Converting all double bytes UTF-8 numbers (０－９) in a String to single bytes (0-9)

gistfile1.java

private static String toHankakuNumbers(String zenkaku) {
    if (zenkaku == null)
        return null;
    StringBuffer ret = new StringBuffer();
    char [] src = zenkaku.toCharArray();
    for (char c : src) {
        if (c >= 0xff10 && c <= 0xff19) {
            ret.append((char)(c - 65248));
        } else {
            ret.append(c);
        }
    }
    return ret.toString();
}