Why do I get a Null Pointer Exception when I try to run rewarded ads?

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SUMMARY

The discussion centers on a Null Pointer Exception encountered when attempting to display rewarded interstitial ads using the Google Mobile Ads SDK in an Android application. The issue arises when the ad is called upon a button click, specifically due to the rewardedInterstitialAd variable being null at the time of invocation. The user is advised to ensure that the ad is successfully loaded before attempting to show it, as indicated by the stack trace pointing to the onClick method in MainActivity.

PREREQUISITES
  • Familiarity with Android development and Java programming
  • Understanding of the Google Mobile Ads SDK, specifically version 20.4.0 or later
  • Knowledge of handling asynchronous tasks in Android
  • Experience with debugging and interpreting stack traces in Android applications
NEXT STEPS
  • Review the Google Mobile Ads SDK documentation for RewardedInterstitialAd implementation
  • Learn about asynchronous programming in Android to manage ad loading
  • Investigate error handling techniques for ad loading failures
  • Explore best practices for managing ad lifecycle events in Android applications
USEFUL FOR

Android developers, mobile app marketers, and anyone integrating rewarded ads into their applications will benefit from this discussion.

Darkmisc
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TL;DR
I copied code from the Admob developer page for a rewarded ad and it doesn't work. I get a Null Pointer Exception.
Hi everyone

I've cut and pasted code from here to test rewarded interstitial ads. https://developers.google.com/admob/android/rewarded-interstitial
The only change I've made is to call the ad upon a button click.

When I click the button to call the ad, the program crashes and makes reference to a Null Pointer Exception caused by the following lines:
[CODE lang="java" title="line with error"]rewardedInterstitialAd.show(/* Activity */ MainActivity.this,/*
OnUserEarnedRewardListener */ MainActivity.this);
[/CODE]

Does anyone know why this is happening?

Full code and stacktrace below.
Thanks
[CODE lang="java" title="MainActivity" highlight="62-63"]import static androidx.constraintlayout.helper.widget.MotionEffect.TAG;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.google.android.gms.ads.AdError;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.FullScreenContentCallback;
import com.google.android.gms.ads.LoadAdError;
import com.google.android.gms.ads.MobileAds;
import com.google.android.gms.ads.OnUserEarnedRewardListener;
import com.google.android.gms.ads.initialization.InitializationStatus;
import com.google.android.gms.ads.initialization.OnInitializationCompleteListener;
import com.google.android.gms.ads.rewarded.RewardItem;
import com.google.android.gms.ads.rewardedinterstitial.RewardedInterstitialAd;
import com.google.android.gms.ads.rewardedinterstitial.RewardedInterstitialAdLoadCallback;

public class MainActivity extends AppCompatActivity implements OnUserEarnedRewardListener {
private RewardedInterstitialAd rewardedInterstitialAd;
private String TAG = "MainActivity";

TextView textView;
Button button;
int value;

@Override
public void onUserEarnedReward(@NonNull RewardItem rewardItem) {
Log.i(TAG, "User earned reward.");
value++;
textView.setText(String.valueOf(value));
}

@SuppressLint("MissingInflatedId")
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

int value =0;
textView = findViewById(R.id.textView);
textView.setText(String.valueOf(value));
button = findViewById(R.id.button); MobileAds.initialize(this, new OnInitializationCompleteListener() {
@Override
public void onInitializationComplete(InitializationStatus initializationStatus) {
}
});

loadAd();
}

public void onClick (View view){
**rewardedInterstitialAd.show(/* Activity */ MainActivity.this,/*
OnUserEarnedRewardListener */ MainActivity.this);**
}

public void loadAd(){
RewardedInterstitialAd.load(MainActivity.this, "ca-app-pub-3940256099942544/5354046379",
new AdRequest.Builder().build(), new RewardedInterstitialAdLoadCallback() {
@Override
public void onAdLoaded(RewardedInterstitialAd ad) {
rewardedInterstitialAd = ad;
rewardedInterstitialAd.setFullScreenContentCallback(new FullScreenContentCallback() {
@Override
public void onAdClicked() {
// Called when a click is recorded for an ad.
Log.d(TAG, "Ad was clicked.");
}

@Override
public void onAdDismissedFullScreenContent() {
// Called when ad is dismissed.
// Set the ad reference to null so you don't show the ad a second time.
Log.d(TAG, "Ad dismissed fullscreen content.");
rewardedInterstitialAd = null;
}

@Override
public void onAdFailedToShowFullScreenContent(AdError adError) {
// Called when ad fails to show.
Log.e(TAG, "Ad failed to show fullscreen content.");
rewardedInterstitialAd = null;
}

@Override
public void onAdImpression() {
// Called when an impression is recorded for an ad.
Log.d(TAG, "Ad recorded an impression.");
}

@Override
public void onAdShowedFullScreenContent() {
// Called when ad is shown.
Log.d(TAG, "Ad showed fullscreen content.");
}
});
}
@Override
public void onAdFailedToLoad(LoadAdError loadAdError) {
Log.d(TAG, loadAdError.toString());
rewardedInterstitialAd = null;
}
});
}
}
[/CODE]

Here is the stacktrace.[CODE lang="java" title="stacktrace" highlight="34-35"]E FATAL EXCEPTION: main
Process: com.xxmassdeveloper.rewardedads2, PID: 20217
java.lang.IllegalStateException: Could not execute method for android:onClick
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:473)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1202)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:468)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1202)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
**Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.ads.rewardedinterstitial.RewardedInterstitialAd.show(android.app.Activity, com.google.android.gms.ads.OnUserEarnedRewardListener)' on a null object reference
at com.xxmassdeveloper.rewardedads2.MainActivity.onClick(MainActivity.java:64)**
at java.lang.reflect.Method.invoke(Native Method)
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:468)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1202)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
2023-02-01 10:30:03.908 20217-20217 Process com.xxmassdeveloper.rewardedads2 I Sending signal. PID: 20217 SIG: 9
---------------------------- PROCESS ENDED (20217) for package com.xxmassdeveloper.rewardedads2 ----------------------------
2023-02-01 10:30:04.207 3675-5653 WindowManager pid-3675 E win=Window{81ddba7 u0 com.xxmassdeveloper.rewardedads2/com.xxmassdeveloper.rewardedads2.MainActivity EXITING} destroySurfaces: appStopped=false win.mWindowRemovalAllowed=true win.mRemoveOnExit=true win.mViewVisibility=0 caller=com.android.server.wm.AppWindowToken.destroySurfaces:1200 com.android.server.wm.AppWindowToken.destroySurfaces:1181 com.android.server.wm.WindowState.onExitAnimationDone:5030 com.android.server.wm.-$$Lambda$01bPtngJg5AqEoOWfW3rWfV7MH4.accept:2 java.util.ArrayList.forEach:1262 com.android.server.wm.AppWindowToken.onAnimationFinished:3549 com.android.server.wm.AppWindowToken.commitVisibility:860
[/CODE]
 
Last edited:
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Darkmisc said:
When I click the button to call the ad, the program crashes and makes reference to a Null Pointer Exception caused by the following lines:
[CODE lang="java" title="line with error"]rewardedInterstitialAd.show(/* Activity */ MainActivity.this,/*
OnUserEarnedRewardListener */ MainActivity.this);
[/CODE]

Does anyone know why this is happening?
You were supposed to pass an instance of the Activity class and OnUserEarnedRewardListener interface. Why are you passing MainActivity.this? Just pass this, you are already inside the MainActivity class, which is also an instance of the interface as you have implemented it.

The correct method call would be:
[CODE lang="java" title="Corrected code"]rewardedInterstitialAd.show(this, this);
[/CODE]

Classname.this is a different thing. See https://stackoverflow.com/q/45637427/8387076

Also, your deleted question on Stack Overflow is still popping up in search queries o0)
 
Last edited:
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Thanks. It worked, but I can only click on the button once and see an ad. If I try to watch more than one ad in a row, the program crashes (stacktrace below).

If I wait a bit and then try, I can see the ad again, but I can't click the button again without a crash.

Does this have anything to do with load times? I don't understand why the button shouldn't work all the time.

[CODE lang="java" title="stacktrace" highlight="34-35"]FATAL EXCEPTION: main
Process: com.xxmassdeveloper.rewardedads2, PID: 6960
java.lang.IllegalStateException: Could not execute method for android:onClick
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:473)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1202)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Method.invoke(Native Method)
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:468)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1202)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.ads.rewardedinterstitial.RewardedInterstitialAd.show(android.app.Activity, com.google.android.gms.ads.OnUserEarnedRewardListener)' on a null object reference
at com.xxmassdeveloper.rewardedads2.MainActivity.onClick(MainActivity.java:65)
at java.lang.reflect.Method.invoke(Native Method)
at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:468)
at android.view.View.performClick(View.java:7870)
at android.widget.TextView.performClick(TextView.java:14970)
at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1202)
at android.view.View.performClickInternal(View.java:7839)
at android.view.View.access$3600(View.java:886)
at android.view.View$PerformClick.run(View.java:29363)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
2023-02-02 11:45:28.182 6960-6960 Process com.xxmassdeveloper.rewardedads2 I Sending signal. PID: 6960 SIG: 9
---------------------------- PROCESS ENDED (6960) for package com.xxmassdeveloper.rewardedads2 ----------------------------
2023-02-02 11:45:28.304 3675-5805 WindowManager pid-3675 E win=Window{e12d993 u0 com.xxmassdeveloper.rewardedads2/com.xxmassdeveloper.rewardedads2.

Activity EXITING} destroySurfaces: appStopped=false win.mWindowRemovalAllowed=true win.mRemoveOnExit=true win.mViewVisibility=0 caller=com.android.server.wm.AppWindowToken.destroySurfaces:1200 com.android.server.wm.AppWindowToken.destroySurfaces:1181 com.android.server.wm.WindowState.onExitAnimationDone:5030 com.android.server.wm.-$$Lambda$01bPtngJg5AqEoOWfW3rWfV7MH4.accept:2 java.util.ArrayList.forEach:1262 com.android.server.wm.AppWindowToken.onAnimationFinished:3549 com.android.server.wm.AppWindowToken.commitVisibility:860
[/CODE]
 
One error I see is that in the full code you posted in the OP, the loadAd() method call at line 58 should be inside the nested class, i.e. between lines 54 and 55.

Note that in the loadAd() method, you are inside a nested class, you have to use MainActivity.this instead of just this, which is what you've already done.

If this change doesn't solve the issue, then I think you have to initialize the ad again once it has been showed (call the nested class callback from line 52 again). Separate this portion out in a method to call easily.
 
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