Unique random alpha num string

Question

Hi Guys,
I have a unique question.
I need to generate a unique random Alpha Num string.
I cannot use 0,5,8 and 0,S,B in the generated output string.
The string generated should be no more than length 7 max
Pls help.

Answer 1

Following solution generates random unique strings combinations
from any given base set. Here is a sample output.
$ java UniqueStringGenerator 7 100 10
FCT2RYT
2M31BU8
VV8FU4Z
BON2MKE
OKUJZT0
68E8X8S
YTNLZAY
CPEKPFP
C176M24
GQB9QE9The idea is quite simple.
Pre-work:
1. The base set is first mixed to increase entropy.
2. All possible combinations are divide up into random buckets.
Work for every unique string:
1. Pick a bucket at random.
2. Get next number in bucket.
3. Convert the number to a unique string token.
It is possible to adjust how much entropy you want in your
sequence of generated combinations by choosing how many buckets
the generator contains. However, every bucket will take up some
memory.
import java.util.*;
import java.math.*;
public class UniqueStringGenerator {
    public static String[] ALPHA_NUM =
     {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P",
      "Q","R","S","T","U","V","X","Y","Z",
      "0","1","2","3","4","5","6","7","8","9"};
    private List _parts = null;
    private List _buckets = null;
    private int _length;
    private BigInteger _range = null;
    public UniqueStringGenerator(String[] parts, int length,
                     int numberOfBuckets) {
     _length = length;
     // mixing the base set to increase entropy
     _parts = new Vector();
     List all = new Vector();
     for (int i = 0;i < parts.length;i++) {
         all.add(parts);
     while (all.size()>0) {
     int p = (int)(((double)all.size())*Math.random());
     _parts.add(all.get(p));
     all.remove(p);
     // dividing all possible numbers into given number of buckets
     BigInteger numParts = BigInteger.valueOf(parts.length);
     _range = BigInteger.ONE;
     for (int i = 0;i<_length;i++) {
     range = range.multiply(numParts);
     TreeSet starts = new TreeSet();
     Random rand = new Random();
     while (starts.size()<numberOfBuckets-1) {
     BigInteger bi = new BigInteger(_range.bitLength(), rand);
     if (bi.compareTo(_range)<=0) {
          starts.add(bi);
     _buckets = new Vector();
     Bucket last = new Bucket(BigInteger.ZERO);
     _buckets.add(last);
     for (Iterator i = starts.iterator();i.hasNext();) {
     Bucket next = new Bucket((BigInteger)i.next());
     _buckets.add(next);
     last.setTo(next.getStart());
     last = next;
     last.setTo(_range);
private class Bucket {
     private BigInteger _start;
     private BigInteger _next;
     private BigInteger _to;
     public Bucket(BigInteger start) {
     _start = start;
     _next = start;
     public void setTo(BigInteger to) {
     _to = to;
     public BigInteger getStart() {
     return _start;
     public boolean hasNext() {
     return next.compareTo(to)<0;
     public BigInteger next() {
     BigInteger ret = _next;
     next = next.add(BigInteger.ONE);
     return ret;
     public String toString() {
     return "["+start+", "+next+", "+_to+"]:"+hasNext();
public String nextUnique() {
     // getting next big integer
     BigInteger left = null;
     while (left==null) {
     Bucket b = (Bucket)
          buckets.get((int)(Math.random()*buckets.size()));
     if (b.hasNext()) {
          left = b.next();
     // converting it to a combination
     StringBuffer unique = new StringBuffer();
     BigInteger base = BigInteger.valueOf(_parts.size());
     for (int i = 0;i<_length;i++) {
     BigInteger[] resRem = left.divideAndRemainder(base);
     left = resRem[0];
     unique.append(_parts.get(resRem[1].intValue()));
     return unique.toString();
public static void main(String[] args) {
     if (args.length!=3) {
     System.out.println("Usage: UniqueStringGenerator "+
               "[string size] [# buckets] [# prints]");
     return;
     int length = Integer.parseInt(args[0]);
     int numBuckets = Integer.parseInt(args[1]);
     int num = Integer.parseInt(args[2]);
     UniqueStringGenerator usg = new UniqueStringGenerator
     (UniqueStringGenerator.ALPHA_NUM, length, numBuckets);
     for (int i=0;i<num;i++) {
     System.out.println(usg.nextUnique());