布隆过滤器(Bloom Filter)的Java实现方法

import java.util.BitSet; 
 
/** 
 * 
 * @author xkey 
 */ 
public class BloomFilter { 
 
  private static final int DEFAULT_SIZE = 2 << 24;//布隆过滤器的比特长度 
  private static final int[] seeds = {3,5,7, 11, 13, 31, 37, 61};//这里要选取质数，能很好的降低错误率 
  private static BitSet bits = new BitSet(DEFAULT_SIZE); 
  private static SimpleHash[] func = new SimpleHash[seeds.length]; 
 
  public static void addValue(String value) 
  { 
    for(SimpleHash f : func)//将字符串value哈希为8个或多个整数，然后在这些整数的bit上变为1 
      bits.set(f.hash(value),true); 
  } 
   
  public static void add(String value) 
  { 
    if(value != null) addValue(value); 
  } 
   
  public static boolean contains(String value) 
  { 
    if(value == null) return false; 
    boolean ret = true; 
    for(SimpleHash f : func)//这里其实没必要全部跑完，只要一次ret==false那么就不包含这个字符串 
      ret = ret && bits.get(f.hash(value)); 
    return ret; 
  } 
   
  public static void main(String[] args) { 
    String value = "www.jb51.net"; 
    for (int i = 0; i < seeds.length; i++) { 
      func[i] = new SimpleHash(DEFAULT_SIZE, seeds[i]); 
    } 
    add(value); 
    System.out.println(contains(value)); 
  } 
} 
 
class SimpleHash {//这玩意相当于C++中的结构体 
 
  private int cap; 
  private int seed; 
 
  public SimpleHash(int cap, int seed) { 
    this.cap = cap; 
    this.seed = seed; 
  } 
 
  public int hash(String value) {//字符串哈希，选取好的哈希函数很重要 
    int result = 0; 
    int len = value.length(); 
    for (int i = 0; i < len; i++) { 
      result = seed * result + value.charAt(i); 
    } 
    return (cap - 1) & result; 
  } 
}