Implementation of java.util.Random.nextInt

Question

This function is from java.util.Random. It returns a pseudorandom int uniformly distributed between 0 and the given n. Unfortunately I

Answer 1

next generates random bits.

1. When n is a power of 2, a random integer in that range can be generated just by generating random bits (I assume that always generating 31 and throwing some away is for reproducibility). This code path is simpler and I guess it's a more commonly used case so it's worth making a special "fast path" for this case.

2. When n isn't a power of 2, it throws away numbers at the "top" of the range so that the random number is evenly distributed. E.g. imagine we had n=3, and imagine we were using 3 bits rather than 31 bits. So bits is a randomly generated number between 0 and 7. How can you generate a fair random number there? Answer: if bits is 6 or 7, we throw it away and generate a new one.