public CachingWrapperFilter(Filter filter) 
{
    this.filter = filter;
}

Parameters:

filter - Filter to cache results of

 public BitSet bits(IndexReader reader) throws IOException 
{
    if (cache == null) {
      cache = new WeakHashMap();
    }
    Object cached = null;
    synchronized (cache) {  // check cache
      cached = cache.get(reader);
    }
    if (cached != null) {
      if (cached instanceof BitSet) {
        return (BitSet) cached;
      } else if (cached instanceof DocIdBitSet)
        return ((DocIdBitSet) cached).getBitSet();
      // It would be nice to handle the DocIdSet case, but that's not really possible
    }
    final BitSet bits = filter.bits(reader);
    synchronized (cache) {  // update cache
      cache.put(reader, bits);
    }
    return bits;
}

 public boolean equals(Object o) 
{
    if (!(o instanceof CachingWrapperFilter)) return false;
    return this.filter.equals(((CachingWrapperFilter)o).filter);
}

 public DocIdSet getDocIdSet(IndexReader reader) throws IOException 
{
    if (cache == null) {
      cache = new WeakHashMap();
    }
    Object cached = null;
    synchronized (cache) {  // check cache
      cached = cache.get(reader);
    }
    if (cached != null) {
      if (cached instanceof DocIdSet)
        return (DocIdSet) cached;
      else
        return new DocIdBitSet((BitSet) cached);
    }
    final DocIdSet docIdSet = filter.getDocIdSet(reader);
    synchronized (cache) {  // update cache
      cache.put(reader, docIdSet);
    }
    return docIdSet;
}

 public int hashCode() 
{
    return filter.hashCode() ^ 0x1117BF25;
}

 public String toString() 
{
    return "CachingWrapperFilter("+filter+")";
}