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Why is my code not displaying notifications

So I am trying to create a notification at the top of the phone, but when I run my code it does nothing. Not even a single error. ??? What am I doing wrong here?
public void createNotification(Context ctx) {
SharedPreferences settings = ctx.getApplicationContext().getSharedPreferences(ContactsContract.CommonDataKinds.StructuredName.DISPLAY_NAME, 0);
String contactName = settings.getString("curName", String.valueOf(0));
String contactEmail = settings.getString("contactEmail", String.valueOf(0));
String contactNumber = settings.getString("curPhone", String.valueOf(0));
String dueAmount = String.valueOf(settings.getInt("amountDue", 0));
NotificationCompat.Builder builder = new NotificationCompat.Builder(ctx);
builder.setSmallIcon(R.drawable.myanlogo);
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("https://www.journaldev.com/"));
PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE);
builder.setContentIntent(pendingIntent);
builder.setLargeIcon(BitmapFactory.decodeResource(ctx.getResources(), R.mipmap.ic_launcher));
builder.setContentTitle("LETTING YOU KNOW");
builder.setContentText("Your notification content here.");
NotificationManager notificationManager = (NotificationManager) ctx.getSystemService(NOTIFICATION_SERVICE);
// Will display the notification in the notification bar
notificationManager.notify(1, builder.build());
}
I have it inside a class like this:
class ContactRVAdapter extends RecyclerView.Adapter<ContactRVAdapter.ViewHolder>
And I am calling the function here:
holder.textDueTomorrow.setVisibility(View.GONE);
if (Integer.valueOf(getFirstDateNumber) == Integer.valueOf(getSecondDateNumber) - 1) {
holder.textDueTomorrow.setVisibility(View.VISIBLE);
createNotification(context);
}
Some advice would be nice because I have been working at this for a while now.

It looks like you're missing Notification Channels for Build.VERSION_CODE.O and greater
https://developer.android.com/develop/ui/views/notifications/channels
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
int importance = NotificationManager.IMPORTANCE_LOW;
NotificationChannel notificationChannel = new NotificationChannel(channelId, channelName, importance);
notificationManager.createNotificationChannel(notificationChannel);
// Make notification show big text.
NotificationCompat.BigTextStyle bigTextStyle = new NotificationCompat.BigTextStyle();
bigTextStyle.setBigContentTitle(title);
bigTextStyle.bigText(notificationText);
// Set big text style.
builder.setStyle(bigTextStyle);
} else {
builder.setContentTitle(title);
builder.setContentText(notificationText);
}

This Chanel link below hellped me out along with the example provided below.
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
int importance = NotificationManager.IMPORTANCE_LOW;
NotificationChannel notificationChannel = new NotificationChannel(channelId, channelName, importance);
notificationManager.createNotificationChannel(notificationChannel);
// Make notification show big text.
NotificationCompat.BigTextStyle bigTextStyle = new NotificationCompat.BigTextStyle();
bigTextStyle.setBigContentTitle(title);
bigTextStyle.bigText(notificationText);
// Set big text style.
builder.setStyle(bigTextStyle);
} else {
builder.setContentTitle(title);
builder.setContentText(notificationText);
}
This link makess it more clear. notification channel is not sending notifications Its basically 100% fine he just forgot to call function createNotificationChannel() before creating the notification.

For Android 8 and higher versions, you need to create a notification channel, write before your notification builder and pass same channel id to NotificationCompat.Builder
Copy/Paste this:
public void createNotification(Context ctx) {
createNotificationChannel();
SharedPreferences settings = ctx.getApplicationContext().getSharedPreferences(ContactsContract.CommonDataKinds.StructuredName.DISPLAY_NAME, 0);
String contactName = settings.getString("curName", String.valueOf(0));
String contactEmail = settings.getString("contactEmail", String.valueOf(0));
String contactNumber = settings.getString("curPhone", String.valueOf(0));
String dueAmount = String.valueOf(settings.getInt("amountDue", 0));
NotificationCompat.Builder builder = new NotificationCompat.Builder(ctx, "CHANNEL_ID");
builder.setSmallIcon(R.drawable.myanlogo);
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("https://www.journaldev.com/"));
PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE);
builder.setContentIntent(pendingIntent);
builder.setLargeIcon(BitmapFactory.decodeResource(ctx.getResources(), R.mipmap.ic_launcher));
builder.setContentTitle("LETTING YOU KNOW");
builder.setContentText("Your notification content here.");
NotificationManager notificationManager = (NotificationManager) ctx.getSystemService(NOTIFICATION_SERVICE);
notificationManager.notify(1, builder.build());
}
I have update this line NotificationCompat.Builder builder = new NotificationCompat.Builder(ctx, "CHANNEL_ID");
JAVA
private void createNotificationChannel() {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
NotificationChannel notificationChannel = new NotificationChannel("CHANNEL_ID", "Updates", NotificationManager.IMPORTANCE_DEFAULT);
NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
notificationManager.createNotificationChannel(notificationChannel);
}
}
KOTLIN
private fun createNotificationChannel() {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
val importance = NotificationManager.IMPORTANCE_DEFAULT
val notificationChannel = NotificationChannel("CHANNEL_ID", "Updates", NotificationManager.IMPORTANCE_DEFAULT)
val notificationManager = getSystemService(NOTIFICATION_SERVICE) as NotificationManager
notificationManager.createNotificationChannel(notificationChannel)
}
}

error: cannot find symbol method getSystemService(Class<NotificationManager>)

I get this code from google android developer website, but I'm not sure why I'm getting this error. First I did this in my MainActivity and it works fine, and then I created another at AlarmReceiver.java but error occurred.
error: cannot find symbol method getSystemService(Class)
public class AlarmReceiver extends BroadcastReceiver{
private final String CHANNEL_ID = "Notification";
#Override
public void onReceive(Context context, Intent intent) {
createNotificationChannel();
Intent notificationIntent = new Intent(context, MapsActivity.class);
TaskStackBuilder stackBuilder = TaskStackBuilder.create(context);
stackBuilder.addParentStack(MapsActivity.class);
stackBuilder.addNextIntent(notificationIntent);
PendingIntent pendingIntent = stackBuilder.getPendingIntent(0, PendingIntent.FLAG_UPDATE_CURRENT);
NotificationCompat.Builder builder = new NotificationCompat.Builder(context);
Notification notification = builder.setContentTitle("Demo App Notification")
.setContentText("New Notification From Demo App..")
.setTicker("New Message Alert!")
.setSmallIcon(R.mipmap.ic_launcher)
.setContentIntent(pendingIntent).build();
NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
notificationManager.notify(0, notification);
}
private void createNotificationChannel(){
// Create the NotificationChannel, but only on API 26+ because
// the NotificationChannel class is new and not in the support library
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
CharSequence name = "NOTIFICATION";
String description = "DESCRIPTION";
int importance = NotificationManager.IMPORTANCE_DEFAULT;
NotificationChannel channel = new NotificationChannel(CHANNEL_ID, name, importance);
channel.setDescription(description);
// Register the channel with the system; you can't change the importance
// or other notification behaviors after this
NotificationManager notificationManager = getSystemService(NotificationManager.class);
notificationManager.createNotificationChannel(channel);
}
}
}

Attempt to invoke virtual method 'android.app.Notification androidx.core.app.NotificationCompat$Builder.build()' on a null object reference

I am using Firebase Push notification to send notification to app. Notification is working in Latest android version's but when i tried to test in Lolipop version .When i test the notification app crashed . I don't know where is the problem. Please help me to solve the issue. Thanks in advance .
public class MyFirebaseMessagingService extends FirebaseMessagingService {
#Override
public void onMessageReceived(#NonNull RemoteMessage remoteMessage) {
String title = Objects.requireNonNull(remoteMessage.getNotification()).getTitle();
String message = remoteMessage.getNotification().getBody();
//33
sendNotification(title,message);
}
private void sendNotification(String messageTitle,String messageBody) {
NotificationManager notificationManager =
(NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
Intent intent = new Intent(this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_ONE_SHOT);
Uri soundUri= RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
#SuppressLint("WrongConstant")
NotificationChannel notificationChannel=new NotificationChannel("my_notification","n_channel",NotificationManager.IMPORTANCE_MAX);
notificationChannel.setDescription("description");
notificationChannel.setName("Channel Name");
assert notificationManager != null;
notificationManager.createNotificationChannel(notificationChannel);
}
NotificationCompat.Builder notificationBuilder = null;
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
notificationBuilder = new NotificationCompat.Builder(this)
.setSmallIcon(R.drawable.sicont)
.setLargeIcon(BitmapFactory.decodeResource(getResources(), R.drawable.sicont))
.setContentTitle(messageTitle)
.setContentText(messageBody)
.setAutoCancel(true)
.setSound(soundUri)
.setContentIntent(pendingIntent)
.setDefaults(Notification.DEFAULT_ALL)
.setPriority(NotificationManager.IMPORTANCE_MAX)
.setOnlyAlertOnce(true)
.setChannelId("my_notification")
.setColor(Color.parseColor("#3F5996"));
}
//.setProgress(100,50,false);
assert notificationManager != null;
int m = (int) ((new Date().getTime() / 1000L) % Integer.MAX_VALUE);
assert notificationBuilder != null;
//76 number line
notificationManager.notify(m, notificationBuilder.build());
}
}
Logcat:
java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.Notification androidx.core.app.NotificationCompat$Builder.build()' on a null object reference
at p.MyFirebaseMessagingService.sendNotification(MyFirebaseMessagingService.java:76)
at p.MyFirebaseMessagingService.onMessageReceived(MyFirebaseMessagingService.java:33)
at com.google.firebase.messaging.FirebaseMessagingService.zzc(com.google.firebase:firebase-messaging##20.0.0:68)
at com.google.firebase.messaging.zze.run(com.google.firebase:firebase-messaging##20.0.0:2)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at com.google.android.gms.common.util.concurrent.zza.run(Unknown Source)

Try this method:
private void sendNotification(String messageTitle,String messageBody) {
NotificationManager notificationManager =
(NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
Intent intent = new Intent(this, MainActivity.class);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_ONE_SHOT);
Uri soundUri= RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
#SuppressLint("WrongConstant")
NotificationChannel notificationChannel=new NotificationChannel("my_notification","n_channel",NotificationManager.IMPORTANCE_MAX);
notificationChannel.setDescription("description");
notificationChannel.setName("Channel Name");
assert notificationManager != null;
notificationManager.createNotificationChannel(notificationChannel);
}
NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this)
.setSmallIcon(R.drawable.sicont)
.setLargeIcon(BitmapFactory.decodeResource(getResources(), R.drawable.sicont))
.setContentTitle(messageTitle)
.setContentText(messageBody)
.setAutoCancel(true)
.setSound(soundUri)
.setContentIntent(pendingIntent)
.setDefaults(Notification.DEFAULT_ALL)
.setOnlyAlertOnce(true)
.setChannelId("my_notification")
.setColor(Color.parseColor("#3F5996"));
//.setProgress(100,50,false);
assert notificationManager != null;
int m = (int) ((new Date().getTime() / 1000L) % Integer.MAX_VALUE);
notificationManager.notify(m, notificationBuilder.build());
}

Failed to post notification on channel “null” Target Api is 27

After upgrading my app target api to 27 notification is no more working and i get a toast message on android studio emulator:
Failed to post notification on channel “null”
Here is my notification code:
private void sendNotification(Context context, String sysModel) {
Intent intentAction = new Intent(context, MainActivity.class);
intentAction.putExtra("sysModel", sysModel);
PendingIntent pendingIntent = PendingIntent.getActivity(context, 1, intentAction, PendingIntent.FLAG_ONE_SHOT);
notificationBuilder = new NotificationCompat.Builder(context)
.setSmallIcon(R.drawable.changer_ico)
.setContentTitle(context.getString(R.string.service_ready))
.setContentIntent(pendingIntent)
.addAction(R.drawable.ic_key_black_24dp, context.getString(R.string.turn_on), pendingIntent)
.setAutoCancel(true);
Notification notification = notificationBuilder.build();
notificationManager.notify(1903, notification);
}

Can you test the code below?
Note that new NotificationCompat.Builder(context, NOTIFICATION_CHANNEL); instead of new NotificationCompat.Builder(context);
private final static String NOTIFICATION_CHANNEL = "channel_name";
private final static String CHANNEL_DESCRIPTION = "channel_description";
public final static int NOTIFICATION_ID = 1903;
private void sendNotification(Context context, String sysModel) {
// Intent
Intent intentAction = new Intent(context, MainActivity.class);
intentAction.putExtra("sysModel", sysModel);
// Pending Intent
PendingIntent pendingIntent = PendingIntent.getActivity(context, 1, intentAction, PendingIntent.FLAG_ONE_SHOT);
// Notification Manager
NotificationManager notificationManager = (NotificationManager) context.getSystemService(Service.NOTIFICATION_SERVICE);
// Create the notification channel if android >= 8
if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {
NotificationChannel channel = new NotificationChannel(NOTIFICATION_CHANNEL, CHANNEL_DESCRIPTION, NotificationManager.IMPORTANCE_DEFAULT);
notificationManager.createNotificationChannel(channel);
}
// Notification builder.. Note that I added the notification channel on this constructor
NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(context, NOTIFICATION_CHANNEL);
notificationBuilder.setSmallIcon(R.drawable.changer_ico)
notificationBuilder.setContentTitle(context.getString(R.string.service_ready))
notificationBuilder.setContentIntent(pendingIntent)
notificationBuilder.addAction(R.drawable.ic_key_black_24dp, context.getString(R.string.turn_on), pendingIntent)
notificationBuilder.setAutoCancel(true);
// Notify
notificationManager.notify(NOTIFICATION_ID, notificationBuilder.build());
}

In android O you should dispatch notice like follow:
NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(context)
.setSmallIcon(R.drawable.changer_ico)
.setContentTitle(context.getString(R.string.service_ready))
.setContentIntent(pendingIntent)
.addAction(R.drawable.ic_key_black_24dp, context.getString(R.string.turn_on), pendingIntent)
.setAutoCancel(true);
Notification notification = notificationBuilder.build();
NotificationChannel channel = new NotificationChannel("1903",
context.getString(R.string.user_notify), NotificationManager.IMPORTANCE_DEFAULT);
notificationManager.createNotificationChannel(channel);
notificationManager.notify(1903, notification);

How to open a Fragment after notification click? Firebase

I created a a function "send_notification" with firebase function and node.js and now at the end it's difficult to figure out how to open a Fragment instead of an Activity in my FirebaseMessagingService class.
this is my code
public class FirebaseMessagingService extends com.google.firebase.messaging.FirebaseMessagingService {
#Override
public void onMessageReceived(RemoteMessage remoteMessage) {
super.onMessageReceived(remoteMessage);
String notification_title = remoteMessage.getNotification().getTitle();
String notification_message = remoteMessage.getNotification().getBody();
NotificationCompat.Builder mBuilder =
(NotificationCompat.Builder) new NotificationCompat.Builder(this)
.setSmallIcon(R.mipmap.logo_login)
.setContentTitle(notification_title)
.setContentText(notification_message);
//i need to open a fragment not Main Activiti
Intent resultIntent = new Intent(this, MainActivity.class);
PendingIntent resultPendingIntent =
PendingIntent.getActivity(
this,
0,
resultIntent,
PendingIntent.FLAG_UPDATE_CURRENT
);
mBuilder.setContentIntent(resultPendingIntent);
//----------------------------------------------------
int mNotificationId = (int) System.currentTimeMillis();
NotificationManager mNotifyMgr =
(NotificationManager) getSystemService(NOTIFICATION_SERVICE);
mNotifyMgr.notify(mNotificationId, mBuilder.build());
}
}