gistfile1.txt

class Fragment {
    private void displayAdMob() {
        initializeMobileAdsAsyncIfPossible(() -> {
            ...

            adView.loadAd(adRequest);
        });
    }
}

public class Utils {
    // volatile keyword is required : https://en.wikipedia.org/wiki/Double-checked_locking
    // http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
    private static volatile boolean initializeMobileAds = false;

    private static final Object monitor = new Object();

    public static void initializeMobileAdsAsyncIfPossible(Runnable runnable) {
        if (!initializeMobileAds) {
            // getCommonExecutor is a single thread executor.
            getCommonExecutor().execute(() -> {
                synchronized (monitor) {
                    if (!initializeMobileAds) {
                        // https://stackoverflow.com/questions/58084706/should-we-first-call-mobileads-setrequestconfiguration-or-mobileads-initialize
                        RequestConfiguration conf = new RequestConfiguration.Builder()
                                .setMaxAdContentRating(
                                        MAX_AD_CONTENT_RATING_T)
                                .build();

                        MobileAds.setRequestConfiguration(conf);

                        MobileAds.initialize(WeNoteApplication.instance());

                        initializeMobileAds = true;
                    }
                }

                if (runnable != null) {
                    Assert(initializeMobileAds);
                    runInUiThread(runnable);
                }
            }); // getCommonExecutor().execute
        } else {
            if (runnable != null) {
                Assert(initializeMobileAds);
                runnable.run();
            }
        }
    }
}