Question Detail

You have to use NotificationChannel Class to implement Notification till you Android Oreo Version.

public void displayNotification(Context activity, int notiCode) {
    String title = activity.getString(R.string.app_name) + " is Active";

    Intent tapIntent = new Intent(activity, DashboardActivity.class);
    notificationManager = (NotificationManager) activity.getSystemService(NOTIFICATION_SERVICE);
    PendingIntent pendingIntent = PendingIntent.getActivity(activity, 1, tapIntent, 0);

    Notification.Builder builder = new Notification.Builder(activity)
            .setContentTitle(title)
            .setSmallIcon(R.drawable.notification_icon)
            .setAutoCancel(false)
            .setSound(null)
            .setContentIntent(pendingIntent).setOngoing(true)
            .setContentText("This is Content!");
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel mChannel = new NotificationChannel(notiCode + "", title, NotificationManager.IMPORTANCE_HIGH);
        builder.setChannelId(notiCode + "");
        mChannel.setSound(null, null);
        notificationManager.createNotificationChannel(mChannel);
    }
    if (Pref.getBoolean(activity, "showcurrentRej_pref")) {
        if (Pref.getInteger(activity, "messageCount_pref") > 0 || Pref.getInteger(activity, "callsCount_pref") > 0 || Pref.getInteger(activity, "notificationCount_pref") > 0) {
            builder.setStyle(new Notification.BigTextStyle(builder)
                    .setBigContentTitle("Rejected Events")
                    .bigText("Total Rejected Calls: " + Pref.getInteger(activity, "callsCount_pref") +
                            "\nTotal Rejected SMS : " + Pref.getInteger(activity, "messageCount_pref") +
                            "\nTotal Rejected Notification : " + Pref.getInteger(activity, "notificationCount_pref"))
                    .setSummaryText("More")).setContentTitle(title);
        }
    }
    Notification notification = builder.build();
    notificationManager.notify(notiCode, notification);
}

You have to use NotificationChannel Class to implement Notification till you Android Oreo Version.

public void displayNotification(Context activity, int notiCode) {
    String title = activity.getString(R.string.app_name) + " is Active";

    Intent tapIntent = new Intent(activity, DashboardActivity.class);
    notificationManager = (NotificationManager) activity.getSystemService(NOTIFICATION_SERVICE);
    PendingIntent pendingIntent = PendingIntent.getActivity(activity, 1, tapIntent, 0);

    Notification.Builder builder = new Notification.Builder(activity)
            .setContentTitle(title)
            .setSmallIcon(R.drawable.notification_icon)
            .setAutoCancel(false)
            .setSound(null)
            .setContentIntent(pendingIntent).setOngoing(true)
            .setContentText("This is Content!");
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel mChannel = new NotificationChannel(notiCode + "", title, NotificationManager.IMPORTANCE_HIGH);
        builder.setChannelId(notiCode + "");
        mChannel.setSound(null, null);
        notificationManager.createNotificationChannel(mChannel);
    }
    if (Pref.getBoolean(activity, "showcurrentRej_pref")) {
        if (Pref.getInteger(activity, "messageCount_pref") > 0 || Pref.getInteger(activity, "callsCount_pref") > 0 || Pref.getInteger(activity, "notificationCount_pref") > 0) {
            builder.setStyle(new Notification.BigTextStyle(builder)
                    .setBigContentTitle("Rejected Events")
                    .bigText("Total Rejected Calls: " + Pref.getInteger(activity, "callsCount_pref") +
                            "\nTotal Rejected SMS : " + Pref.getInteger(activity, "messageCount_pref") +
                            "\nTotal Rejected Notification : " + Pref.getInteger(activity, "notificationCount_pref"))
                    .setSummaryText("More")).setContentTitle(title);
        }
    }
    Notification notification = builder.build();
    notificationManager.notify(notiCode, notification);
}

Here is the Notification Receiver Class

public class FirebaseMessageReceiver extends FirebaseMessagingService {
    String channel_id = "";

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        //handle when receive notification via data event
        channel_id = getString(R.string.default_notification_channel_id);
        String title = "Notification";
        String messageStr = "We are missing you, Please check our latest Offers to grow more.";
        String imagePath = "https://www.glassdoor.com/employers/app/uploads/sites/2/2021/03/GoogleDrive_640X469_How-to-Rescind-an-Offer-Letter-02.png";
        if (remoteMessage.getData().size() > 0) {
            title = remoteMessage.getData().get("title");
            messageStr = remoteMessage.getData().get("message");
            imagePath = remoteMessage.getData().get("Image");
            isLogPrintValue("NotificaXtionsData", "" + remoteMessage.getData().toString());
        }

        //handle when receive notification
        if (remoteMessage.getNotification() != null) {
            title = remoteMessage.getNotification().getTitle();
            messageStr = remoteMessage.getNotification().getBody();
            isLogPrintValue("NotificaXtions_Data", "" + remoteMessage.getData().toString());
        }
        showNewNotification(title, messageStr, imagePath);
    }


    public void showNewNotification(String title, String bodyString, String image) {
        Intent intent = new Intent(getApplicationContext(), SplashActivity.class);
        Uri alarmSound = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
        PendingIntent pendingIntent = PendingIntent.getActivity(getApplicationContext(), 0, intent, 0);
        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        String NOTIFICATION_CHANNEL_ID = channel_id;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel notificationChannel = new NotificationChannel(
                    NOTIFICATION_CHANNEL_ID,
                    "Notification",
                    NotificationManager.IMPORTANCE_DEFAULT);
            //Configure Notification Channel
            notificationChannel.setDescription(title);
            notificationChannel.enableLights(true);
            notificationChannel.setLightColor(Color.GREEN);
            notificationChannel.setVibrationPattern(
                    new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400}
            );
            notificationChannel.enableVibration(true);
            notificationManager.createNotificationChannel(notificationChannel);
        }

        long notificatioId = System.currentTimeMillis();
        NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID);
        notificationBuilder.setSmallIcon(R.drawable.notification_icon);
        notificationBuilder.setLargeIcon(BitmapFactory.decodeResource(this.getResources(), R.drawable.boy));
        notificationBuilder.setContentTitle(checkString(title));
        notificationBuilder.setAutoCancel(true);
        notificationBuilder.setContentText(checkString(bodyString));
        notificationBuilder.setSound(alarmSound);
        if (image != null && !image.isEmpty()) {
            Bitmap bitmap = getBitmapfromUrl(image);
            notificationBuilder.setStyle(
                    new NotificationCompat.BigPictureStyle()
                            .bigPicture(bitmap)
                            .setSummaryText(bodyString));
        } else {
            notificationBuilder.setContentText(bodyString);
            notificationBuilder.setStyle((new NotificationCompat.BigTextStyle().setSummaryText(bodyString)));
        }
        notificationBuilder.setAutoCancel(true);
        notificationBuilder.setContentIntent(pendingIntent);
        notificationBuilder.setWhen(notificatioId);
        notificationBuilder.setPriority(Notification.PRIORITY_HIGH);
        notificationManager.notify((int) notificatioId, notificationBuilder.build());
    }

    public Bitmap getBitmapfromUrl(String imageUrl) {
        try {
            URL url = new URL(imageUrl);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setDoInput(true);
            connection.connect();
            InputStream input = connection.getInputStream();
            return BitmapFactory.decodeStream(input);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }
    }
}