code: public synchronized NativeAd getAdForIndex(final int index) {
          NativeAd adNative = null;
          if(index >= 0)
              adNative = (NativeAd) adMapAtIndex.get(index);
  
          if (adNative == null && mPrefetchedAdList.size() > 0) {
              adNative = mPrefetchedAdList.remove(0);
  
              if (adNative != null) {
                  adMapAtIndex.put(index, adNative);
              }
          }
  
          ensurePrefetchAmount(); // Make sure we have enough pre-fetched ads
          return adNative;
      }
docs: Gets native ad at a particular index in the fetched ads list.
  
  @param index the index of ad in the fetched ads list
  @return the native ad in the list
  @see #getFetchedAdsCount()