Question

Usually in a Swing application if I want to display JDialogs to the user, I create them once and then reuse them.
Usually the main JFrame has references to these JDialogs.
Sometimes though I need to display a JDialog from parts of code outside the JFrame class.
Usually I make the JDialog member variables (of JFrame) public and either they are static or pass a reference of the JFrame arround to be able to access the JDialog.
It works but it seems messy to me.
I was wondering is there a standard design approach for something like this?

Answer 1

My own philosophy is to make almost all fields private, including the dialog and only expose that which needs exposing, no more and no less. For instance if I have a JDialog called from another class and I get information from that dialog, I expose only the information via public getter methods, not the dialog itself.

Edit 1
Sorry for some gross over-kill and much longer than expected code, but this is kind of what I meant in an MVC sort of way and created in a great hurry:

import java.awt.*;
import java.awt.Dialog.ModalityType;
import java.awt.event.*;

import javax.swing.*;

public class DialogEg {
   private static void createAndShowUI() {
      JFrame frame = new JFrame("Dialog Eg");

      MainPanel mainPanel = new MainPanel();
      MainControl mainControl = new MainControl(frame);
      mainControl.setMainPanel(mainPanel);
      mainPanel.setControl(mainControl);

      frame.getContentPane().add(mainPanel.getMainPanel());
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

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

class MainPanel {
   private JPanel mainPanel = new JPanel();
   private JButton showDlgBtn = new JButton("Show Dialog");
   private JTextField field = new JTextField(10);
   private MainControl mainControl;

   public MainPanel() {
      mainPanel.add(showDlgBtn);
      mainPanel.add(field);
      field.setEditable(false);

      showDlgBtn.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent e) {
            if (mainControl != null) {
               mainControl.showDialog();
            }
         }
      }); 
   }

   public void setControl(MainControl mainControl) {
      this.mainControl = mainControl;
   }

   public void setFieldText(String text) {
      field.setText(text);
   }

   public JPanel getMainPanel() {
      return mainPanel;
   }
}

class DialogPanel {
   private static final String[] COMBO_DATA = {"", "one", "two", "three", "four"};
   private JPanel dialogPanel = new JPanel();
   private JComboBox combo = new JComboBox(COMBO_DATA);

   public DialogPanel() {
      dialogPanel.add(combo);
      combo.addActionListener(new ActionListener() {
         public void actionPerformed(ActionEvent e) {
            Window window = SwingUtilities.getWindowAncestor(dialogPanel);
            window.dispose();
         }
      });
   }

   public JPanel getDialogPanel() {
      return dialogPanel;
   }

   public String getSelectedText() {
      return combo.getSelectedItem() == null ? "" : combo.getSelectedItem().toString();
   }
}

class MainControl {
   private DialogPanel dlgPanel = new DialogPanel();
   private MainPanel mainPanel;
   private JFrame frame;

   public MainControl(JFrame frame) {
      this.frame = frame;
   }

   public void setMainPanel(MainPanel mainPanel) {
      this.mainPanel = mainPanel;
   }

   public void showDialog() {
      if (mainPanel != null) {
         JDialog dialog = new JDialog(frame, "Dialog", ModalityType.APPLICATION_MODAL);
         dialog.getContentPane().add(dlgPanel.getDialogPanel());
         dialog.pack();
         dialog.setLocationRelativeTo(null);
         dialog.setVisible(true);

         String text = dlgPanel.getSelectedText();
         if (text != null) {
            mainPanel.setFieldText(text);
         }
      }
   }

}

The key here is that the MainPanel has no knowledge about the DialogPanel and visa-versa since it’s all connected by the MainControl through the marvels of MVC.