code: private static int hashCapacity(int min) {
          BigInteger capacity = BigInteger.valueOf(min * 2 + 1);
          while (!capacity.isProbablePrime(10)) {
              capacity = capacity.add(BigInteger.valueOf(2));
          }
          return capacity.intValue();
      }
docs: Returns a prime number, at least twice as large as needed. This should
  minimize hash collisions. Since all the hash keys are known up front,
  the capacity could be tweaked until there are no collisions, but this
  technique is easier and deterministic.