我得到这个错误无法调用“java.sql.Connection.prepareStatement（String）”因为“dbConnection”是null

在com.claim.classes.Connection.selectRecordsFromTable（Connection.java：24）在com.claim.classes.Connection.main（Connection.java：12）

看看我的代码，一切看起来都是正确的，所以我不知道为什么我得到这个错误。

连接.java

package com.claim.classes;

        import java.sql.PreparedStatement;
        import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class Connection {

    
 public static void main(String args[])throws Exception{  
 selectRecordsFromTable();
 } 
 
 private static void selectRecordsFromTable() throws SQLException {

 Connection dbConnection = null;
 PreparedStatement preparedStatement = null;

 String selectSQL = "SELECT * FROM USER WHERE id = ?";

 try {
 dbConnection = (Connection) DbConnection.getConnection();
 preparedStatement = ((java.sql.Connection) dbConnection).prepareStatement(selectSQL);
 preparedStatement.setInt(1, 16);

 // execute select SQL statement
 ResultSet rs = preparedStatement.executeQuery();

 while (rs.next()) {

   int id  = rs.getInt("id");          
          String email = rs.getString("email");
          String fname = rs.getString("first_name");
          String lname = rs.getString("lanst_name");

          //Display values
          System.out.print("\nID: " + id +"\nEmail: " + email+ "\nfirst name: " + fname+"\nlast Name: " + lname);          
       
 }

 } catch (SQLException e) {

 System.out.println(e.getMessage());

 } finally {

 if (preparedStatement != null) {
 preparedStatement.close();
 }

 if (dbConnection != null) {
 ((Statement) dbConnection).close();
 }

 }

 }
}

DbConnection.java

package com.claim.classes;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DbConnection{

// public Connection conn;
//static reference to itself
    private static DbConnection instance = new DbConnection();    
    public static final String URL = "jdbc:mysql://localhost:3306/root";
    public static final String USER = "root";
    public static final String PASSWORD = "root";
    public static final String DRIVER_CLASS = "com.mysql.cj.jdbc.Driver"; 
     
    //private constructor
    private DbConnection() {
        try {
            Class.forName(DRIVER_CLASS);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
     
    private Connection createConnection() {
        Connection connection = null;
        try {                  
            connection = DriverManager.getConnection(URL, USER, PASSWORD);
            System.out.println("Connected to Database.");
        } catch (SQLException e) {
            System.out.println("ERROR: Unable to Connect to Database.");
        }
        return connection;
    }   
     
    public static Connection getConnection() {
        return instance.createConnection();
    }
 
}

有一个相关的问题在这里，但那个人有一个不同的问题比我。谢谢你的帮助：）