NotificationCompat with API 26

Create the NotificationChannel only if API >= 26

public void initChannels(Context context) {
    if (Build.VERSION.SDK_INT < 26) {
        return;
    }
    NotificationManager notificationManager =
            (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
    NotificationChannel channel = new NotificationChannel("default",
                                                          "Channel name",
                                                          NotificationManager.IMPORTANCE_DEFAULT);
    channel.setDescription("Channel description");
    notificationManager.createNotificationChannel(channel);
}

And then just use:

NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(context, "default");

So your notifications are working with both API 26 (with channel) and below (without).

Declare Notification Manager:

   final NotificationManager mNotific=            
   (NotificationManager)getSystemService(Context.NOTIFICATION_SERVICE);

    CharSequence name="Ragav";
    String desc="this is notific";
    int imp=NotificationManager.IMPORTANCE_HIGH;
    final String ChannelID="my_channel_01";

Notification Channel

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) 
    {
      NotificationChannel mChannel = new NotificationChannel(ChannelID, name, 
     imp);
            mChannel.setDescription(desc);
            mChannel.setLightColor(Color.CYAN);
            mChannel.canShowBadge();
            mChannel.setShowBadge(true);
            mNotific.createNotificationChannel(mChannel);
        }

    final int ncode=101;

    String Body="This is testing notific";

Notification Builder

        Notification n= new Notification.Builder(this,ChannelID)
                .setContentTitle(getPackageName())
                .setContentText(Body)
                .setBadgeIconType(R.mipmap.ic_launcher)
                .setNumber(5)
                .setSmallIcon(R.mipmap.ic_launcher_round)
                .setAutoCancel(true)
                .build();

NotificationManager notify to User:

            mNotific.notify(ncode, n);

NotificationChannel actually groups multiple notifications into channels. It basically gives more control of the notification behavior to the user. You can read more about Notification Channel and its implementation at Working with Notification Channel | With Example

Notification Channel is only applicable for Android Oreo.

 //Notification channel should only be created for devices running Android 26
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {

  NotificationChannel notificationChannel = new NotificationChannel("unique_channel_id","channel_name",NotificationManager.IMPORTANCE_DEFAULT);

  //Boolean value to set if lights are enabled for Notifications from this Channel
  notificationChannel.enableLights(true);

  //Boolean value to set if vibration is enabled for Notifications from this Channel
  notificationChannel.enableVibration(true);

  //Sets the color of Notification Light
  notificationChannel.setLightColor(Color.GREEN);

  //Set the vibration pattern for notifications. Pattern is in milliseconds with the format {delay,play,sleep,play,sleep...}
  notificationChannel.setVibrationPattern(new long[]{500,500,500,500,500});

  //Sets whether notifications from these Channel should be visible on Lockscreen or not
  notificationChannel.setLockscreenVisibility( Notification.VISIBILITY_PUBLIC);
}  

Note that Channel ID passed to the constructor acts as the unique identifier for that Notification Channel. Now create the Notification as shown below

// Creating the Channel
NotificationManager notificationManager = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
notificationManager.createNotificationChannel(notificationChannel);

To add any Notification to this Channel just pass the Channel ID as shown below

//We pass the unique channel id as the second parameter in the constructor
NotificationCompat.Builder notificationCompatBuilder=new NotificationCompat.Builder(this,NOTIFICATION_CHANNEL_ID);

//Title for your notification
notificationCompatBuilder.setContentTitle("This is title");

//Subtext for your notification
notificationCompatBuilder.setContentText("This is subtext");

//Small Icon for your notificatiom
notificationCompatBuilder.setSmallIcon(R.id.icon);

//Large Icon for your notification 
notificationCompatBuilder.setLargeIcon(  BitmapFactory.decodeResource(getResources(),R.id.icon));

notificationManager.notify( NOTIFICATION_ID,notificationCompatBuilder.build());