Sieve implementation in Java

Helper.java

public class Helper {
      public static BitSet generatePrimes(final int maxNum) {
        final BitSet set = new BitSet(maxNum+1);
        set.clear(0, 1);
        set.set(2, maxNum, true);
        for (int i = 2; i < maxNum; i++) {
            boolean isPrime = true;
            for (int j = 0; j < (int) Math.sqrt(i); j++) {
                if (!set.get(j)) {
                    continue;
                }
                if (i % j == 0) {
                    isPrime = false;
                    break;
                }
            }
            if (isPrime) {
                int idx = i;
                while (idx < maxNum + 1) {
                    idx += i;
                    set.set(idx, false);
                }
            }
        }

        return set;
    }
}