求质数的BitSet算法

import java.util.*;

public class BitSetTest {

	public static void main(String[] args) {

		long begin = System.currentTimeMillis();

		BitSet sieve = new BitSet(54115297);

		int size = sieve.size();

		for (int i = 2; i < size; i++)

			sieve.set(i);

		int finalBit = (int) Math.sqrt(sieve.size());

		//这个for if 写的太风骚
		for (int i = 2; i < finalBit; i++)

			if (sieve.get(i))

				for (int j = 2 * i; j < size; j += i)

					sieve.clear(j);

		int counter = 0;

		for (int i = 1; i < size; i++) {

			if (sieve.get(i)) {
				++counter;
			}
			//求 54115291是第几个质数
			if (sieve.get(i) && i == 54115291) {

				System.out.printf("%5d", i);
				System.out.println();
				long end = System.currentTimeMillis();

				System.out.println("求第" + counter + "个质数耗时：" + (end - begin)
						+ "毫秒");
			}

		}

	}

}