Java Swing: main class wait until JFrame is closed

Question

I need some help with a simple java application which makes use of two jframe to get some input parameters. Here's a sketch of my code:

//second jframe, called when the button OK of the first frame is clicked
public class NewParamJFrame extends JFrame{
  ...
}

//first jframe
public class StartingJFrame extends JFrame{
  private static  NewParamJFrame newPFrame = null;
  private JTextField gnFilePath;
  private JButton btnOK;

  public StartingJFrame(){
            //..
    initComponents();
  }

  private void initComponents(){
     btnOK.addActionListener(new ActionListener(){
      public void actionPerformed(ActionEvent e){
        try{
        EventQueue.invokeAndWait(new Runnable(){
           public void run() {
               try {
               newPFrame = new NewParamJFrame();
               newPFrame.setVisible(true);
               } catch (Exception e) {
               e.printStackTrace();
               }
           }
         });
        }
        catch(InvocationTargetException e2) {} 
        catch(InterruptedException e1){}
        dispose();
      }
  }

  public String getText(){
       return gnFilePath.getText();
  }
}

public class Main {
  private static StartingJFrame begin = null;
  public static void main(String[] args) {
     try{
        EventQueue.invokeAndWait(new Runnable(){
            public void run() {
                try {
                    begin = new StartingJFrame();
                    begin.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
    catch(InvocationTargetException e) {} 
    catch(InterruptedException e1){}

    String s= begin.getText();

    //...use s ...
  }
}

The call to getText() causes a NullPointerException. I want the main class to wait until the frames are closed but I don't know how to do. I'm using swing for the first time.

Answer 1

I want the main class to wait until the frames are closed but I don't know how to do. I'm using swing for the first time.

If I understand your problem correctly, you need StartingJFrame to stay waiting until NewParamJFrame is closed and then continue its execution. If this is the case then it won't happen because JFrame doesn't support modality. But JDialog does, so you can have just one JFrame and do the parameters request in a JDialog whose parent is this JFrame.

For a better explanation about modality, take a read to How to Use Modality in Dialogs.

Also take a look to this topic: The Use of Multiple JFrames, Good/Bad Practice?

In any case you'll probably face a new problem: what should the JFrame do if the user closes/cancels the dialog withouth input any parameter? How could this JFrame know what just happened in that dialog? One approach is described in this answer. You'll see the example is about a login dialog but the problem is similar to this one: How could a dialog notify to its parent frame on how the process went?

Answer 2

The easiest way to wait for close without modifying the code flow is to use a modal JDialog. So you have to change your StartingJFrame class to make it a subclass of JDialog instead of JFrame, and add the following to the begin of its constructor:

super((Window)null);
setModal(true);

Then the setVisible(true); invocation on the StartingJFrame instance will wait until the dialog has been closed and hence the invokeAndWait invocation will wait too.