public class MainApplication extends Application {
    private static final String ONESIGNAL_APP_ID = "########-####-####-####-############";
  
    @Override
    public void onCreate() {
        super.onCreate();
        
        // Uncomment to enable verbose OneSignal logging to debug issues if needed.
        // OneSignal.setLogLevel(OneSignal.LOG_LEVEL.VERBOSE, OneSignal.LOG_LEVEL.NONE);
        
        OneSignal.initWithContext(this);
        OneSignal.setAppId(ONESIGNAL_APP_ID);
    }
}

OneSignal.setNotificationWillShowInForegroundHandler(new NotificationWillShowInForegroundHandler() {
  @Override
  void notificationWillShowInForeground(OSNotificationReceivedEvent notificationReceivedEvent) {    
    // Get custom additional data you sent with the notification
    JSONObject data = notificationReceivedEvent.notification.getAdditionalData();

    if (/* some condition */ ) {
      // Complete with a notification means it will show
     notificationReceivedEvent.complete(notification);
    }
    else {
      // Complete with null means don't show a notification.
     notificationReceivedEvent.complete(null);
    }
  }
});

import com.onesignal.OSNotification;
import com.onesignal.OSMutableNotification;
import com.onesignal.OSNotificationReceivedEvent;
import com.onesignal.OneSignal.OSRemoteNotificationReceivedHandler;

public class NotificationServiceExtension implements OSRemoteNotificationReceivedHandler {

   @Override
   public void remoteNotificationReceived(Context context, OSNotificationReceivedEvent notificationReceivedEvent) {
      OSNotification notification = notificationReceivedEvent.getNotification();

      // Example of modifying the notification's accent color
      OSMutableNotification mutableNotification = notification.mutableCopy();
      mutableNotification.setExtender(builder -> builder.setColor(context.getResources().getColor(R.color.colorPrimary)));

      // If complete isn't call within a time period of 25 seconds, OneSignal internal logic will show the original notification
      // To omit displaying a notifiation, pass `null` to complete()
      notificationReceivedEvent.complete(mutableNotification);
   }

}

<application ...> 
    <meta-data android:name="com.onesignal.NotificationServiceExtension"
               android:value="com.company.NotificationServiceExtension" />
</application>

<!-- Remove this Service with the following action if you added in from OneSignal 3.x.x -->
<service
        android:name="YOUR_NAMESPACE_AND_CLASS_NAME_HERE"
        android:permission="android.permission.BIND_JOB_SERVICE"
        android:exported="false">
  <intent-filter>
    <action android:name="com.onesignal.NotificationExtender" />
  </intent-filter>
</service>

OneSignal.setInAppMessageClickHandler(
   new OneSignal.OSInAppMessageClickHandler() {
       @Override
       public void inAppMessageClicked(OSInAppMessageAction result) {
         String clickName = result.getClickName();
         String clickUrl = result.getClickUrl();
              
         boolean closesMessage = result.doesCloseMessage();
         boolean firstClick = result.isFirstClick();
});

OneSignal.setNotificationOpenedHandler(
   new OneSignal.OSNotificationOpenedHandler() {
     @Override
     public void notificationOpened(OSNotificationOpenedResult result) {
       String actionId = result.getAction().getActionId();
       String type = result.getAction().getType(); // "ActionTaken" | "Opened"
  
       String title = result.getNotification().getTitle();
     }
});

OneSignal.unsubscribeWhenNotificationsAreDisabled(false);

OSDeviceState device = OneSignal.getDeviceState();

String email = device.getEmailAddress();
String emailId = device.getEmailUserId();
String pushToken = device.getPushToken();
String userId = device.getUserId();

boolean enabled = device.areNotificationsEnabled();
boolean subscribed = device.isSubscribed();
boolean subscribedToOneSignal = device.isUserSubscribed();