How to generate unique positive Long using UUID

java uuid
Saurabh Kumar picture Saurabh Kumar · Mar 3, 2013 · Viewed 56.9k times · Source

I have a requirement to generate unique Long ids for my database primary key column.

I thought i can use UUID.randomUUID().getMostSignificantBits() but sometimes its generating some negative long also which is problem for me.

Is it possible to generate only positive long from UUID ?There will be like billions of entries so i want that each generated key must be unique.

Answer

Sasha Pechenyi picture Sasha Pechenyi · Jan 12, 2017 
UUID.randomUUID().getMostSignificantBits() & Long.MAX_VALUE

The reason why this works is, when you do bitwise & with 1 it allows the same digit to pass as it is and when you do bitwise & with 0 it blocks it and result is 0. Now, Long.MAX_Value in binary is 

0111111111111111111111111111111111111111111111111111111111111111

this is 0 followed by 63 1s (total is 64 bits, it's long in java)

So when you bitwise & a number X with this above number then you will get the same number X except that the leftmost bit is now turned into a zero. Which means you've only changed the sign of that number and not the value.

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