@Override
 public BloomFilter getFilter(int size, double falsePositiveRate, long tweak) {
   lock.lock();
   try {
     checkArgument(size >= numBloomFilterEntries());
     maybeLookAhead();
     return basicKeyChain.getFilter(size, falsePositiveRate, tweak);
   } finally {
     lock.unlock();
   }
 }