code: protected synchronized void fetchAd(final NativeExpressAdView adView) {
          if(mFetchFailCount > MAX_FETCH_ATTEMPT)
              return;
  
          Context context = mContext.get();
  
          if (context != null) {
              Log.i(TAG, "Fetching Ad now");
              new Handler(context.getMainLooper()).post(new Runnable() {
                  @Override
                  public void run() {
                      adView.loadAd(getAdRequest()); //Fetching the ads item
                  }
              });
          } else {
              mFetchFailCount++;
              Log.i(TAG, "Context is null, not fetching Ad");
          }
      }
docs: Fetches a new native ad.