code: public void addResponse(CheckRequest req, CheckResponse resp) {
      if (cache == null) {
        return;
      }
      String signature = sign(req).toString();
      long now = ticker.read();
      int quotaScale = 0; // WIP
      synchronized (cache) {
        CachedItem item = cache.getIfPresent(signature);
        if (item == null) {
          cache.put(signature, new CachedItem(resp, req, kinds, now, quotaScale));
        } else {
          item.lastCheckTimestamp = now;
          item.response = resp;
          item.quotaScale = quotaScale;
          item.isFlushing = false;
          cache.put(signature, item);
        }
      }
    }
docs: Adds the response from sending {@code req} to this instances cache.
  
  @param req a {@link CheckRequest}
  @param resp the response from sending {@link CheckResponse}