How to get android notifications when the app was closed?

This example demonstrate about How to get android notifications when the app was closed
Step 1 − Create a new project in Android Studio, go to File ⇒ New Project and fill all required details to create a new project.
Step 2 − Add the following code to res/layout/activity_main.xml.
<? xml version = "1.0" encoding = "utf-8" ?>
<RelativeLayout xmlns: android = "http://schemas.android.com/apk/res/android"
   xmlns: tools = "http://schemas.android.com/tools"
   android :layout_width = "match_parent"
   android :layout_height = "match_parent"
   android :padding = "16dp"
   tools :context = ".MainActivity" >
   <Button
      android :layout_width = "match_parent"
      android :layout_height = "wrap_content"
      android :layout_centerInParent = "true"
      android :onClick = "closeApp"
      android :text = "close App for notification" />
</RelativeLayout>
Step 3 − Add the following code to src/MainActivity
package app.tutorialspoint.com.notifyme ;
import android.content.Intent ;
import android.os.Bundle ;
import android.support.v7.app.AppCompatActivity ;
import android.view.View ;
public class MainActivity extends AppCompatActivity {
   @Override
   protected void onCreate (Bundle savedInstanceState) {
      super .onCreate(savedInstanceState) ;
      setContentView(R.layout. activity_main ) ;
   }
   @Override
   protected void onStop () {
      super .onStop() ;
      startService( new Intent( this, NotificationService. class )) ;
   }
   public void closeApp (View view) {
      finish() ;
   }
}
Step 4 − Add the following code to src/NotificationService
package app.tutorialspoint.com.notifyme ;
import android.app.NotificationChannel ;
import android.app.NotificationManager ;
import android.app.Service ;
import android.content.Intent ;
import android.os.Handler ;
import android.os.IBinder ;
import android.support.v4.app.NotificationCompat ;
import android.util.Log ;
import java.util.Timer ;
import java.util.TimerTask ;
public class NotificationService extends Service {
   public static final String NOTIFICATION_CHANNEL_ID = "10001" ;
   private final static String default_notification_channel_id = "default" ;
   Timer timer ;
   TimerTask timerTask ;
   String TAG = "Timers" ;
   int Your_X_SECS = 5 ;
   @Override
   public IBinder onBind (Intent arg0) {
      return null;
   }
   @Override
   public int onStartCommand (Intent intent , int flags , int startId) {
      Log. e ( TAG , "onStartCommand" ) ;
      super .onStartCommand(intent , flags , startId) ;
      startTimer() ;
      return START_STICKY ;
   }
   @Override
   public void onCreate () {
      Log. e ( TAG , "onCreate" ) ;
   }
   @Override
   public void onDestroy () {
      Log. e ( TAG , "onDestroy" ) ;
      stopTimerTask() ;
      super .onDestroy() ;
   }
   //we are going to use a handler to be able to run in our TimerTask
   final Handler handler = new Handler() ;
   public void startTimer () {
      timer = new Timer() ;
      initializeTimerTask() ;
      timer .schedule( timerTask , 5000 , Your_X_SECS * 1000 ) ; //
   }
   public void stopTimerTask () {
      if ( timer != null ) {
         timer .cancel() ;
         timer = null;
      }
   }
   public void initializeTimerTask () {
      timerTask = new TimerTask() {
         public void run () {
            handler .post( new Runnable() {
               public void run () {
                  createNotification() ;
               }
            }) ;
         }
      } ;
   }
   private void createNotification () {
      NotificationManager mNotificationManager = (NotificationManager) getSystemService( NOTIFICATION_SERVICE ) ;
      NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(getApplicationContext() , default_notification_channel_id ) ;
      mBuilder.setContentTitle( "My Notification" ) ;
      mBuilder.setContentText( "Notification Listener Service Example" ) ;
      mBuilder.setTicker( "Notification Listener Service Example" ) ;
      mBuilder.setSmallIcon(R.drawable. ic_launcher_foreground ) ;
      mBuilder.setAutoCancel( true ) ;
      if (android.os.Build.VERSION. SDK_INT >= android.os.Build.VERSION_CODES. O ) {
         int importance = NotificationManager. IMPORTANCE_HIGH ;
         NotificationChannel notificationChannel = new NotificationChannel( NOTIFICATION_CHANNEL_ID , "NOTIFICATION_CHANNEL_NAME" , importance) ;
         mBuilder.setChannelId( NOTIFICATION_CHANNEL_ID ) ;
         assert mNotificationManager != null;
         mNotificationManager.createNotificationChannel(notificationChannel) ;
      }
      assert mNotificationManager != null;
      mNotificationManager.notify(( int ) System. currentTimeMillis () , mBuilder.build()) ;
   }
}
Step 5 − Add the following code to AndroidManifest.xml
<? xml version = "1.0" encoding = "utf-8" ?>
<manifest xmlns: android = "http://schemas.android.com/apk/res/android"
   package = "app.tutorialspoint.com.notifyme" >
   <uses-permission android :name = "android.permission.VIBRATE" />
   <application
      android :allowBackup = "true"
      android :icon = "@mipmap/ic_launcher"
      android :label = "@string/app_name"
      android :roundIcon = "@mipmap/ic_launcher_round"
      android :supportsRtl = "true"
      android :theme = "@style/AppTheme" >
      <activity android :name = ".MainActivity" >
         <intent-filter>
            <action android :name = "android.intent.action.MAIN" />
            <category android :name = "android.intent.category.LAUNCHER" />
         </intent-filter>
      </activity>
      <service
         android :name = ".NotificationService"
         android :label = "@string/app_name" >
         <intent-filter>
            <action
               android :name = "app.tutorialspoint.com.notifyme.NotificationService" />
            <category android :name = "android.intent.category.DEFAULT" />
         </intent-filter>
      </service>
   </application>
</manifest>
Let's try to run your application. I assume you have connected your actual Android Mobile device with your computer. To run the app from android studio, open one of your project's activity files and click Run  icon from the toolbar. Select your mobile device as an option and then check your mobile device which will display your default screen −
Click  here  to download the project code