Here is working code for all android versions as of API LEVEL 26+ with backward compatibility.

 NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(getContext(), "M_CH_ID");

 notificationBuilder.setAutoCancel(true)
 .setDefaults(Notification.DEFAULT_ALL)
 .setWhen(System.currentTimeMillis())
 .setSmallIcon(R.drawable.ic_launcher)
 .setTicker("Hearty365")
 .setPriority(Notification.PRIORITY_MAX) // this is deprecated in API 26 but you can still use for below 26. check below update for 26 API
 .setContentTitle("Default notification")
 .setContentText("Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
 .setContentInfo("Info");

NotificationManager notificationManager = (NotificationManager) getContext().getSystemService(Context.NOTIFICATION_SERVICE);
notificationManager.notify(1, notificationBuilder.build());

UPDATE for API 26 to set Max priority

 NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
 String NOTIFICATION_CHANNEL_ID = "my_channel_id_01";

 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) 
 NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "My Notifications", NotificationManager.IMPORTANCE_MAX);

 // Configure the notification channel.
 notificationChannel.setDescription("Channel description");
 notificationChannel.enableLights(true);
 notificationChannel.setLightColor(Color.RED);
 notificationChannel.setVibrationPattern(new long[]0, 1000, 500, 1000);
 notificationChannel.enableVibration(true);
 notificationManager.createNotificationChannel(notificationChannel);
 


 NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID);

 notificationBuilder.setAutoCancel(true)
 .setDefaults(Notification.DEFAULT_ALL)
 .setWhen(System.currentTimeMillis())
 .setSmallIcon(R.drawable.ic_launcher)
 .setTicker("Hearty365")
 // .setPriority(Notification.PRIORITY_MAX)
 .setContentTitle("Default notification")
 .setContentText("Lorem ipsum dolor sit amet, consectetur adipiscing elit.")
 .setContentInfo("Info");

 notificationManager.notify(/*notification id*/1, notificationBuilder.build());

Call the 2-arg constructor: For compatibility with Android O, call support-v4 NotificationCompat.Builder(Context context, String channelId). When running on Android N or earlier, the channelId will be ignored. When running on Android O, also create a NotificationChannel with the same channelId.

Out of date sample code: The sample code on several JavaDoc pages such as Notification.Builder calling new Notification.Builder(mContext) is out of date.

Deprecated constructors: Notification.Builder(Context context) and v4 NotificationCompat.Builder(Context context) are deprecated in favor of Notification[Compat].Builder(Context context, String channelId). (See Notification.Builder(android.content.Context) and v4 NotificationCompat.Builder(Context context).)

Deprecated class: The entire class v7 NotificationCompat.Builder is deprecated. (See v7 NotificationCompat.Builder.) Previously, v7 NotificationCompat.Builder was needed to support NotificationCompat.MediaStyle. In Android O, there's a v4 NotificationCompat.MediaStyle in the media-compat library's android.support.v4.media package. Use that one if you need MediaStyle.

API 14+: In Support Library from 26.0.0 and higher, the support-v4 and support-v7 packages both support a minimum API level of 14. The v# names are historical.

See Recent Support Library Revisions.

Instead of checking for Build.VERSION.SDK_INT >= Build.VERSION_CODES.O as many answers suggest, there is a slightly simpler way -

Add the following line to the application section of AndroidManifest.xml file as explained in the Set Up a Firebase Cloud Messaging Client App on Android doc:

 <meta-data
 android:name="com.google.firebase.messaging.default_notification_channel_id" 
 android:value="@string/default_notification_channel_id" />

Then add a line with a channel name to the values/strings.xml file:

<string name="default_notification_channel_id">default</string>

After that you will be able to use the new version of NotificationCompat.Builder constructor with 2 parameters (since the old constructor with 1 parameter has been deprecated in Android Oreo):

private void sendNotification(String title, String body) 
 Intent i = new Intent(this, MainActivity.class);
 i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
 PendingIntent pi = PendingIntent.getActivity(this,
 0 /* Request code */,
 i,
 PendingIntent.FLAG_ONE_SHOT);

 Uri sound = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);

 NotificationCompat.Builder builder = new NotificationCompat.Builder(this, 
 getString(R.string.default_notification_channel_id))
 .setSmallIcon(R.mipmap.ic_launcher)
 .setContentTitle(title)
 .setContentText(body)
 .setAutoCancel(true)
 .setSound(sound)
 .setContentIntent(pi);

 NotificationManager manager = 
 (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

 manager.notify(0, builder.build());

Here is the sample code, which is working in Android Oreo and less than Oreo.

 NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
 NotificationCompat.Builder builder = null;
 if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) 
 int importance = NotificationManager.IMPORTANCE_DEFAULT;
 NotificationChannel notificationChannel = new NotificationChannel("ID", "Name", importance);
 notificationManager.createNotificationChannel(notificationChannel);
 builder = new NotificationCompat.Builder(getApplicationContext(), notificationChannel.getId());
 else 
 builder = new NotificationCompat.Builder(getApplicationContext());
 

 builder = builder
 .setSmallIcon(R.drawable.ic_notification_icon)
 .setColor(ContextCompat.getColor(context, R.color.color))
 .setContentTitle(context.getString(R.string.getTitel))
 .setTicker(context.getString(R.string.text))
 .setContentText(message)
 .setDefaults(Notification.DEFAULT_ALL)
 .setAutoCancel(true);
 notificationManager.notify(requestCode, builder.build());

Simple Sample

 public void showNotification (String from, String notification, Intent intent) 
 PendingIntent pendingIntent = PendingIntent.getActivity(
 context,
 Notification_ID,
 intent,
 PendingIntent.FLAG_UPDATE_CURRENT
 );


 String NOTIFICATION_CHANNEL_ID = "my_channel_id_01";
 NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);


 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) 
 NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "My Notifications", NotificationManager.IMPORTANCE_DEFAULT);

 // Configure the notification channel.
 notificationChannel.setDescription("Channel description");
 notificationChannel.enableLights(true);
 notificationChannel.setLightColor(Color.RED);
 notificationChannel.setVibrationPattern(new long[]0, 1000, 500, 1000);
 notificationChannel.enableVibration(true);
 notificationManager.createNotificationChannel(notificationChannel);
 


 NotificationCompat.Builder builder = new NotificationCompat.Builder(context, NOTIFICATION_CHANNEL_ID);
 Notification mNotification = builder
 .setContentTitle(from)
 .setContentText(notification)

// .setTicker("Hearty365")
// .setContentInfo("Info")
 // .setPriority(Notification.PRIORITY_MAX)

 .setContentIntent(pendingIntent)

 .setAutoCancel(true)
// .setDefaults(Notification.DEFAULT_ALL)
// .setWhen(System.currentTimeMillis())
 .setSmallIcon(R.mipmap.ic_launcher)
 .setLargeIcon(BitmapFactory.decodeResource(context.getResources(), R.mipmap.ic_launcher))
 .build();

 notificationManager.notify(/*notification id*/Notification_ID, mNotification);

 

Notification notification = new Notification.Builder(MainActivity.this)
 .setContentTitle("New Message")
 .setContentText("You've received new messages.")
 .setSmallIcon(R.drawable.ic_notify_status)
 .setChannelId(CHANNEL_ID)
 .build(); 

Right code will be :

Notification.Builder notification=new Notification.Builder(this)

with dependency 26.0.1 and new updated dependencies such as 28.0.0.

Some users use this code in the form of this :

Notification notification=new NotificationCompat.Builder(this)//this is also wrong code.

So Logic is that which Method you will declare or initilize then the same methode on Right side will be use for Allocation. if in Leftside of = you will use some method then the same method will be use in right side of = for Allocation with new.

Try this code...It will sure work

This constructor was deprecated in API level 26.1.0.
use NotificationCompat.Builder(Context, String) instead. All posted Notifications must specify a NotificationChannel Id.