Question

Please give me sample code to generate UUID of long type in java without using timestamp.

Thanks

Answer 1

A real UUID is 128 bits. A long is 64 bits.

This is not just pedantry. UUID stands for Universal Unique IDentifier.

The “universal uniqueness” of the established UUID schemes are based on:

• encoding a MAC address and a timestamp,
• encoding a hash of a DNS name and a timestamp, or
• using a 122 bit random number … which is large enough that the probability of a collision is very very small.

With 64 bits, there are simply not enough bits for “universal uniqueness”. For instance, the birthday paradox means that if we had a number of computers generating random 64 bit numbers, the probability of a potentially detectable collision would be large enough to be of concern.

Now if you just want a UID (not a UUID), then any 64-bit sequence generator will do the job, provided that you take steps to guard against the sequence repeating. (If the sequence repeats, then the IDs are not unique in time; i.e. over time a given ID may denote different entities.)