Random.nextInt() % n has

• a 1/n chance of being 0
• a 1/2n chance of being each number from 1 to n-1 inclusive
• a 1/2n chance of being each number from -1 to -(n-1) inclusive

Many users expect a uniformly distributed random integer between 0 and n-1 inclusive, but you must use random.nextInt(n) to get that behavior. If the original behavior is truly desired, use (random.nextBoolean() ? 1 : -1) * random.nextInt(n).