOUTH);
      cP2.add(bP2, BorderLayout.SOUTH);
      cP3.add(bP3, BorderLayout.SOUTH);
      cP4.add(bP4, BorderLayout.SOUTH);
      jtp.addTab("Administrator", cP1);
      jtp.addTab("New User", cP2);
      jtp.addTab("Existing User", cP3);
      jtp.addTab("Exit the system", cP4);
		
      JFrame jf = new JFrame("Home Entertainment");		
      jf.getContentPane().add(jtp, BorderLayout.CENTER);
      jf.setSize(500, 500);
      jf.setVisible(true);
		
      jbAdministratorLogOn.addActionListener(this);
      jbNewUserRegister.addActionListener(this);
      jbExistingUserLogOn.addActionListener(this);
      jbExitTheSystem.addActionListener(this);
   }
	
   public void actionPerformed(ActionEvent e){
      if(e.getSource() == jbAdministratorLogOn){
         LogOn logOn = new LogOn();
         logOn.setVisible(true);
      }
      if(e.getSource() == jbNewUserRegister){
         RegistrationForm r = new RegistrationForm();
         r.setVisible(true);
      }
      if(e.getSource() == jbExistingUserLogOn){ 
         LogOn logOn = new LogOn();
         logOn.setVisible(true);
      }
      if(e.getSource() == jbExitTheSystem){ 
         System.exit(0);
      }
   }
	
   public static void main(String[] args){
      new HomeEntertainment();
   }
}

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class LogOn extends JFrame{
   JPanel pnlBody, pnlFooter;
   JLabel unLabel = new JLabel("Username: ");
   JLabel pwLabel = new JLabel("Password: "); 
   JTextField jtfUN = new JTextField(20);
   JPasswordField jtfPW = new JPasswordField(20);
   JButton jbOK = new JButton("OK");
   JButton jbCancel = new JButton("Cancel");
   Container contentpane;
	
   public LogOn(){
      super("Welcome to Home Entertainment");
      contentpane = getContentPane();
      contentpane.setLayout(new BorderLayout());
      pnlBody = new JPanel();
      pnlFooter = new JPanel();
      pnlBody.add(unLabel);
      pnlBody.add(jtfUN);
      pnlBody.add(pwLabel);
      pnlBody.add(jtfPW);
      pnlFooter.add(jbOK);
      pnlFooter.add(jbCancel);
      contentpane.add(pnlBody,BorderLayout.NORTH);
      contentpane.add(pnlFooter,BorderLayout.CENTER);
      pack();
      setVisible(true);
 
      jbOK.addActionListener(new ActionListener(){
         public void actionPerformed(ActionEvent e){
            User u = new User();
            u.doLogOn();
            u.username = jtfUN.getText().trim();
            u.password = new String(jtfPW.getPassword());
            Administrator a = new Administrator();
            a.doLogOn();
            a.username = jtfUN.getText().trim();
            a.password = new String(jtfPW.getPassword());
            setVisible(false);
         }
      });
 
      jbCancel.addActionListener(new ActionListener(){
         public void actionPerformed(ActionEvent e){
            setVisible(false);
         }
      });
   }
}
 
class User{
   String username;
   String password;
   String[] userUsernameArray = {"Ann Smyth", "John Murphy"};
   String[] userPasswordArray = {"1", "2"};
		
   public void doLogOn(){
      int i, j;
	
      for(i = 0; i < userUsernameArray.length; i++){ 
         if(username.equals(userUsernameArray[i])){
            UserMainMenu umm = new UserMainMenu();
            umm.setVisible(true); 
         } 
         else{ 
              JOptionPane.showMessageDialog(null, "Error\n\nYou have entered an incorrect username and/or password\nPlease try again", null, JOptionPane.ERROR_MESSAGE);
         } 
      } 
		
      for(j = 0; j < userPasswordArray.length; j++){ 
         if(password.equals(userPasswordArray[j])){ 
            UserMainMenu umm = new UserMainMenu();
            umm.setVisible(true); 
         } 
         else{ 
              JOptionPane.showMessageDialog(null, "Error\n\nYou have entered an incorrect username and/or password\nPlease try again", null, JOptionPane.ERROR_MESSAGE);
         } 
      }
   }
}
 
class Administrator{
   String username;
   String password;
   String[] adminUsernameArray = {"Administrator"};
   String[] adminPasswordArray = {"0"};
 
   public void doLogOn(){
      int k, l;
	
      for(k = 0; k < adminUsernameArray.length; k++){ 
         if(username.equals(adminUsernameArray[k])){
            AdminMainMenu amm = new AdminMainMenu();
            amm.setVisible(true); 
         } 
         else{ 
              JOptionPane.showMessageDialog(null, "Error\n\nYou have entered an incorrect username and/or password\nPlease try again", null, JOptionPane.ERROR_MESSAGE);
         } 
      } 
		
      for(l = 0; l < adminPasswordArray.length; l++){ 
         if(password.equals(adminPasswordArray[l])){
            AdminMainMenu amm = new AdminMainMenu();
            amm.setVisible(true); 
         } 
         else{ 
              JOptionPane.showMessageDialog(null, "Error\n\nYou have entered an incorrect username and/or password\nPlease try again", null, JOptionPane.ERROR_MESSAGE);
         } 
      } 
   } 
}