Change JLabel value of void GUI constructor from external method

Question

I am trying to create a method in order to change the value of a JLabel in my creatAndShowGUI() method. What I want to do is change the value of some JLabels after importing a file (according to the file information).

private static void createAndShowGUI(){
    // Create and set up window
    final JFrame frame = new JFrame("SongApp");


    // Display the window
    frame.setSize(400, 150);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    // Set flowlayout
    frame.getContentPane().setLayout(new FlowLayout());
    try{
        UIManager.setLookAndFeel(
                UIManager.getSystemLookAndFeelClassName());      
    } catch(Exception e){
        System.err.println("Cannot set look and feel: " + e);
    }

    JButton loadFile = new JButton("Choose a new file ...");
    JLabel songLength = new JLabel("0:00");
    JLabel songName = new JLabel("No song selected yet");

    loadFile.addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent e){

            createFileChooser(frame);

        }
    });

    frame.getContentPane().add(loadFile);
    frame.getContentPane().add(songLength);
    frame.getContentPane().add(songName);
}

My method needs to be void since I call it later on in my main method in

    javax.swing.SwingUtilities.invokeLater(new Runnable(){
        public void run(){
            createAndShowGUI();                
        }
    });

After this I created a method as following:

private static void changeWindowInformation(String filename, int time) {

    createAndShowGUI().songName.setText(filename);
    // code to check time of song and return it in format xx:xx
    createAndShoGUI().songLength.setText(time);

}

This method is called in a createFileChooser method which is called in the createAndShowGUI() method after clicking a JButton.

I get to see an error message: void cannot be dereferenced. I am pretty new in Java programming and don't really know what I am doing wrong. Should I place the creation of my frame somewhere else in my code (maybe under my main method?) or am I not accessing the JLabel in the correct way? I have tried to place the method call in my loadFile.addActionListener method and make it only return strings so I can access the JLabels in loadFile.addActionListener but I still get the same error message.

If you need any further information, just tell me.

Thanks already for your help!!

Answer 1

You should make your own Frame-class which inherits JFrame and add the changing labels as class members, because as gparyani already commented

You can't access local variables declared in a method from another method.

For example:

public class MyFrame extends JFrame {
  private JLabel songName, songLength;

  public MyFrame() {
    super("SongApp");
    songLength = new JLabel("0:00");
    songName = new JLabel("No song selected yet");
    JButton loadFile = new JButton("Choose a new file ...");

    //other UI settings

    loadFile.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent e){
        createFileChooser(this);
      }
    });

    getContentPane().add(loadFile);
    getContentPane().add(songLength);
    getContentPane().add(songName);
  }

  public void changeWindowInformation(String filename, int time) {
    this.songName.setText(filename);
    this.songLength.setText(yourFormattedTime); 
  }
}

Further you could pass the reference of this class to your createFileChooser()

public void createFileChooser(MyFrame callingFrame) {
  //your filechooser logic

  callingFrame.changeWindowInformation(filename, time);
}

If createFileChooser() is a method of MyFrame, passing an instance of MyFrame is unnecessary, because changeWindowInformation and createFileChooser would be members of the same instance so they share the same class-members. In this case your code should be:

public void createFileChooser() {
  //your filechooser logic

  changeWindowInformation(filename, time);
}

Using this, you can replace your whole createAndShowGUI()-call with this:

javax.swing.SwingUtilities.invokeLater(new Runnable(){
    public void run(){
        MyFrame frame = new MyFrame();
        frame.setVisible(true);               
    }
});