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If I’m using Long uuid = UUID.randomUUID().getMostSignificantBits() how likely is it to get a collision. It cuts off the least significant bits, so there is a possibility that you run into a collision, right?
According to the documentation, the static method
UUID.randomUUID()generates a type 4 UUID.This means that six bits are used for some type information and the remaining 122 bits are assigned randomly.
The six non-random bits are distributed with four in the most significant half of the UUID and two in the least significant half. So the most significant half of your UUID contains 60 bits of randomness, which means you on average need to generate 2^30 UUIDs to get a collision (compared to 2^61 for the full UUID).
So I would say that you are rather safe. Note, however that this is absolutely not true for other types of UUIDs, as Carl Seleborg mentions.
Incidentally, you would be slightly better off by using the least significant half of the UUID (or just generating a random long using SecureRandom).