Question

I have an issue setting a JLabel‘s text using a method in the class creating the GUI from a different class calling that method. The method to set the JLabel is called outside the GUI but when called from inside the GUI class it works. I have tested the getText() method on the label after it is called from outside the GUI class and it shows that the label has been updated. I get that it is probably a paint issue or update issue with Swing but I’m at a loss of what to do. I have tried repaint() and revalidate() on the label and then panel that it is within. Here is my current code:

public void setStatusLabel(String statusEntered) {
    //Shows the variable statusEntered has been received 
    System.out.println(statusEntered);

    //Not working
    status_label.setText(statusEntered);

    //Used this to check if the label receives the data. It does.
    String status = status_label.getText();
    System.out.println(status);
}

And the context in which I am calling it. Setups a database connection

//GUI Class reference
MainWindow mainwindow = new MainWindow();

public void connect(){
    Connection conn = null;
    try {
        String userName = "root";
        String password = "";
        String url = "jdbc:mysql://localhost:3306";
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        conn = DriverManager.getConnection(url, userName, password);

        //This works
        System.out.println("Connection Established");
        //The issue is with this guy
        mainwindow.setStatusLabel("Connection");
    }
    catch(Exception e) {
        System.err.println("Failed to connect to database");
        mainwindow.setStatusLabel("No connection");
    }
}

Any help with this would be awesome or if you have some links to suggestions, that would be awesome too! Thanks for the help.

Here is my main:

public static void main(String[] args) {
    java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new MainWindow().setVisible(true);
        }
    });
}

Answer 1

Your problem is possibly one of reference — your mainwindow variable in your GUI class may not be referring to the MainWindow object that is being displayed. Do you call new MainWindow(); anywhere else in your code? If so, then you’d best pass a reference to the visualized MainWindow into this class above so that you can call methods on it that will result in something that can be seen.

For e.g.,

public class DatabaseConnection {
   // MainWindow mainwindow = new MainWindow();  *** don't do this ***
   MainWindow mainwindow;

   public DatabaseConnection(MainWindow mainwindow) {
      this.mainwindow = mainwindow; // pass in the reference in the constructor
   }

   public void connect() {
      Connection conn = null;

      // ... etc

      // now we can call methods on the actual visualized object
      mainwindow.setStatusLabel("Connection");
   }