How to show interstitial Ad before launching new activity after click a button?

The answer suggested by @user8240773 is correct but there is a more efficient way of handling your problem. Here is my code:

// Has the interstitial loaded successfully?
// If it has loaded, perform these actions
if(mInterstitialAd.isLoaded()) {
    // Step 1: Display the interstitial
    mInterstitialAd.show();
    // Step 2: Attach an AdListener
    mInterstitialAd.setAdListener(new AdListener() {
        @Override
        public void onAdClosed() {
            // Step 2.1: Load another ad
            AdRequest adRequest = new AdRequest.Builder()
                                    .addTestDevice(AdRequest.DEVICE_EMULATOR_ID)
                                    .build();
            mInterstitialAd.loadAd(adRequest);

            // Step 2.2: Start the new activity
            startActivity(new Intent(Activity1.this, Activity2.class));
        }
    });
}
// If it has not loaded due to any reason simply load the next activity
else {
    startActivity(new Intent(Activity1.this, Activity2.class));
}

That way you will also do not have to worry about the ad not loading due to no internet connection or anything else. Everything would be handled by this code in the way you described your problem.

Maybe something Like this? Use the onAdClosed function to start activity

interstitial.setAdListener(new AdListener() {
 public void onAdLoaded() {
  if (interstitial.isLoaded()) {
      interstitial.show();
   }
 }
     @Override
        public void onAdClosed() {
             startActivity(new Intent(this, secondactivity.class ));    
            // Code to be executed when when the interstitial ad is closed.
            Log.i("Ads", "onAdClosed");
        } 
 });

Read more about this here: https://developers.google.com/admob/android/interstitial