public final class FileNameExtensionFilter extends FileFilter {
    // ...
    public boolean accept(File f) {
        if (f != null) {
            if (f.isDirectory()) {
                return true;
            }
            // NOTE: we tested implementations using Maps, binary search
            // on a sorted list and this implementation. All implementations
            // provided roughly the same speed, most likely because of
            // overhead associated with java.io.File. Therefor we've stuck
            // with the simple lightweight approach.
            String fileName = f.getName();
            int i = fileName.lastIndexOf('.');
            if (i > 0 && i < fileName.length() - 1) {
                String desiredExtension = fileName.substring(i+1).
                        toLowerCase(Locale.ENGLISH);
                for (String extension : lowerCaseExtensions) {
                    if (desiredExtension.equals(extension)) {
                        return true;
                    }
                }
            }
        }
        return false;
    }
    // ...