DBC problem - extractFilename

/** A function that extracts the filename from an URL (taken from the Mozilla project).
  * Examine the code and explain whether the code represents an "error checking" solution,
  * a Design by Contract solution, or neither.
  */
public class DBCdemo
{
    // function that extracts the filename from a url
    public static String getFileNameFromUrl(String urlstr)
    {
        if (urlstr.length() == 0)
        {
            return null;
        }
        // For "http://foo/bar/cheese.jpg", return "cheese.jpg".
        // For "imap://[email protected]:143/fetch>UID>/INBOX>951?part=1.2&type=image/gif&filename=foo.jpeg", return "foo.jpeg".
        // The 2nd url (ie, "imap://...") is generated for inline images by MimeInlineImage_parse_begin() in mimeiimg.cpp.

        String tail = urlstr.substring(urlstr.lastIndexOf("/")+1);

        if (tail.length()>0)
        {
            int nameIndex = tail.lastIndexOf( "filename=" );
            if (nameIndex != -1)
            {
                return (tail.substring(nameIndex+9));
            }
            else
            {
                return tail;
            }
        }
        return null;
    }
}