Java BitSet 找素數

import java.util.BitSet;  

public class Sieve
{  
    public static void main(String[] args){  
        int n = 20000000;  
        long start = System.currentTimeMillis();  
        BitSet b = new BitSet(n+1);  
 
        int i = 2;  
        for(i = 2; i <= n;i++)
        {  
            b.set(i);  
        }  
        i = 2;  
       
        while(i*i <=n)
        {  
            if(b.get(i))
            {  
                //set current num's multiple num to false.
                int k = 2 * i;  
                while(k <= n)
                {  
                    b.clear(k);  
                    k +=i;  
                }  
            }  
            i++;  
        }
        long end = System.currentTimeMillis();  
        System.out.println("count = " + b.cardinality());
        System.out.println((end-start) +" milliseconds");  
    }  
}