EratosthenesSieve.java

    import java.util.BitSet;

    public class EratosthenesSieve {
        private BitSet primes = null;

        public void init(int maxNum) {
            final long start = System.currentTimeMillis();
            primes = new BitSet(maxNum+1);

            primes.clear(0, 1);
            primes.set(2, maxNum, true);
            for (int i = 2; i < maxNum; i++) {
                if (primes.get(i)) {
                    int mul = 2;
                    while (mul * i > 0 && mul * i <= maxNum) {
                        primes.set(i * mul, false);
                        mul++;
                    }
                }
            }

            final long duration = System.currentTimeMillis() - start;

            System.out.println(String.format("Generated primes up to %s in %s ms.", maxNum, duration));

        }

        public boolean isPrime(int num) {
            return primes.get(num);
        }
    }