如何摆脱jframe中的nullpointerexception？

Question

我的意图是使用Jframe创建登录页面。 我已经在一个单独的文件中创建了数据库连接类，如下所示，当我运行登录Jframe时，出现一个指出nullpointer异常的错误。

请帮助我:)

dbConnection class ----------------------------------- -----------

package vehicle_renting; 

    import java.sql.*; 
    import javax.swing.JOptionPane; 
    public class dbConnection { 

     Connection con; 
     Statement stmt; 
     ResultSet rs; 

     public dbConnection() {} 

     public void connect() { 
     try { 
      Class.forName("com.jdbc.mysql.Driver"); 
     con=DriverManager.getConnection("jdbc:mysql://localhost:3306/vehicle_renting_1","root","qwer1234"); 

     } catch (Exception e) { 
      e.printStackTrace(); 
     } 
    } 
    } 

而下面的代码放置在jframe源代码中。

package vehicle_renting; 


import java.sql.* ; 
import javax.swing.* ; 
public class Login extends javax.swing.JFrame { 

    Connection conn ; 
    ResultSet rs; 
    PreparedStatement pst; 

    /** 
    * Creates new form Login 
    */ 
    public Login() { 
     initComponents(); 
    } 

    /** 
    * This method is called from within the constructor to initialize the form. 
    * WARNING: Do NOT modify this code. The content of this method is always 
    * regenerated by the Form Editor. 
    */ 
    @SuppressWarnings("unchecked") 
    // <editor-fold defaultstate="collapsed" desc="Generated Code">       
    private void initComponents() { 

     jLabel1 = new javax.swing.JLabel(); 
     jLabel2 = new javax.swing.JLabel(); 
     jButton1 = new javax.swing.JButton(); 
     txt_username = new javax.swing.JTextField(); 
     txt_password = new javax.swing.JPasswordField(); 

     setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE); 

     jLabel1.setText("Username"); 

     jLabel2.setText("Password"); 

     jButton1.setText("Ok"); 
     jButton1.addActionListener(new java.awt.event.ActionListener() { 
      public void actionPerformed(java.awt.event.ActionEvent evt) { 
       jButton1ActionPerformed(evt); 
      } 
     }); 

     javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane()); 
     getContentPane().setLayout(layout); 
     layout.setHorizontalGroup(
      layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
      .addGroup(layout.createSequentialGroup() 
       .addGap(110, 110, 110) 
       .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false) 
        .addGroup(layout.createSequentialGroup() 
         .addComponent(jLabel2) 
         .addGap(18, 18, 18) 
         .addComponent(txt_password)) 
        .addGroup(layout.createSequentialGroup() 
         .addComponent(jLabel1) 
         .addGap(18, 18, 18) 
         .addComponent(txt_username, javax.swing.GroupLayout.PREFERRED_SIZE, 180, javax.swing.GroupLayout.PREFERRED_SIZE))) 
       .addContainerGap(44, Short.MAX_VALUE)) 
      .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup() 
       .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE) 
       .addComponent(jButton1) 
       .addGap(143, 143, 143)) 
     ); 
     layout.setVerticalGroup(
      layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING) 
      .addGroup(layout.createSequentialGroup() 
       .addGap(60, 60, 60) 
       .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) 
        .addComponent(jLabel1) 
        .addComponent(txt_username, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)) 
       .addGap(18, 18, 18) 
       .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE) 
        .addComponent(jLabel2) 
        .addComponent(txt_password, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)) 
       .addGap(18, 18, 18) 
       .addComponent(jButton1) 
       .addContainerGap(141, Short.MAX_VALUE)) 
     ); 

     pack(); 
    }// </editor-fold>       

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {           
     // TODO add your handling code here: 



    String username=txt_username.getText(); 
    String password=txt_password.getText(); 





    String sql = "select * from login where username = ? and password = ?"; 
     try 
     { 




      pst = conn.prepareStatement(sql); 
      pst.setString(1, username);     // Passing the values to the username from the textbox 
      pst.setString(2, password);     // Passing the values to the password from the textbox 
      rs = pst.executeQuery();     // Storing the results retrieved from the query 
      if (rs.next()) 
      { 
       JOptionPane.showMessageDialog(null, "Username and Password correct"); 
      } 
      else 
      { 
       JOptionPane.showMessageDialog(null, "invalid username and password"); 
      } 


     } 

     catch (Exception e) 
     { 
      JOptionPane.showMessageDialog(null, e); 
     } 






    }           

    /** 
    * @param args the command line arguments 
    */ 
    public static void main(String args[]) { 
     /* Set the Nimbus look and feel */ 
     //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) "> 
     /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel. 
     * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
     */ 
     try { 
      for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) { 
       if ("Nimbus".equals(info.getName())) { 
        javax.swing.UIManager.setLookAndFeel(info.getClassName()); 
        break; 
       } 
      } 
     } catch (ClassNotFoundException ex) { 
      java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } catch (InstantiationException ex) { 
      java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } catch (IllegalAccessException ex) { 
      java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } catch (javax.swing.UnsupportedLookAndFeelException ex) { 
      java.util.logging.Logger.getLogger(Login.class.getName()).log(java.util.logging.Level.SEVERE, null, ex); 
     } 
     //</editor-fold> 

     /* Create and display the form */ 
     java.awt.EventQueue.invokeLater(new Runnable() { 
      public void run() { 
       new Login().setVisible(true); 
      } 
     }); 
    } 

    // Variables declaration - do not modify      
    private javax.swing.JButton jButton1; 
    private javax.swing.JLabel jLabel1; 
    private javax.swing.JLabel jLabel2; 
    private javax.swing.JPasswordField txt_password; 
    private javax.swing.JTextField txt_username; 
    // End of variables declaration     
} 

而当我运行Jframe时，它说“java.lang.NullPointerException”。我找不到这个错误。谢谢

try 
     { pst = conn.prepareStatement(sql); 
      pst.setString(1, username);     // Passing the values to the username from the textbox 
      pst.setString(2, password);     // Passing the values to the password from the textbox 
      rs = pst.executeQuery();     // Storing the results retrieved from the query 
      if (rs.next()) 
      { 
       JOptionPane.showMessageDialog(null, "Username and Password correct"); 
      } 
      else 
      { 
       JOptionPane.showMessageDialog(null, "invalid username and password"); 
      } 


     } 

     catch (Exception e) 
     { 
      JOptionPane.showMessageDialog(null, e); 
     } 

我觉得上面的代码段给了我错误。我不得不编辑JOptionPane来找出它。

Answer 1

我说你有没有在你的JFrame代码