Question

Following my previous two posts here and another one here, the following code
opens the regular file browser instead of the expanded one:

public class GuiHandler extends javax.swing.JFrame {
    // data members
    private DataParser xmlParser = new DataParser();
    private File newFile;
    JFileChooser jfc = new JFileChooser();

    // more code 

    public void launchFileChooser() {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch(Exception e) {
                    e.printStackTrace();
                }
                jfc.setFileSelectionMode(JFileChooser.FILES_ONLY);
                jfc.setAcceptAllFileFilterUsed(false);
                if (jfc.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
                    newFile = jfc.getSelectedFile();
            }
        });
    }

    // more code 

    private void XMLfilesBrowserActionPerformed(java.awt.event.ActionEvent evt) {       
        launchFileChooser();
        xmlParser.getNodeListFromFile(newFile);
        // here the code has the below problems 

Problems:

1. The code opens a regular file browser when I hit a button to open XML file; it still allows me to pick a file.
2. It throws an exception:

Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: File cannot be null
    at javax.xml.parsers.DocumentBuilder.parse(DocumentBuilder.java:198)

Why does it open the regular browser if jfc is a data member, and when it’s a local
variable, the expanded one opens?

Answer 1

Concerning the regular versus expanded file chooser, make sure to call UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); before calling new JFileChooser();. Actually, unless you allow users to change the look and feel (L&F) during application execution, set the L&F close to the beginning of application execution, like in the main method, before creating any Swing components. From my experience, not doing so can cause some odd UI behavior.

When you have JFileChooser as a local variable in launchFileChooser, UIManager.setLookAndFeel is called before new JFileChooser. When JFileChooser is a class member variable (a.k.a. data member), UIManager.setLookAndFeel is called after new JFileChooser; in the latter case, the JFileChooser is created when an instance of GuiHandler is instantiated.

---

Concerning the IllegalArgumentException use SwingUtilities.invokeAndWait in launchFileChooser instead of SwingUtilities.invokeLater. Better yet, if you’re sure launchFileChooser will always occur on the event dispatch thread, there’s no need to call either SwingUtilities.invokeAndWait or SwingUtilities.invokeLater.

---

You also may want to use a file filter:

jfc.setFileFilter(new FileNameExtensionFilter("XML files (*.xml)", "xml"));

---

The following is an SSCE that demonstrates the concepts discussed above:

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;

import javax.swing.JButton;
import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.UIManager;
import javax.swing.filechooser.FileNameExtensionFilter;

public class GuiHandler extends JFrame {
    public static void main(String[] args) throws Exception {
        // call UIManager.setLookAndFeel early in application execution
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        JFrame frame = new GuiHandler();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

    private final JFileChooser jfc;

    public GuiHandler() {
        this.jfc = new JFileChooser();
        this.jfc.setFileSelectionMode(JFileChooser.FILES_ONLY);
        this.jfc.setFileFilter(new FileNameExtensionFilter("XML files (*.xml)", "xml"));

        final JButton button = new JButton("Open XML file");
        button.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                xmlFilesBrowserActionPerformed();
            }
        });
        add(button);

        pack();
    }

    protected void xmlFilesBrowserActionPerformed() {
        final File xmlFile = getXmlFile();
        if (xmlFile != null) {
            System.out.println(xmlFile); // process XML file
        }
    }

    private File getXmlFile() {
        // At this point we should be on the event dispatch thread,
        // so there is no need to call SwingUtilities.invokeLater
        // or SwingUtilities.invokeAndWait.
        if (this.jfc.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) {
            return this.jfc.getSelectedFile();
        }
        return null;
    }
}