Notification not showing up in Android O despite creating a channel

Question

This is my code given below. This is unable to create any notifications on Android O, inspite of creating the Notification Channel.

private void weatherNotification(WeatherInfo weather) {
    Intent intent = new Intent(this, WeatherActivity.class);
    PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);

    String temperatureScale = prefs.getUnits().equals("metric") ? getString(R.string.c) : getString(R.string.f);
    String speedScale = prefs.getUnits().equals("metric") ? getString(R.string.mps) : getString(R.string.mph);

    String temperature = getString(R.string.temperature , weather.getMain().getTemp() , temperatureScale);
    String city = getString(R.string.city , weather.getName() + ", " + weather.getSys().getCountry());
    String wind = getString(R.string.wind_ , weather.getWind().getSpeed(), speedScale);
    String humidity = getString(R.string.humidity , weather.getMain().getHumidity());
    String pressure = getString(R.string.pressure, weather.getMain().getPressure());

    String data = city + "\n" + temperature + "\n" + wind + "\n" + humidity + "\n" + pressure;

    NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        String id = "w01", name = getString(R.string.weather_notification_title);
        int importance = NotificationManager.IMPORTANCE_DEFAULT;
        String desc = getString(R.string.weather_notification_description);

        NotificationChannel channel = new NotificationChannel(id, name, importance);
        channel.setDescription(desc);
        notificationManager.createNotificationChannel(channel);
    }
    Notification.Builder builder = new Notification.Builder(this);

    builder.setAutoCancel(false);
    builder.setContentTitle("Weather Notification");
    builder.setContentText(Math.round(weather.getMain().getTemp()) + temperatureScale + " at " + weather.getName());
    builder.setStyle(new Notification.BigTextStyle().bigText(data));
    builder.setSmallIcon(R.drawable.ic_notification_icon);
    builder.setContentIntent(pendingIntent);
    if (Build.VERSION.SDK_INT >= 24)
        builder.setColor(Color.parseColor("#ff0000"));
    Notification notification = builder.build();
    notificationManager.notify(0 , notification);
}

I believe I have followed all the steps that are required for creating a Notification on Android O - Creating Notification Channel using the notification manager, followed by building the notification on this manager. I don't know where I am going wrong.

EDIT 1: Made the following change in weatherNotification() method, still does not work:

private void weatherNotification(WeatherInfo weather) {

    ....

    NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        String id = "w01", name = getString(R.string.weather_notification_title);
        int importance = NotificationManager.IMPORTANCE_DEFAULT;
        String desc = getString(R.string.weather_notification_description);

        NotificationChannel channel = new NotificationChannel(id, name, importance);
        channel.setDescription(desc);
        notificationManager.createNotificationChannel(channel);
        builder = new Notification.Builder(this , id);
    }
    builder = new Notification.Builder(this);
    builder.setAutoCancel(false);
    builder.setContentTitle("Weather Notification");
    builder.setContentText(Math.round(weather.getMain().getTemp()) + temperatureScale + " at " + weather.getName());
    builder.setStyle(new Notification.BigTextStyle().bigText(data));
    builder.setSmallIcon(R.drawable.ic_notification_icon);
    builder.setContentIntent(pendingIntent);
    if (Build.VERSION.SDK_INT >= 24)
        builder.setColor(Color.parseColor("#ff0000"));
    Notification notification = builder.build();
    notificationManager.notify(0 , notification);
    ....
}

EDIT 2: From Edit 1 Code, I found out that the builder was being regenerated again. So I made the following changes again:

private void weatherNotification(WeatherInfo weather) {
    ....
    NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        String id = "w01", name = getString(R.string.weather_notification_title);
        int importance = NotificationManager.IMPORTANCE_DEFAULT;
        String desc = getString(R.string.weather_notification_description);

        NotificationChannel channel = new NotificationChannel(id, name, importance);
        channel.setDescription(desc);
        notificationManager.createNotificationChannel(channel);
        builder = new Notification.Builder(this , id);
    }
    else
        builder = new Notification.Builder(this);
    ....
}

Answer 1

You should be getting a deprecation warning on this line:

Notification.Builder builder = new Notification.Builder(this);

That is because the new constructor takes your channel ID:

Notification.Builder builder = new Notification.Builder(this, id);

(though you will need to rework your code a bit so that id is still available.

Quoting the JavaDocs for the constructor that you are using right now: "All posted Notifications must specify a NotificationChannel Id." As it stands, you are not using that channel when raising the Notification. AFAIK, that will prevent your Notification from being displayed.