Java Validated FileChooser

Fri, Nov 4, 2011 - 7:55am -- Isaac Sukin

I happened to be working on a Java project recently where I needed to let the user save files through the GUI. The Swing toolkit has a nice JFileChooser class that lets you show a file save dialog so that the user can choose the directory and name of the file to save. However, not all filenames are valid, and Java doesn't validate them for you by default. If you try to save a file with an invalid name, Java will throw an error, and this is often considered the only way to know if the filename is valid. That's bad practice though since you should never rely on an exception being thrown as a condition of your program running correctly; by definition, exceptions are unreliable and sometimes unpredictable. Additionally if your file saves successfully then you have to immediately delete it (because you were only saving it to test the filename) and that's messy. So I wrote a ValidatedFileChooser class that checks various criteria to make sure that filenames are valid before attempting to save the file, and alerts the user if one of the criteria fails. The class is below, and I'm releasing it to the public domain.

The class also allows setting one or more file extensions; if these are set then no other file extensions will be allowed. If a single extension is designated as required then filenames with no extension will be saved with the required extension.

import javax.swing.JFileChooser;
import javax.swing.JOptionPane;
import javax.swing.filechooser.FileFilter;
public class ValidatedFileChooser extends JFileChooser {
	private static final long serialVersionUID = -8192863096623921736L;
        // Invalid names on Windows
	private static final String[] INVALID_NAMES = {
	private String extension;
	private String[] allowed_extensions;
	public void approveSelection() {
		File file = getSelectedFile();
		String filename = file.getName();
		 * We do a series of checks to make sure we have a valid filename:
		 * - The parent directory exists (it might not if a user includes a backslash in the name)
		 * - The name can only use valid characters
		 * - The name can't be too long or short
		 * - The name can't end with a space or period (a Windows restriction)
		 * - The name can't start with a period (this would make it hidden)
		 * - The name can't be a system reserved name
		 * Then, if specified, we add the required extension to the name if it
		 * is not already part of the filename. If a required extension was not
		 * specified, we check the extension against a list of valid
		 * extensions and complain if the filename uses an invalid extension or
		 * no extension at all.
		 * Finally, we check whether a file with that name already exists, and
		 * ask for confirmation to overwrite it if it does.
		if (!(new File(file.getParent()).isDirectory()) && getDialogType() == SAVE_DIALOG) {
			complain("Invalid path - backslashes are not allowed in file names.");
		if (!filename.matches("[\\w \\-_\\.]+") && getDialogType() == SAVE_DIALOG) {
			complain("\""+ filename + "\" is an invalid filename. You can only use alphanumeric characters, spaces, hyphens, underscores, and periods.");
		if (filename.length() > 255 && getDialogType() == SAVE_DIALOG) {
			complain("The file name \""+ filename.substring(0, 64) + "...\" is too long. Please choose a shorter name.");
		if (filename.length() < 1 && getDialogType() == SAVE_DIALOG) {
			complain("Please choose a file name.");
		if (filename.matches(".*(\\.|\\s)+") && getDialogType() == SAVE_DIALOG) {
			complain("You cannot end a file name with a space or period.");
		if (filename.matches("\\.+.*+") && getDialogType() == SAVE_DIALOG) {
			complain("You cannot start a file name with a period (hidden files are not allowed here).");
		if (inArray(filename, INVALID_NAMES) && getDialogType() == SAVE_DIALOG) {
			complain("\""+ filename +"\" is an invalid filename.");
		if (extension != null) {
			String ext = getExtension(file);
			if (ext == null || !ext.equals(extension)) {
				file = new File(file.getPath() +"."+ extension);
		else {
			String ext = getExtension(file);
			if (ext == null) {
				complain("You must specify a file extension.");
			if (!inArray(ext, allowed_extensions)) {
				complain("The file extension "+ ext +" is not allowed.");
		if (file.exists() && getDialogType() == SAVE_DIALOG) {
			int confirm = JOptionPane.showConfirmDialog(
					file.getName() + " already exists! Would you like to overwrite it?",
					"File already exists",
			if (confirm != JOptionPane.YES_OPTION)
	private void complain(String complaint) {
				"Invalid filename",
	public String getRequiredExtension() {
		return extension;
	public void setRequiredExtension(final String extension) {
		this.extension = extension;
		setFileFilter(new FileFilter() {
			public boolean accept(File f) {
				if (f.isDirectory())
					return f.getName().charAt(0) != '.';
				String ext = ValidatedFileChooser.getExtension(f);
				return ext != null && ext.equals(extension);
			public String getDescription() {
				return extension.toUpperCase();
	public void setAllowedExtensions(String[] exts) {
		allowed_extensions = exts;
	public String[] getAllowedExtensions() {
		return allowed_extensions;
	// Based on code from .
    public static String getExtension(File f) {
        String ext = null, s = f.getName();
        int i = s.lastIndexOf('.');
        if (i > 0 &&  i < s.length() - 1) {
            ext = s.substring(i+1).toLowerCase();
        return ext;
    private static boolean inArray(String val, String[] array) {
    	for (String v : array) {
    		if (val.equalsIgnoreCase(v))
    			return true;
    	return false;