How to ensure values produces randomly are unique in Java

Question

When generating a random value, I need to ensure that the generateUserID() method generates a "random and unique" value every time.

private String generateUserID(String prefix, int digitNumber) 
{
    return prefix + String.valueOf(digitNumber < 1 ? 0 : new Random()
            .nextInt((9 * (int) Math.pow(10, digitNumber - 1)) - 1)
            + (int) Math.pow(10, digitNumber - 1));
}

No output from this method should be the same as another value. Thus, I need to refactor the code, but I cannot use any loop.

See also [Create a GUID in Java](https://stackoverflow.com/questions/2982748/create-a-guid-in-java) — andrewJames, Mar 30 '21 at 22:27
That doesn't help me, too many opinions in that post but no concrete solution. — The_Liner, Mar 30 '21 at 22:34
@The_Liner - What is the use of the parameter, `digitNumber`? There is no way to get 100% uniqueness. For all practical purposes, a unique number with nanosecond precision is good enough. — Arvind Kumar Avinash, Mar 30 '21 at 22:54

score 1 · Answer 1 · answered Mar 30 '21 at 22:42

I use random ID's like this when moving data records from one system to another. I generally use a UUID (Universally Unique IDentifier) so I can clearly distinguish any one record from any of its (potentially millions of) counterparts.

Java has pretty good support for them. Try something like this:

import java.util.UUID;

private String generateUserID(String prefix, int digitNumber) {
  String uuid = UUID.randomUUID().toString();
  return prefix + digitNumber + uuid;  // Use the prefix and digitNumber however you want.
}

JavaDocs: https://docs.oracle.com/javase/7/docs/api/java/util/UUID.html#randomUUID()

How to ensure values produces randomly are unique in Java

1 Answers1