16 bytes length becomes sometimes 15 bytes length

Question

I'm using some code to generate a 16 bytes length String and I noticed a strange behaviour while using my code which is:

public static String generateMyUniqueString() {
    return new BigInteger(64,oRandom).toString(16);
}

This is giving me a nice 16 characters length string 99% of the time.
But yes, sometimes, the generated string is 15 characters length and, for now, I did not find why.

I bet it happens 1/16 of the time. If the value is less than 2^56, then it will not require 16 digits. — Raymond Chen, Dec 17 '14 at 07:21
@Makoto oRandom comes from `private static SecureRandom oRandom = new SecureRandom();` — Gregordy, Dec 17 '14 at 07:44
@RaymondChen Ahhh! thanks a lot! I was thinking about something similar, I guess you're right! Thanks for enlighten me! — Gregordy, Dec 17 '14 at 07:48

score 1 · Answer 1 · edited May 23 '17 at 11:58

1

Your code reads:

Generate a random secure 64-bit integer and convert it to a hexadecimal string.

When it's converted to a string, leading zeroes are omitted. If you are really lucky you could also get a result with 14 or less digits.

If you want to always have a 16-digit value, you need to add leading zeros manually.

see: https://stackoverflow.com/a/6185386/3264295 for an example how to pad a string

edited May 23 '17 at 11:58

Community

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1

answered Dec 17 '14 at 07:45

mukunda

2,666
11
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Thank you for this suggestion, I'll give it a try! – Gregordy Dec 17 '14 at 23:30

score 1 · Accepted Answer · answered Dec 17 '14 at 08:09

1

I suggest you try

public static String generateMyUniqueString() {
    return String.format("%016x", new BigInteger(64, oRandom));
}

This will always be 16 digits long as it zero pads the start.

BTW: if you generate 4 billion of these ids there is a 50/50 changes two will be the same.

Have you considered using UUID (128 bit), or a durable counters instead?

answered Dec 17 '14 at 08:09

Peter Lawrey

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72
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Thanks, I will try this out and no I did not considered using UUID. – Gregordy Dec 17 '14 at 23:29

16 bytes length becomes sometimes 15 bytes length

2 Answers2