Example of a service that send a notification
package com.xanderco.serviceapp;
import android.app.IntentService;
import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.app.TaskStackBuilder;
import android.content.Context;
import android.content.Intent;
import android.support.annotation.Nullable;
public class MyService extends IntentService {
    public static final String MESSAGE = "message";
    public static final int NOTIFICATION_ID = 1325;
    public MyService() {
        super("MyService");
    }
    @Override
    protected void onHandleIntent(@Nullable Intent intent) {
        //Syncronized es usado para que java ejecute este codigo obligatoriamente y no ejecute el otro codigo en diferenntes Threads
        synchronized (this) {
            try {
                wait(10000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            String text = intent.getStringExtra(MESSAGE);
            showText(text);
        }
    }
    
    // Este es un metodo creado para recibir una mensaje y enviar la notificacion
    private void showText(final String text) {
        Intent intent = new Intent(this, MainActivity.class);
        TaskStackBuilder stackbuilder = TaskStackBuilder.create(this);
        stackbuilder.addParentStack(MainActivity.class);
        stackbuilder.addNextIntent(intent);
        PendingIntent pendingIntent = stackbuilder.getPendingIntent(0,PendingIntent.FLAG_UPDATE_CURRENT);
        Notification notification = new Notification.Builder(this)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setContentTitle("Hello")
                .setPriority(Notification.PRIORITY_MAX)
                .setDefaults(Notification.DEFAULT_VIBRATE)
                .setDefaults(Notification.DEFAULT_SOUND)
                .setContentIntent(pendingIntent)
                .setContentText(text)
                .build();
        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.notify(NOTIFICATION_ID,notification);
    }
}