UID finder.java

PrintWriter pw = new PrintWriter("uids.txt", "UTF-8");
BigInteger i = new BigInteger("0");
String uidPrefix = "20";
String uid = "";
while (!uid.equals("219999999999999999999")) {
    i = i.add(BigInteger.valueOf(1));
    uid = uidPrefix;
    if (String.valueOf(i).length() < 19) {
        for (int a = 0; a < 19 - String.valueOf(i).length(); a++) {
            uid += "0";
        }
    }
    uid += i;
    if (uid.equals("209999999999999999999")) {
        i = BigInteger.valueOf(0);
        uidPrefix = "21";
    }
    pw.println(uid);
}
pw.close();