Mysql java 不能使用来自另一个类的连接方法

Question

I am currently a fist year student doing software engineering and have currently ran into some problem while developing a project for university. I am currently working on a netbeans project which allows users to login and retrieve information from an mysql database. I currently have a separate class for datahandling where I have written the connection method see below. However when I create the a new object of the datahandler in my Jframe class and try to call the connection method it does not allow me to use insert statements(see below) which I have programmed behind buttons(in order to retrieve information from txtfields). However when I move the connection method to the Jframe class it works perfectly fine. I was wonder if I am missing something really obvious here, or if it is just horrible design.

Thanks for your support!

    private void connection(){
    //from data handler class

    try{

    String mysqlUrl = "jdbc:mysql://localhost/test";
    String username = "root";
    String password = "";
    con = DriverManager.getConnection(mysqlUrl, username, password);
    }
    catch(Exception e){
        System.err.println("Got an exception");
        System.err.println(e.getMessage());

    }
  }
 }

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

    try{
        Datahandler test = new Datahandler();
        test.connection();

        //create my mysql database connection


        String query = "INSERT INTO User(customerStatus ,title, firstname, lastname, contactNo, email, username, password)" 
                + "VALUES (?, ?, ?, ?, ?, ?, ?, ?)";
        pst = con.prepareStatement(query);

        pst.setString(1,"Beginner");
        pst.setString(2,cmdGender.getSelectedItem().toString());
        pst.setString(3,txtFirstname.getText());
        pst.setString(4,txtLastname.getText());
        pst.setString(5,txtPhone.getText());
        pst.setString(6,txtEmail.getText());
        pst.setString(7,txtUsername.getText());
        pst.setString(8,txtPassword.getText());
        pst.execute();

        /* inserts the values username and password into a separate table in the database and
        makes use of the to ensure that the login is sercure(I will possibly make a change to
            how the structure of the databases work since there is an security issue here
            */
            String query2 = "INSERT INTO Login(username, password)" 
                    + "VALUES (?, ?)";
            pst = con.prepareStatement(query2);
            pst.setString(1,txtUsername.getText());
            pst.setString(2,txtPassword.getText());

            pst.execute();


            JOptionPane.showMessageDialog(null, "Registration completed!");

            //reset registration boxes into empty text
            txtFirstname.setText("");
            txtLastname.setText("");
            txtPhone.setText("");
            txtEmail.setText("");
            txtUsername.setText("");
            txtPassword.setText("");

        }
        catch (Exception e){

            System.err.println("Got an exception");
            System.err.println(e.getMessage());
            JOptionPane.showMessageDialog(null, "Something went wrong try to register again");

        }
}                                           

Answer 1

your getConnection() method is not returning anything. instead it is setting connection object to obejct con in that specific class.

con = DriverManager.getConnection(mysqlUrl, username, password);

This is why you are able to use it in your Jframe class, because it contains con property which gets set in getConnection() method. but in your new class there is no filed con that is having proper connection object.

Modify method getConnection() so that it return connection object, and use that connection object returned from this method to further processing.