当我将 function 分配给它时，如何自行更新变量

Question

So I am trying to update a variable by just using a function call without changing it. So I want to have an object with a variable 'name' which will receive it's value from fetching it from a database:

String name = fetchNameFromDB(); // Name is 'Jim'

Then if I change the value in the database, whenever I refer to the 'name' value, it returns the updated value, example:

String name = fetchNameFromDB(); // Assigns name as 'Jim'
changeNameTo("Steve"); // Changes the value in the database from 'Jim' to 'Steve'
System.out.println(name); // Will print 'Jim' but I want this to be printing 'Steve'

The only solution I have at the moment is this(I am using JDBC and MySQL):

public  class GroundControlToMajorTom {
    static String theName;
    static String ID;

    public static void main(String[] args) throws ClassNotFoundException, SQLException {

        //System.out.println(returnEmployeeSalary("ivy"));
        theName = returnEmployeeName("72913");
        changeName("Kass", "72913");
        System.out.println(theName);
    }

    public static void changeName(String name, String ID) throws ClassNotFoundException, SQLException{
        System.out.println(theName);
        changeEmployeeName(name, ID);
        theName = returnEmployeeName(ID);
        System.out.println(theName);
    }

    public static void changeEmployeeName(String name, String ID) throws ClassNotFoundException, SQLException {
        String url = "jdbc:mysql://localhost:3306/sql_hr";
        String uname = "root";
        String pass = "Culley1max1";
        String query = "UPDATE employees SET first_name = '" + name + "' WHERE employee_ID = '" + ID + "'";

        Class.forName("com.mysql.cj.jdbc.Driver");
        Connection con = DriverManager.getConnection(url, uname, pass);

        Statement st = con.createStatement();
        int rs = st.executeUpdate(query);

        st.close();
        con.close();
    }

    public static String returnEmployeeName(String ID) throws ClassNotFoundException {
        HashMap<String, String> infoHR = connectionInfoHR();

        String query = "SELECT first_name FROM employees WHERE employee_id = '" + ID + "'";

        Class.forName("com.mysql.cj.jdbc.Driver");

        try (Connection con = DriverManager.getConnection(infoHR.get("url"), infoHR.get("uname"), infoHR.get("pass"));
                Statement st = con.createStatement();
                ResultSet rs = st.executeQuery(query)) {

            rs.next();
            return rs.getString("first_name");

        } catch (Exception e) {
            return "ConnectionInfoHR credentials incorrect - Cannot connect to database";
        }
    }

Here I am manually reasigning the value every time through a separate function, how can I avoid this and have it fetch the value from the database and use the fetched value every time I call the variable name?

Answer 1

I don't know why you want to do this, as other said it may be really a design issue.

Anyway, you may want to consider the idea to write custom events for your application. onValueChange() (for example) you could update the value inside your database assigning a function for each registered object to the event handler.

One thing you can do is to design an interface, which satisfy your needs, having as first argument the target of the change and the function associated to the change. In the end, on each change fired, your data would be updated automatically inside your database.

You may want to look at some design patterns like Observer , Event Notifier or EventBus to use with this interface.

IMPORTANT :
I do not recommend doing something like this, it has negative impacts on your application performance and I strongly advise against using it.