My file sorting comparator sometimes invoke java.lang.IllegalArgumentException in android app.

I cannot figure out the reason. could you explain that?

here is my code block:

    Comparator cc = new Comparator<File>() {
        @Override
        public int compare(File f1, File f2) {
            if (f1.isDirectory() && !f2.isDirectory()) {
                return 1;
            }
            if (f2.isDirectory() && !f1.isDirectory()) {
                return -1;
            }
            if ((f1.isFile() && f2.isFile()) || (f1.isDirectory() && f2.isDirectory())) {
                if (sort == 1) {
                    return Util.compareFileName(f2.getName(), f1.getName());
                } else if (sort == 2) {
                    return (int) (f1.lastModified() - f2.lastModified());
                } else if (sort == 3) {
                    return (int) (f2.lastModified() - f1.lastModified());
                } else if (sort == 4) {
                    return (int) (f1.length() - f2.length());
                } else if (sort == 5) {
                    return (int) (f2.length() - f1.length());
                } else
                    return Util.compareFileName(f1.getName(), f2.getName());
            }
            return f1.compareTo(f2);
        }
    };

Util.compareFileName is as followings:

    public static int compareFileName(String s1, String s2) {
    int thisMarker = 0;
    int thatMarker = 0;
    int s1Length = s1.length();
    int s2Length = s2.length();

    while (thisMarker < s1Length && thatMarker < s2Length) {
        String thisChunk = getChunk(s1, s1Length, thisMarker);
        thisMarker += thisChunk.length();

        String thatChunk = getChunk(s2, s2Length, thatMarker);
        thatMarker += thatChunk.length();

        // If both chunks contain numeric characters, sort them numerically
        int result = 0;
        if (isDigit(thisChunk.charAt(0)) && isDigit(thatChunk.charAt(0))) {
            // Simple chunk comparison by length.
            int thisChunkLength = thisChunk.length();
            result = thisChunkLength - thatChunk.length();
            // If equal, the first different number counts
            if (result == 0) {
                for (int i = 0; i < thisChunkLength; i++) {
                    result = thisChunk.charAt(i) - thatChunk.charAt(i);
                    if (result != 0) {
                        return result;
                    }
                }
            }
        } else {
            result = thisChunk.compareTo(thatChunk);
        }

        if (result != 0)
            return result;
    }

    return s1Length - s2Length;
}