NotificationService.java

package com.sivsivsree.firetest;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.os.IBinder;
import android.support.v4.app.NotificationCompat;
import android.support.v4.app.NotificationManagerCompat;
import android.util.Log;
import android.widget.Toast;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.concurrent.atomic.AtomicInteger;

public class NotificationService extends Service {
    private static final String TAG = "SERVICE_TAG" ;
    public final AtomicInteger id;
    public NotificationService() {
        id = new AtomicInteger(0);
    }

    @Override
    public void onCreate() {
        super.onCreate();
        FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference myRef = database.getReference("message");


        // Read from the database
        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // This method is called once with the initial value and again
                // whenever data at this location is updated.
                String value = dataSnapshot.getValue(String.class);
                generateNotification(getApplicationContext(), value, "New Notification");
                Log.d(TAG, "Value is: " + value);
            }

            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
                Log.w(TAG, "Failed to read value.", error.toException());
            }
        });

    }

    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
         super.onStartCommand(intent, flags, startId);
        Toast.makeText(getApplicationContext(),"Service Started", Toast.LENGTH_LONG).show();
         return START_STICKY;
    }

    private void generateNotification(Context context, String message,
                                             String key) {
        Intent viewIntent = new Intent(context, MainActivity.class);

        PendingIntent viewPendingIntent =
                PendingIntent.getActivity(context, 0, viewIntent, 0);

        NotificationCompat.Builder notificationBuilder =
                new NotificationCompat.Builder(context)
                        .setSmallIcon(R.drawable.common_ic_googleplayservices)
                        .setContentTitle(key)
                        .setContentText(message)
                        .setGroup(TAG)
                        .setContentIntent(viewPendingIntent);

// Get an instance of the NotificationManager service
        NotificationManagerCompat notificationManager =
                NotificationManagerCompat.from(context);

// Build the notification and issues it with notification manager.

        notificationManager.notify(id.incrementAndGet(), notificationBuilder.build());

    }

}