1) To create notification UI content and action we make use of  NotificationCompat.Builder object and Builder object must include the following:-

• Small icon that would be shown when your notification will arrive at your device and it is done by using "setSmallIcon()".
• The title of your Notification is set by using "setContentTitle()".
• The detail text is set by using "setContentText()".

2) When notification is generated you need to move to a specific activity when user clicks on the notification for this w3e make use of "PendingIntent".

3) To generate the notification 

• Get an instance of NotificationManager.
• Use the notify() method to issue the notification. When you call notify(), specify a notification ID. You can use this ID to update the notification later on. This is described in more detail in Managing Notifications.
• Call build(), which returns a Notification object containing your specifications.

MainActivity.java

public class MainActivity extends AppCompatActivity {
    private WebView mweView;
    private Button btn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn=(Button)findViewById(R.id.btn);
        btn.setOnClickListener(mClickListener);

    }
 View.OnClickListener mClickListener=new View.OnClickListener() {
        @Override
        public void onClick(View v) {
notification();
            
        }
    };

 private void notification()
    {
        Toast.makeText(MainActivity.this,"entered in  Notification",Toast.LENGTH_LONG).show();
        NotificationCompat.Builder mBuilder = (NotificationCompat.Builder) new NotificationCompat.Builder(this).setSmallIcon(R.drawable.go).setContentTitle("WebView").setContentText("Your webView Started working");

//HomeActivity is activity where the user will be redirected when user click notification 
        Intent intent=new Intent(this,HomeActivity.class);
        PendingIntent pendingIntent=PendingIntent.getActivity(MainActivity.this,0,intent,PendingIntent.FLAG_UPDATE_CURRENT);
        mBuilder.setContentIntent(pendingIntent);
 mBuilder.getNotification().flags |= Notification.FLAG_AUTO_CANCEL;
        mBuilder.setAutoCancel(true);
        NotificationManager notificationManager=(NotificationManager)getSystemService(NOTIFICATION_SERVICE);
        notificationManager.notify(0,mBuilder.build());
    }

}