What's up with this broken Java Random.nextInt(long) behavior?

Question

I discovered the hard way that this method seems to fail miserably when you feed it a power of 2. Given two Random objects of different seeds, it seems that the first integer they return when asked to return an integer between 0 (inclusive) and and a power of two (exclusive) is always the same; the seeds don't matter. For example:

public static void main(String[] args) {
    Random mRandom;
    for (int i = 0; i < 10; i++) {
        mRandom = new Random(i);            
        System.out.println(mRandom.nextInt((int) Math.pow(2, 4)));
    }
}

Console:
11
11
11
11
11
11
11
11
11
11

I chose 2^4 arbitrarily, but it seems to work for any power of two. What's going on? Furthermore, how can I avoid this?

Answer 1

This problem occurs due to 2 reasons.

1. Same seed for Random class.
2. At nextInt(int n), if n is power of 2

1. Same seed for Random class.

Because, you have initiated new Random instance with new seed value which has influence on the nextInt value generation. According to Java docs of Random(long seed).

Creates a new random number generator using a single long seed. The seed is the initial value of the internal state of the pseudorandom number generator which is maintained by method next(int).

The invocation new Random(seed) is equivalent to:

 Random rnd = new Random();
 rnd.setSeed(seed);

---

If you try to generate random value, without new seed than it will generate real random value, even though new instance of Random class.

for (int i = 0; i < 10; i++) {
    mRandom = new Random();   // Without seed         
    System.out.println(mRandom.nextInt((int) Math.pow(2, 4)));
}

Output: 2 1 12 4 3 9 9 8 2 9

2. At nextInt(int n), if n is power of 2

Besides this, Random#nextInt has effect of power of 2. If n is a power of 2 it will return (int)((n * (long)next(31)) >> 31) which will always same for same n. According to nextInt Algorithm,

public int nextInt(int n) {
   if (n <= 0)
     throw new IllegalArgumentException("n must be positive");

   if ((n & -n) == n)  // i.e., n is a power of 2
     return (int)((n * (long)next(31)) >> 31);

   int bits, val;
   do {
       bits = next(31);
       val = bits % n;
   } while (bits - val + (n-1) < 0);
   return val;
 }