使用BitSet查找某个范围内的所有的素数的个数（摘自java核心技术.卷I）

import java.util.*;

/**
 * This program runs the Sieve of Erathostenes benchmark. It computes all primes up to 2,000,000.
 * @version 1.21 2004-08-03
 * @author Cay Horstmann
 */
public class Sieve
{
	public static void main(String[] s)
	   {
		 int n = 2000000;
	      long start = System.currentTimeMillis();
	      BitSet b=new BitSet(n+1);
	      int count=0;
	      int i;
	      for(i=2;i<=n;i++)
	      {
	    	  b.set(i);
	      }
	      i=2;
	      while(i*i<=n)
	      {
	    	  if(b.get(i))
	    	  {
	    		  count++;
	    		  int k=2*i;
	    		  while(k<=n)
	    		  {
	    			  b.clear(k);
	    			  k+=i;
	    		  }
	    	  }
	    	  i++;
	      }
	      while(i<=n)
	      {
	    	  if(b.get(i))count++;
	    	  i++;
	      }
	      long end = System.currentTimeMillis();
	      System.out.println(count + " primes");
	      System.out.println((end - start) + " milliseconds");
	   }
	
}