UUID to unique integer id?

Question

I was wondering what the easiest way to convert a UUID to a unique integer would be? I have tried using the hash code but people tell me that it is not going to always be unique if i use the hash code?

So what is the easiest way? Is the hash code unique?

No, it's not - by definition. Also, if it were unique, why then would anybody need UUIDs? — Ingo, Apr 06 '11 at 08:35
Define unique. Globally, or just within your application, or a bit of code? — Bart van Heukelom, Apr 06 '11 at 08:36
Its just that by some silly specification I needed an application unique integer, I wanted to make use of the UUID class, but it turns out I cannot down scale it. — Alex Hope O'Connor, Apr 06 '11 at 08:58
It would be easier if you asked the real question: how can I have an application unique integer ? could I convert a UUID to it ? Then we could answer your problem and not just tell you you can't use UUID for that purpose... — pgras, Apr 06 '11 at 09:04

score 47 · Accepted Answer · answered Apr 06 '11 at 08:34

You are going to have a problem since the UUID is 128 bits and the int is only 32bit. You'll either have to accept the risk of collisions and try to fudge it to a smaller space (hashCode is probably a good way to do that) or find an alternative (use the UUID directly, map to a BigInteger - difficult to tell without knowing why)

score 18 · Answer 2 · edited Mar 01 '17 at 19:54

18

Answering the How can I have a unique application wide Integer:

If it needs to be unique even after restarts or if you application is clustered you may use a Database sequence.

If it just needs to be unique during the runtime use a static AtomicInteger.

EDIT (example added):

public class Sequence {

  private static final AtomicInteger counter = new AtomicInteger();

  public static int nextValue() {
    return counter.getAndIncrement();
  }
}

Usage:

int nextValue = Sequence.nextValue();

This is thread safe (different threads will always receive distinct values, and no values will be "lost")

edited Mar 01 '17 at 19:54

ememorais

329
2
11

answered Apr 06 '11 at 09:07

pgras

12,614
4
38
46

Can you give me an example of how to use AtomicInteger? – Alex Hope O'Connor Apr 06 '11 at 11:29
1

The Usage should be edited; shows "Counter.nextValue();", but the class is named "Sequence" – Thomas Sunderland Jul 20 '16 at 17:57
This usage only can be used by single jvm, the cluster (multiple jvms) could not use this option. – Eric Wei May 23 '17 at 07:39
what happens if the distinct value generated goes beyond the int range? Can that happen? – Coder17 Jul 05 '21 at 10:29
@Coder17 You'd get an Integer/Number Overflow exception. But would you end up getting to 2Billion calls? – TheRealChx101 May 05 '23 at 20:30

Andreas Dolk · Answer 3 · 2011-04-06T08:43:46.663

8

A UUID is a 16-Byte number (128 bit). You can't crunch it into an int (32 bit) while preserving it's uniqueness.

Mathematically spoken: 2⁹⁶ UUIDs will share the same Java-int-size hash value (which is ... a lot ;) )

A way out - some real life UUID often have a rather static part. So in isolated scenarios, the real unique portion of UUIDs may be less then 32 bit.

edited Apr 06 '11 at 08:43

answered Apr 06 '11 at 08:36

Andreas Dolk

113,398
19
180
268

score 6 · Answer 4 · answered Dec 28 '18 at 13:15

We had a requirement to convert all our UUIDs into serial numbers. Finally, we tested and used the next algorithm:

Get CRC64 of uuid(16 bytes) using the ECMA polynomial 0xC96C5795D7870F42. Do not use ISO polynomial because it could cause a lot of collisions for some UUID generation algorithms.
Now we have crc64(8 bytes). Take the first N bytes(in our case 5 in yours it will be 4 bytes for int and all bytes for int64)

We tested this method and it works well for several million UUIDs.

Our additional step: convert 5 bytes number into a number with base 36 and finally we have SN: 4YD3SOJB.

Great! But is any library out there for CRC64 using a special polynominal? Iam using Python and there is a CRC64ISO which I guess is not what you meant. Event a C lib is good since I can use ffi for calling it. — Kamyar, Aug 25 '21 at 18:50

score 3 · Answer 5 · edited Nov 17 '13 at 05:01

3

No, the hash code is not (and cannot be) unique. The thing with a GUID/UUID is that you need all 128 bits to guarantee uniqueness, therefore scaling it down in any way will give problems, see e.g. GUIDs are globally unique, but substrings of GUIDs aren't.

Honestly, I think you're better off with just using sequential integers and skip the GUID thing altogether. If you need GUIDs for any reason, then use them and don't try generating an integer from them.

edited Nov 17 '13 at 05:01

Dan Is Fiddling By Firelight

5,981
17
77
128

answered Apr 06 '11 at 08:35

Joey

344,408
85
689
683

6

Comments: 1) The hash code might be unique. You don't know. [Don't rely on it though](http://security.stackexchange.com/a/52881/94827). 2) You don't have a guarantee of UUID uniqueness. Just a very good probability of never seeing a collision. 3) Even if you did "scaling it down in any way", you don't necessarily create any problems. You increase the risk of collisions, yes, and you need to be aware of the consequences of breaking the UUID standard, but that may be perfectly fine in some situations. 4) The link is dead. – Zero3 Jan 11 '16 at 09:26

score -3 · Answer 6 · answered Jan 15 '20 at 08:43

-3

You may convert uuid to BigInteger and keep it unique.

              BigInteger  big = new BigInteger(uuid, 16);

answered Jan 15 '20 at 08:43

yangjun

7

UUID to unique integer id?

6 Answers6

Linked