code: @Override
    public Object get(final ByteBuffer buf, final int position)
    {
      final Lock lock = stripedLock.getAt(lockIndex(position)).readLock();
      lock.lock();
      try {
        return sketchCache.get(buf).get(position).copy();
      }
      finally {
        lock.unlock();
      }
    }
docs: This method uses locks because it can be used during indexing,
  and Druid can call aggregate() and get() concurrently
  See https://github.com/druid-io/druid/pull/3956