code: public synchronized NativeExpressAdView getAdForIndex(int adPos) {
          if(adPos >= 0 && mPrefetchedAds.size() > adPos)
              return mPrefetchedAds.get(adPos);
          return null;
      }
docs: Gets native ad at a particular index in the fetched ads list.
  
  @param adPos the index of ad in the fetched ads list
  @return the native ad in the list
  @see #getFetchedAdsCount()