Java异常nullpointerexception错误(Java Exception nullpointerexception error)

 什么导致null异常？ 我根本无法看到它..请第二组眼睛将帮助这个初学者java学生..这个应用程序基本上捕获有关游戏团队的5条信息并插入到mySQL数据库中。 
 
 错误如下所示：InsertTeamInfo.access $ 100中InsertTeamInfo.insertRecord（InsertTeamInfo.java:130）中的线程“AWT-EventQueue-0”java.lang.NullPointerException异常（InsertTeamInfo.java:11） 
 
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.sql.*;

public class InsertTeamInfo extends JFrame{
  private String sql;
  private Connection con;
  private Statement stat;
  private JButton btnSave;

  private JTextField txtTeam;  
  private JTextField txtCity;
  private JTextField txtYear;
  private JTextField txtLoserTeam;
  private JTextField txtLoserCity;

  private JLabel lblTeam;
  private JLabel lblCity;
  private JLabel lblYear;
  private JLabel lblLoserTeam;
  private JLabel lblLoserCity;    

  public InsertTeamInfo(){
    super("Favorite Team");

    btnSave = new JButton("Save");

    txtTeam = new JTextField("");
    txtCity = new JTextField("");
    txtYear = new JTextField("");
    txtLoserTeam = new JTextField("");
    txtLoserCity = new JTextField("");

    lblTeam = new JLabel("Team");
    lblCity = new JLabel("City");
    lblYear = new JLabel("Year Played");
    lblLoserTeam = new JLabel("Loser Team");
    lblLoserCity = new JLabel("Loser City");

    txtYear.setEditable(true);
    txtLoserTeam.setEditable(true);
    txtLoserCity.setEditable(true);
    txtTeam.setEditable(true);
    txtCity.setEditable(true);  

  }

  public void launchJFrame(){
    //width - height
    setSize(500, 300);
    getContentPane().setLayout(null);
    getContentPane().setBackground(Color.white);

    setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
    getContentPane().add(btnSave);

    getContentPane().add(txtTeam);
    getContentPane().add(txtCity);
    getContentPane().add(txtYear);
    getContentPane().add(txtLoserTeam);
    getContentPane().add(txtLoserCity);

    getContentPane().add(lblTeam);
    getContentPane().add(lblCity);    
    getContentPane().add(lblYear);
    getContentPane().add(lblLoserTeam);
    getContentPane().add(lblLoserCity);

    lblTeam.setBounds(new Rectangle(65,20,100,30));
    lblCity.setBounds(new Rectangle(65,55,100,30));
    lblYear.setBounds(new Rectangle(65, 85, 100, 30));
    lblLoserTeam.setBounds(new Rectangle(65, 115, 100, 30));
    lblLoserCity.setBounds(new Rectangle(65, 145, 100, 30));

    txtTeam.setBounds(new Rectangle(210, 20, 150, 30));
    txtCity.setBounds(new Rectangle(210, 54, 150, 30));
    txtYear.setBounds(new Rectangle(210, 86, 150, 30));
    txtLoserTeam.setBounds(new Rectangle(210, 119, 150, 30));
    txtLoserCity.setBounds(new Rectangle(210, 150, 150, 30));

    btnSave.setBounds(new Rectangle(65, 190, 90, 30));
    setVisible(true);

    addWindowListener(new java.awt.event.WindowAdapter() {
      public void windowClosing(java.awt.event.WindowEvent evt) {
        shutDown();
      }
    });

    btnSave.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){ //Handle Save button click event
            insertRecord();
          }
     });
 }

 private void insertRecord(){
    try {
        DriverManager.registerDriver(new com.mysql.jdbc.Driver());
        con = (Connection) DriverManager.getConnection("jdbc:mysql://localhost:3306/week5", "root","Saras231");
    }
    catch(Exception ex){
      JOptionPane.showMessageDialog( null, "Error connection to database.");         
      System.exit(0);
    }

    try {
       sql = "INSERT INTO Wins_S001 (Team, City, Year_T, LoserTeam, LoserCity) VALUES (?, ?, ?, ?, ?)";
        PreparedStatement preparedStatement = con.prepareStatement(sql);
        preparedStatement.setString(1, txtTeam.getText());
        preparedStatement.setString(2, txtCity.getText());
        preparedStatement.setString(3, txtYear.getText());
        preparedStatement.setString(4, txtLoserTeam.getText());
        preparedStatement.setString(5, txtLoserCity.getText());
        preparedStatement.executeUpdate(); 
       JOptionPane.showMessageDialog( null, "Data Inserted Successfully");          
    }
    catch(SQLException ex){
      ex.printStackTrace();
      JOptionPane.showMessageDialog( null, "Data Insert failed.");          
    }

    try{
      stat.close();
      con.close();
    }
    catch(SQLException ex){
      JOptionPane.showMessageDialog( null, "All active connection closed to the database.");          
    }

 }

  private void shutDown(){
    int returnVal = JOptionPane.showConfirmDialog(this, "Are you sure you want to quit?");

    if(returnVal == JOptionPane.YES_OPTION){

      System.exit(0);
    }
  }

  public static void main(String[] args){
    InsertTeamInfo gui = new InsertTeamInfo();
    gui.launchJFrame();
  }
}

What is causing the null exception here? I am simply unable to see it.. please second set of eyes would help this beginner java student..This application basically captures 5 pieces of info about a game team and insert into mySQL database. 
 
The error is as below: Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at InsertTeamInfo.insertRecord(InsertTeamInfo.java:130) at InsertTeamInfo.access$100(InsertTeamInfo.java:11)
 
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.sql.*;

public class InsertTeamInfo extends JFrame{
  private String sql;
  private Connection con;
  private Statement stat;
  private JButton btnSave;

  private JTextField txtTeam;  
  private JTextField txtCity;
  private JTextField txtYear;
  private JTextField txtLoserTeam;
  private JTextField txtLoserCity;

  private JLabel lblTeam;
  private JLabel lblCity;
  private JLabel lblYear;
  private JLabel lblLoserTeam;
  private JLabel lblLoserCity;    

  public InsertTeamInfo(){
    super("Favorite Team");

    btnSave = new JButton("Save");

    txtTeam = new JTextField("");
    txtCity = new JTextField("");
    txtYear = new JTextField("");
    txtLoserTeam = new JTextField("");
    txtLoserCity = new JTextField("");

    lblTeam = new JLabel("Team");
    lblCity = new JLabel("City");
    lblYear = new JLabel("Year Played");
    lblLoserTeam = new JLabel("Loser Team");
    lblLoserCity = new JLabel("Loser City");

    txtYear.setEditable(true);
    txtLoserTeam.setEditable(true);
    txtLoserCity.setEditable(true);
    txtTeam.setEditable(true);
    txtCity.setEditable(true);  

  }

  public void launchJFrame(){
    //width - height
    setSize(500, 300);
    getContentPane().setLayout(null);
    getContentPane().setBackground(Color.white);

    setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
    getContentPane().add(btnSave);

    getContentPane().add(txtTeam);
    getContentPane().add(txtCity);
    getContentPane().add(txtYear);
    getContentPane().add(txtLoserTeam);
    getContentPane().add(txtLoserCity);

    getContentPane().add(lblTeam);
    getContentPane().add(lblCity);    
    getContentPane().add(lblYear);
    getContentPane().add(lblLoserTeam);
    getContentPane().add(lblLoserCity);

    lblTeam.setBounds(new Rectangle(65,20,100,30));
    lblCity.setBounds(new Rectangle(65,55,100,30));
    lblYear.setBounds(new Rectangle(65, 85, 100, 30));
    lblLoserTeam.setBounds(new Rectangle(65, 115, 100, 30));
    lblLoserCity.setBounds(new Rectangle(65, 145, 100, 30));

    txtTeam.setBounds(new Rectangle(210, 20, 150, 30));
    txtCity.setBounds(new Rectangle(210, 54, 150, 30));
    txtYear.setBounds(new Rectangle(210, 86, 150, 30));
    txtLoserTeam.setBounds(new Rectangle(210, 119, 150, 30));
    txtLoserCity.setBounds(new Rectangle(210, 150, 150, 30));

    btnSave.setBounds(new Rectangle(65, 190, 90, 30));
    setVisible(true);

    addWindowListener(new java.awt.event.WindowAdapter() {
      public void windowClosing(java.awt.event.WindowEvent evt) {
        shutDown();
      }
    });

    btnSave.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){ //Handle Save button click event
            insertRecord();
          }
     });
 }

 private void insertRecord(){
    try {
        DriverManager.registerDriver(new com.mysql.jdbc.Driver());
        con = (Connection) DriverManager.getConnection("jdbc:mysql://localhost:3306/week5", "root","Saras231");
    }
    catch(Exception ex){
      JOptionPane.showMessageDialog( null, "Error connection to database.");         
      System.exit(0);
    }

    try {
       sql = "INSERT INTO Wins_S001 (Team, City, Year_T, LoserTeam, LoserCity) VALUES (?, ?, ?, ?, ?)";
        PreparedStatement preparedStatement = con.prepareStatement(sql);
        preparedStatement.setString(1, txtTeam.getText());
        preparedStatement.setString(2, txtCity.getText());
        preparedStatement.setString(3, txtYear.getText());
        preparedStatement.setString(4, txtLoserTeam.getText());
        preparedStatement.setString(5, txtLoserCity.getText());
        preparedStatement.executeUpdate(); 
       JOptionPane.showMessageDialog( null, "Data Inserted Successfully");          
    }
    catch(SQLException ex){
      ex.printStackTrace();
      JOptionPane.showMessageDialog( null, "Data Insert failed.");          
    }

    try{
      stat.close();
      con.close();
    }
    catch(SQLException ex){
      JOptionPane.showMessageDialog( null, "All active connection closed to the database.");          
    }

 }

  private void shutDown(){
    int returnVal = JOptionPane.showConfirmDialog(this, "Are you sure you want to quit?");

    if(returnVal == JOptionPane.YES_OPTION){

      System.exit(0);
    }
  }

  public static void main(String[] args){
    InsertTeamInfo gui = new InsertTeamInfo();
    gui.launchJFrame();
  }
}

原文：