Infinite loop error after using Java Sun Tutorial for Learning Swing

Question

I have been attempting to create a GUI following Sun's learning swing by example (example two): http://java.sun.com/docs/books/tutorial/uiswing/learn/example2.html
In particular, the following lines were used almost word-for-word to avoid a non-static method call problem:
SwingApplication app = new SwingApplication();
Component contents = app.createComponents();
frame.getContentPane().add(contents, BorderLayout.CENTER);I believe that I am accidentally creating a new instance of the gui class repeatedly (since it shows new GUI's constantly and then crashes my computer), possibly because I am creating an instance in the main class, but creating another instance in the GUI itself. I am not sure how to avoid this, given that the tutorials I have seen do not deal with having a main class as well as the GUI. I have googled (a nice new verb) this problem and have been through the rest of the swing by example tutorials, although I am sure I am simply missing a website that details this problem. Any pointers on websites to study to avoid this problem would be appreciated.
Thanks for your time-
Danielle
/** GUI for MicroMerger program
* Created July/06 at IARC
*@ author Danielle
package micromerger;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import javax.swing.JFileChooser;
import java.lang.Object;
public class MGui
     private static File inputFile1, inputFile2;
     private static File sfile1, sfile2;
     private static String file1Name, file2Name;
     private String currFile1, currFile2;
     private JButton enterFile1, enterFile2;
     private JLabel enterLabel1, enterLabel2;
     private static MGui app;
     public MGui()
          javax.swing.SwingUtilities.invokeLater(new Runnable()
               public void run()
                    System.out.println("About to run create GUI method");
                    app = new MGui();
                    System.out.println("declared a new MGui....");
                    createAndShowGUI();
     //initialize look and feel of program
     private static void initLookAndFeel() {
        String lookAndFeel = null;
     lookAndFeel = UIManager.getSystemLookAndFeelClassName();
     try
          UIManager.setLookAndFeel(lookAndFeel);
     catch (ClassNotFoundException e) {
                System.err.println("Couldn't find class for specified look and feel:"
                                   + lookAndFeel);
                System.err.println("Did you include the L&F library in the class path?");
                System.err.println("Using the default look and feel.");
            } catch (UnsupportedLookAndFeelException e) {
                System.err.println("Can't use the specified look and feel ("
                                   + lookAndFeel
                                   + ") on this platform.");
                System.err.println("Using the default look and feel.");
            } catch (Exception e) {
                System.err.println("Couldn't get specified look and feel ("
                                   + lookAndFeel
                                   + "), for some reason.");
                System.err.println("Using the default look and feel.");
                e.printStackTrace();
     // Make Components--
     private Component createLeftComponents()
          // Make panel-- grid layout
     JPanel pane = new JPanel(new GridLayout(0,1));
        //Add label
        JLabel welcomeLabel = new JLabel("Welcome to MicroMerger.  Please Enter your files.");
        pane.add(welcomeLabel);
     //Add buttons to enter files:
     enterFile1 = new JButton("Please click to enter the first file.");
     enterFile1.addActionListener(new enterFile1Action());
     pane.add(enterFile1);
     enterLabel1 = new JLabel("");
     pane.add(enterLabel1);
     enterFile2 = new JButton("Please click to enter the second file.");
     enterFile2.addActionListener(new enterFile2Action());
     pane.add(enterFile2);
     enterLabel2 = new JLabel("");
     pane.add(enterLabel2);
     return pane;
     /** Make GUI:
     private static void createAndShowGUI()
     System.out.println("Creating a gui...");
        JFrame.setDefaultLookAndFeelDecorated(true);
        //Create and set up the window.
        JFrame frame = new JFrame("MicroMerger");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     //Add stuff to the frame
     //MGui app = new MGui();
     Component leftContents = app.createLeftComponents();
     frame.getContentPane().add(leftContents, BorderLayout.WEST);
        //Display the window.
        frame.pack();
        frame.setVisible(true);
private class enterFile1Action implements ActionListener
     public void actionPerformed(ActionEvent evt)
          JFileChooser chooser = new JFileChooser();
          int rVal = chooser.showOpenDialog(enterFile1);
          if(rVal == JFileChooser.APPROVE_OPTION)
               inputFile1 = chooser.getSelectedFile();
               PrintWriter outputStream;
               file1Name = inputFile1.getName();
               enterLabel1.setText(file1Name);
private class enterFile2Action implements ActionListener
     public void actionPerformed(ActionEvent evt)
          JFileChooser chooser = new JFileChooser();
          int rVal = chooser.showOpenDialog(enterFile1);
          if(rVal == JFileChooser.APPROVE_OPTION)
               inputFile2 = chooser.getSelectedFile();
               PrintWriter outputStream;
               file2Name = inputFile2.getName();
               enterLabel2.setText(file2Name);
} // end classAnd now the main class:
* Main.java
* Created on June 13, 2006, 2:29 PM
* @author Danielle
package micromerger;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
public class Main
    /** Creates a new instance of Main */
    public Main()
     * @param args the command line arguments
    public static void main(String[] args)
        MGui mainScreen = new MGui();
        //mainScreen.setVisible(true);
        /**Starting to get file choices and moving them into GPR Handler:
         System.out.println("into main method");
     String file1Name = new String("");
         file1Name = MGui.get1Name();
     System.out.println("good so far- have MGui.get1Name()");
    }// end main(String[] args)
}// end class Main

Answer 1

um, yeah, you definitely have a recursion problem, that's going to create an infinite loop. you will eventually end up an out of memory error, if you don't first get the OS telling you you have too many windows. interestingly, because you are deferring execution, you won't get a stack overflow error, which you expect in an infinite recursion.
lets examine why this is happening:
in main, you call new MGui().
new MGui() creates a runnable object which will be run on the event dispatch thread. That method ALSO calls new MGui(), which in turn ALSO creates a new object which will be run on the event dispatch thead. That obejct ALSO calls new MGui(), which ...
well, hopefully you get the picture.
you should never unconditionally call a method from within itself. that code that you have put in the constructor for MGui should REALLY be in the main method, and the first time you create the MGui in the main method as it currently exists is unnecessary.
here's what you do: get rid of the MGui constructor altogether. since it is the implicit constructor anyway, if it doesn't do anything, you don't need to provide it.
now, your main method should actually look like this:
public static void main( String [] args ) {
  SwingUtilities.invokeLater( new Runnable() {
    public void run() {
      MGui app = new MGui();
      app.createAndShowGUI();
}// end mainyou could also declare app and call the constructor before creating the Runnable, as many prefer, because you would have access to it from outside of the Runnable object. The catch there, though, is that app would need to be declared final.
- Adam