Bloom Filter --- java简单实现

gistfile1.java

/*布隆过滤器（Bloom Filter）是1970年由Burton Howard Bloom提出的。
*它实际上是一个很长的二进制向量和一系列随机映射函数。
*布隆过滤器可以用于检索一个元素是否在一个集合中。
*它的优点是空间效率和查询时间都远远超过一般的算法，缺点是有一定的误识别率和删除困难。
*/

import java.util.BitSet;

public class SimpleBloomFilter { 

      private static final int DEFAULT_SIZE = 2 << 64;
      private static final int[] seeds = new int[] { 7, 113, 213, 3111, 397, 611,532 };  //不同的函数seed
      private BitSet bits = new BitSet(DEFAULT_SIZE);

      private SimpleHash[] func = new SimpleHash[seeds.length];   //func 函数数组
       
      public SimpleBloomFilter() {
          for (int i = 0; i < seeds.length; i++) func[i] = new SimpleHash(DEFAULT_SIZE, seeds[i]);   //初始化func函数数组
      } 

       public static class SimpleHash { 
           
     	  private int cap; 
          private int seed;
          public SimpleHash(int cap, int seed) { 
               this.cap = cap;
               this.seed = seed;  
           } 

	       public int hash(String value) {
	            int result = 0;
	            int len = value.length(); 
	           for (int i = 0; i < len; i++)result = seed * result + value.charAt(i); //一般的hash算法
	           return (cap - 1) & result;   //把值的范围控制在cap内
	       }
       } 
       
      
      public void add(String url) {    //bloom filter 添加url值
          for (SimpleHash f : func)bits.set(f.hash(url), true); 
     }      

      public boolean contains(String value) {   //判断 bloom filter 是否包含url值
         if (value == null) {return false;}
         boolean ret = true; 
         for (SimpleHash f : func)ret = ret && bits.get(f.hash(value));
         return ret;
       }
       
      
       public static void main(String[] args) { 
           String[] urls = new String[]{"www.example.com","www.renren.com","www.baidu.com","www.baidu.com"};  //测试数据
           SimpleBloomFilter filter = new SimpleBloomFilter(); 
           for(String value : urls){
        	   value = value.trim();
	           System.out.println("filter.contains("+value+"):"+filter.contains(value));
	           filter.add(value); 
	           System.out.println("filter.add("+value+"):"+filter.contains(value));
	           System.out.println("----------------------------------------------------");
	        }
       }
       
  }  

filter.contains(www.example.com):false
filter.add(www.example.com):true

why first result is false?