This tutorial demonstrates how to send notifications from an Android app.

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1. MainActivity.java
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import android.app.Activity;
import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

    Button btn;
    String CHANNEL_ID = "10001";

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btn = (Button)findViewById(R.id.button1);
        btn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                createNotificationChannel();
                Notification.Builder builder = new Notification.Builder(MainActivity.this);
                builder.setChannelId(CHANNEL_ID);
                Intent MyIntent = new Intent(Intent.ACTION_VIEW);
                PendingIntent pendingIntent = PendingIntent.getActivity(getApplicationContext(),0,MyIntent, PendingIntent.FLAG_CANCEL_CURRENT);
                builder.setSmallIcon(R.drawable.notification_template_icon_bg)
                        .setContentTitle("Hello World Notification")
                        .setContentText("Hi, this is the notification details!")
                        .setContentIntent(pendingIntent);
                NotificationManager notificationManager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
                Notification notification = builder.getNotification();
                notificationManager.notify(R.drawable.notification_template_icon_bg, notification);

            }
        });

    }
    private void createNotificationChannel() {
        // Create the NotificationChannel, but only on API 26+ because
        // the NotificationChannel class is new and not in the support library
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            CharSequence name = "Channel Name";
            String description = "Channel Descripton";
            int importance = NotificationManager.IMPORTANCE_DEFAULT;
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, name, importance);
            channel.setDescription(description);
            // Register the channel with the system; you can't change the importance
            // or other notification behaviors after this
            NotificationManager notificationManager = getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(channel);
        }
    }
}

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2. activity_main.xml
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<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="#A9BCF5"
    android:orientation="vertical" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:layout_gravity="center_horizontal"
        android:text="Create Notification" >
    </Button>
</LinearLayout>