How to generate unique positive Long using UUID

by 
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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