Question

当我尝试在数据库中单击注册按钮时INSERT RECORD（如用户名和密码）时，它通过JOptionPane显示错误“java.lang.NullPointerException”...请告诉我解决方法如何插入记录**

import java.sql.*;
import javax.swing.*;

public class Newuser extends javax.swing.JInternalFrame {
Connection cn;
PreparedStatement st;
ResultSet rs;
String url,sql;
public Newuser() {
    initComponents();
}

public void MyConnection()
{
    try
    {
        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
        url="jdbc:sqlserver://localhost;instance=LALLYDB;databaseName=Menu;user=kd;password=123";
        cn=DriverManager.getConnection(url);     //above instance ,i.e,LALLYDB is DatabaseServer Name
    }
    catch(Exception ee)
    {
        JOptionPane.showMessageDialog(this, ee);
    }
}

public void Insert_Record(String username,String passwd)
{
    try
    {
        MyConnection();      //calling MyConnection function 
        st=cn.prepareStatement(sql,ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
        rs=st.executeQuery();
        sql="Insert into Project values(?,?)";
        st=cn.prepareStatement(sql);
        st.setString(1,username);
        st.setString(2,passwd);
        st.executeUpdate();
        JOptionPane.showMessageDialog(this,"Record successfully inserted");
        cn.close();
    }

    catch(Exception ee)
    {
        JOptionPane.showMessageDialog(this, ee);
    }
}

@SuppressWarnings("unchecked")
// <editor-fold defaultstate="collapsed" desc="Generated Code">                          
private void initComponents() {

    nlbl1 = new javax.swing.JLabel();
    nlbl2 = new javax.swing.JLabel();
    ntxt1 = new javax.swing.JTextField();
    ntxt2 = new javax.swing.JTextField();
    nlbl3 = new javax.swing.JLabel();
    nlbl4 = new javax.swing.JLabel();
    nlbl5 = new javax.swing.JLabel();
    ntxt3 = new javax.swing.JTextField();
    ntxt4 = new javax.swing.JTextField();
    regitser = new javax.swing.JButton();
    jcombo1 = new javax.swing.JComboBox<>();

    setClosable(true);
    setIconifiable(true);
    setMaximizable(true);
    setResizable(true);
    setTitle("New User Registration");
    setPreferredSize(new java.awt.Dimension(488, 425));
    setVisible(true);
    getContentPane().setLayout(new org.netbeans.lib.awtextra.AbsoluteLayout());

    nlbl1.setText("Username");
    getContentPane().add(nlbl1, new org.netbeans.lib.awtextra.AbsoluteConstraints(84, 44, -1, -1));

    nlbl2.setText("Password");
    getContentPane().add(nlbl2, new org.netbeans.lib.awtextra.AbsoluteConstraints(80, 80, -1, -1));
    getContentPane().add(ntxt1, new org.netbeans.lib.awtextra.AbsoluteConstraints(210, 40, 140, -1));
    getContentPane().add(ntxt2, new org.netbeans.lib.awtextra.AbsoluteConstraints(210, 80, 140, -1));

    nlbl3.setText("Email-id");
    getContentPane().add(nlbl3, new org.netbeans.lib.awtextra.AbsoluteConstraints(90, 120, -1, -1));

    nlbl4.setText("Hint Question");
    getContentPane().add(nlbl4, new org.netbeans.lib.awtextra.AbsoluteConstraints(80, 160, -1, -1));

    nlbl5.setText("Hint Answer");
    getContentPane().add(nlbl5, new org.netbeans.lib.awtextra.AbsoluteConstraints(80, 200, -1, -1));
    getContentPane().add(ntxt3, new org.netbeans.lib.awtextra.AbsoluteConstraints(210, 120, 140, -1));
    getContentPane().add(ntxt4, new org.netbeans.lib.awtextra.AbsoluteConstraints(210, 200, 140, -1));

    regitser.setText("Register");
    regitser.addActionListener(new java.awt.event.ActionListener() {
        public void actionPerformed(java.awt.event.ActionEvent evt) {
            regitserActionPerformed(evt);
        }
    });
    getContentPane().add(regitser, new org.netbeans.lib.awtextra.AbsoluteConstraints(390, 200, -1, -1));

    jcombo1.setModel(new javax.swing.DefaultComboBoxModel<>(new String[] { "Select", "What is your pet name ?", "What is your school name ?", "What is your favourite sport ?" }));
    getContentPane().add(jcombo1, new org.netbeans.lib.awtextra.AbsoluteConstraints(210, 160, -1, -1));

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


private void regitserActionPerformed(java.awt.event.ActionEvent evt) {                                         
    Insert_Record(ntxt1.getText(),ntxt2.getText());
}                                        


// Variables declaration - do not modify                     
private javax.swing.JComboBox<String> jcombo1;
private javax.swing.JLabel nlbl1;
private javax.swing.JLabel nlbl2;
private javax.swing.JLabel nlbl3;
private javax.swing.JLabel nlbl4;
private javax.swing.JLabel nlbl5;
private javax.swing.JTextField ntxt1;
private javax.swing.JTextField ntxt2;
private javax.swing.JTextField ntxt3;
private javax.swing.JTextField ntxt4;
private javax.swing.JButton regitser;
// End of variables declaration

}

Answer 1

您通过JOptionPane获得的错误包含一个亚麻，其中发生异常（您正在捕获它并显示它）。在MessageDialog中查看linenumber并查看原因是什么行，或者在catch-blocks中键入ee.printStackTrace（），然后您可以看到抛出异常的语句。也在st = cn.prepareStatement（sql，...）;变量sql尚未设置，这是故意的吗？

NullPointerException通常意味着您正在尝试访问之前尚未初始化的变量。

如何解决错误“java.lang.NullPointerException”？

1 个答案: