How to call multiple interstitial ad?

Question

I added interstitial ad (+banner) to my app that contains one activity, no problem with banner but the interstitial ad shows only at app launch one time, my app don't contain any buttons or actions just a pdfview, what I want to do is showing the ad every 5mins when the user reading the pdf. I searched the similar questions but couldn't get it. here my code to load and show the ad :

    AdView mAdView = (AdView) findViewById(R.id.adView);
    AdRequest adRequest = new AdRequest.Builder().build();
    mAdView.loadAd(adRequest);

    // Prepare the Interstitial Ad
    interstitial = new InterstitialAd(PDFViewActivity.this);
    // Insert the Ad Unit ID
    interstitial.setAdUnitId("ca-app-pub-3940256099942544/1033173712");
    interstitial.loadAd(adRequest);
    // Prepare an Interstitial Ad Listener
    interstitial.setAdListener(new AdListener() {
        public void onAdLoaded() {
            // Call displayInterstitial() function
            displayInterstitial();
        }
    });
}

public void displayInterstitial() {
// If Ads are loaded, show Interstitial else show nothing.
    if (interstitial.isLoaded()) {
        interstitial.show();
    }
}

Answer 1

I did it this way :

AdView mAdView = (AdView) findViewById(R.id.adView);
    AdRequest adRequest = new AdRequest.Builder().build();
    mAdView.loadAd(adRequest);

    // Prepare the Interstitial Ad
    interstitial = new InterstitialAd(this);
    interstitial.setAdUnitId("ca-app-pub-3940256099942544/1033173712");

    interstitial.loadAd(adRequest);

    interstitial.setAdListener(new AdListener() {
        public void onAdLoaded() {
            displayInterstitial();
        }
        public void onAdClosed() {
            requestNewInterstitial();
        }

    });
}

public void displayInterstitial() {
// If Ads are loaded, show Interstitial else show nothing.
    if (interstitial.isLoaded()) {
        interstitial.show();
    }
}

public void requestNewInterstitial() {
    mHandler.postDelayed(new Runnable() {
        @Override
        public void run() {
            AdRequest adRequest = new AdRequest.Builder()
                    .build();
            interstitial.loadAd(adRequest);

            if (mHandler != null) {
                mHandler.postDelayed(this, 100000); //time (ms)
            }
        }
    }, 100000); //time (ms)
}