“ java.lang.NullPointerException：无法调用“ java.sql.Connection.prepareStatement（String）”，因为“ this.con”为空”

Question

**“ java.lang.NullPointerException：无法调用“ java.sql.Connection.prepareStatement（String）”，因为“ this.con”为空”

我想使用mySQL作为数据库，我从youtube找到了此代码。但这没有用。为什么呢？

这个'con'是我的连接变量**

这是我的代码

package com.mycompany.dbcon;
import java.sql.Connection;
import java.sql.DriverManager;


public class DBconnect {
    
    public static Connection connect(){
        Connection conn = null;
        try{  
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/sample","root","");
        }catch (Exception e){
            System.out.println(e);
        }
        return conn;
    }
    
}

这是我的主班

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package com.mycompany.dbcon;

import java.sql.Connection;
import java.sql.PreparedStatement;

/**
 *
 * @author tthar
 */
public class inta extends javax.swing.JFrame {

    Connection con = null;
    PreparedStatement pst = null;
    
    public inta() {
        initComponents();
         con = DBconnect.connect();
    }

    /**
     * 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() {

        namebox = new javax.swing.JTextField();
        agebox = new javax.swing.JTextField();
        idbox = new javax.swing.JTextField();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        namebox.setText("jTextField1");
        namebox.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                nameboxActionPerformed(evt);
            }
        });

        agebox.setText("jTextField2");

        idbox.setText("jTextField3");

        jButton1.setText("jButton1");
        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(123, 123, 123)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jButton1)
                    .addComponent(idbox, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(agebox, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(namebox, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addContainerGap(204, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(59, 59, 59)
                .addComponent(namebox, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(18, 18, 18)
                .addComponent(agebox, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(18, 18, 18)
                .addComponent(idbox, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(54, 54, 54)
                .addComponent(jButton1)
                .addContainerGap(68, Short.MAX_VALUE))
        );

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

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

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         

     String name = namebox.getText();
     String age = agebox.getText();
    // String id = idbox.getText();
     
     try {
         String q = "INSERT INTO name (name,age) values (' "+name+" ', '"+age+"'  )";
     pst = con.prepareStatement(q);
     pst.execute();
     }catch(Exception e){
         System.out.println(e);
     }        // TODO add your handling code here:
    }                                        

    /**
     * @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(inta.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(inta.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(inta.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(inta.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 inta().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JTextField agebox;
    private javax.swing.JTextField idbox;
    private javax.swing.JButton jButton1;
    private javax.swing.JTextField namebox;
    // End of variables declaration                   
}

*当我运行上述程序时，将显示错误。