showOpenDialog() of JFileChooser doesn't open dialog box for opening files

I'm writing a GUI utility for merging pdf files but the showOpenDialog() method of JFileChooser doesn't open the dialog box for opening files. There is no response when the "Open" item in the File menu is clicked. For your convenience, the download link for the Apache pdfbox library is:
https://pdfbox.apache.org/download.cgi

I tried to move the guts of the openFiles() method into the corresponding location in the actionPerformed() method, but it still didn't work. However, in the code of another person for a different purpose, the showOpenDialog() method works in a method called from within the actionPerformed method. The code is as follows:

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import javax.swing.*;
import javax.swing.filechooser.FileFilter;
import org.apache.pdfbox.multipdf.PDFMergerUtility;

public class PDFMerger extends JFrame implements ActionListener {
	private static final int DEFAULT_WIDTH = 500;
	private static final int DEFAULT_HEIGHT = 500;
	private JMenuItem openItem;
	private JMenuItem saveItem;
	private JMenuItem exitItem;
	private JTextArea textArea;
	private JTextField textField;
	private JButton bindButton;
	private File[] files;
	private File mergedFile;

	public static void main(String[] args) {
		PDFMerger pdfMerger = new PDFMerger();
		pdfMerger.setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
		pdfMerger.setDefaultCloseOperation(EXIT_ON_CLOSE);
		pdfMerger.setVisible(true);
	}

	public PDFMerger () {
		JMenu menu = new JMenu("File");
		JMenuItem openItem = new JMenuItem("Open");
		openItem.addActionListener(this);
		saveItem = new JMenuItem("Save");
		saveItem.setEnabled(false);
		saveItem.addActionListener(this);
		exitItem = new JMenuItem("Exit");
		exitItem.addActionListener(this);
		menu.add(openItem);
		menu.add(saveItem);
		menu.add(exitItem);
		JMenuBar menuBar = new JMenuBar();
		menuBar.add(menu);
		setJMenuBar(menuBar);
		textArea = new JTextArea(40, 50);
		JScrollPane scrollPane = new JScrollPane(textArea);
		add(scrollPane, BorderLayout.CENTER);
		JPanel operationPanel = new JPanel();
		BoxLayout layout = new BoxLayout(operationPanel, BoxLayout.LINE_AXIS);
		operationPanel.setLayout(layout);
		JLabel label = new JLabel("Result: ");
		textField = new JTextField(30);
		textField.setEditable(false);
		bindButton = new JButton("Bind");
		bindButton.addActionListener(this);
		bindButton.setEnabled(false);
		operationPanel.add(label);
		operationPanel.add(textField);
		operationPanel.add(bindButton);
		add(operationPanel, BorderLayout.NORTH);
	}

	public void actionPerformed (ActionEvent event) {
		
		if (event.getSource() == exitItem) 
			System.exit(0);
		else if (event.getSource() == openItem) {
			files = openFiles();
			bindButton.setEnabled(false);
		}
		else if (event.getSource() == bindButton) {
			mergedFile = mergeFiles();
			saveItem.setEnabled(true);
		}
		else if (event.getSource() == saveItem)
			saveFile();
	}

	public File[] openFiles () {
		File[] selectedFiles = null;
		JFileChooser chooser = new JFileChooser();
		chooser.setCurrentDirectory(new File("."));
		chooser.setMultiSelectionEnabled(true);
		int option = chooser.showOpenDialog(PDFMerger.this);
		
		if (option == JFileChooser.APPROVE_OPTION) 
			selectedFiles = chooser.getSelectedFiles();
		return selectedFiles;
	}

	public File mergeFiles () {
		File merged = null;
		// TODO

		return merged;
	}
	public void saveFile () {
		//TODO
	}
}

Thank you for your help.

I fixed your code and got the JFileChooser to open.

The major changes I made are:

1. I started the Swing GUI with a call to the SwingUtilities invokeLater method. This method ensures that the Swing components are created and executed on the Event Dispatch Thread.

2. I used a JFrame. I did not extend a JFrame. You only extend Swing components when you intend to override one or more of the class methods.

3. Your code was confusing to read. I divided the code into logical methods. I left your confusing code alone, but when you create Swing layouts, you really should create the components in row, column order. You should also group all of the method calls for a particular component together. It makes finding and solving problems easier.

4. You shouldn't have just one action listener for everything. You should have separate action listeners for each button or menu item.

Anyway, here's your code. Once I separated things logically, it was easier to find the mistakes which prevented your JFileChooser from displaying.

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import javax.swing.*;
public class PDFMerger implements ActionListener {
private JFrame frame;
private JMenuItem openItem;
private JMenuItem saveItem;
private JMenuItem exitItem;
private JTextArea textArea;
private JTextField textField;
private JButton bindButton;
private File[] files;
private File mergedFile;
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
new PDFMerger();
}
});
}
public PDFMerger() {
frame = new JFrame("PDF Merger");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JMenuBar menuBar = createJMenuBar();
frame.setJMenuBar(menuBar);
JPanel operationPanel = createOperationPanel();
frame.add(operationPanel, BorderLayout.NORTH);
JPanel textPanel = createTextPanel();
frame.add(textPanel, BorderLayout.CENTER);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
private JPanel createOperationPanel() {
JPanel operationPanel = new JPanel();
BoxLayout layout = new BoxLayout(operationPanel, BoxLayout.LINE_AXIS);
operationPanel.setLayout(layout);
JLabel label = new JLabel("Result: ");
textField = new JTextField(30);
textField.setEditable(false);
bindButton = new JButton("Bind");
bindButton.addActionListener(this);
bindButton.setEnabled(false);
operationPanel.add(label);
operationPanel.add(textField);
operationPanel.add(bindButton);
return operationPanel;
}
private JPanel createTextPanel() {
JPanel panel = new JPanel();
panel.setLayout(new BorderLayout());
textArea = new JTextArea(40, 50);
JScrollPane scrollPane = new JScrollPane(textArea);
panel.add(scrollPane, BorderLayout.CENTER);
return panel;
}
private JMenuBar createJMenuBar() {
JMenu menu = new JMenu("File");
openItem = new JMenuItem("Open");
openItem.addActionListener(this);
saveItem = new JMenuItem("Save");
saveItem.setEnabled(false);
saveItem.addActionListener(this);
exitItem = new JMenuItem("Exit");
exitItem.addActionListener(this);
menu.add(openItem);
menu.add(saveItem);
menu.add(exitItem);
JMenuBar menuBar = new JMenuBar();
menuBar.add(menu);
return menuBar;
}
@Override
public void actionPerformed (ActionEvent event) {
if (event.getSource() == exitItem) {
frame.dispose();
System.exit(0);
} else if (event.getSource() == openItem) {
files = openFiles();
bindButton.setEnabled(false);
} else if (event.getSource() == bindButton) {
mergedFile = mergeFiles();
saveItem.setEnabled(true);
} else if (event.getSource() == saveItem)
saveFile();
}
public File[] openFiles () {
File[] selectedFiles = null;
JFileChooser chooser = new JFileChooser();
chooser.setCurrentDirectory(new File("."));
chooser.setMultiSelectionEnabled(true);
int option = chooser.showOpenDialog(frame);
if (option == JFileChooser.APPROVE_OPTION) 
selectedFiles = chooser.getSelectedFiles();
return selectedFiles;
}
public File mergeFiles () {
File merged = null;
// TODO
return merged;
}
public void saveFile () {
//TODO
}
}