How to add a push notification in my own android app

I am posting demo application of Google Cloud Messaging.

Make sure you create demo application with API level equal or higher than Android OS 2.2 with Google API

User have to signed in at-least one Google Account to use this service.

First you have to add GCM library.

Than create on class which I named GCMIntentService which extends GCMBaseIntentService as follows:

package com.example.gcmdemo;

import android.content.Context;
import android.content.Intent;
import android.util.Log;

import com.google.android.gcm.GCMBaseIntentService;
import com.google.android.gcm.GCMConstants;

public class GCMIntentService extends GCMBaseIntentService {

     private static final String TAG = "Push Notification Demo GCMIntentService";

    @Override
    protected void onError(Context context, String errorId) {

        if(GCMConstants.ERROR_ACCOUNT_MISSING.equalsIgnoreCase(errorId)) {
            Log.v(TAG, "Error Account Missing");
        } else if(GCMConstants.ERROR_AUTHENTICATION_FAILED.equalsIgnoreCase(errorId)) {
            Log.v(TAG, "Error Authentication Failed");
        } else if(GCMConstants.ERROR_INVALID_PARAMETERS.equalsIgnoreCase(errorId)) {
            Log.v(TAG, "Error Invalid Parameters");
        } else if(GCMConstants.ERROR_INVALID_SENDER.equalsIgnoreCase(errorId)) {
            Log.v(TAG, "Error Invalid Sender");
        } else if(GCMConstants.ERROR_PHONE_REGISTRATION_ERROR.equalsIgnoreCase(errorId)) {
            Log.v(TAG, "Error Phone Registration Error");
        } else if(GCMConstants.ERROR_SERVICE_NOT_AVAILABLE.equalsIgnoreCase(errorId)) {
            Log.v(TAG, "Error Service Not Available");
        } 
    }

    @Override
    protected void onMessage(Context context, Intent intent) {

        // App Server Sends message as key value pairs 
        String value1 = intent.getStringExtra("key1");
        String value2 = intent.getStringExtra("key2");

        Log.v(TAG, "key1: "+value1 );
        Log.v(TAG, "key2: "+value2 );
    }

    @Override
    protected void onRegistered(Context context, String regId) {

        Log.v(TAG, "Successfull Registration : "+regId);
    }

    @Override
    protected void onUnregistered(Context context, String regId) {

        Log.v(TAG, "Successfully Unregistred : "+regId);
    }

    @Override
    protected String[] getSenderIds(Context context) {
        return super.getSenderIds(context);
    }

    @Override
    protected void onDeletedMessages(Context context, int total) {
        super.onDeletedMessages(context, total);
    }

    @Override
    protected boolean onRecoverableError(Context context, String errorId) {
        return super.onRecoverableError(context, errorId);
    }
}

Here is how you should check registration in following demo activity :

package com.example.gcmdemo;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;

import com.google.android.gcm.GCMRegistrar;

public class MainActivity extends Activity {

    private static final String TAG = "Push Notification Demo Activity";
    private static final String SENDER_ID = "1069713227710";

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

        GCMRegistrar.checkDevice(this);
        GCMRegistrar.checkManifest(this);
        final String regId = GCMRegistrar.getRegistrationId(this);
        if (regId.equals("")) {
          GCMRegistrar.register(this, SENDER_ID);
        } else {
          Log.v(TAG, "Already registered : "+regId);
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
}

And finally the demo manifest :

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.gcmdemo"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="8" />

    <permission
        android:name="com.example.gcmdemo.permission.C2D_MESSAGE"
        android:protectionLevel="signature" />

    <uses-permission android:name="com.example.gcmdemo.permission.C2D_MESSAGE" />

    <!-- App receives GCM messages. -->
    <uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />
    <!-- GCM connects to Google Services. -->
    <uses-permission android:name="android.permission.INTERNET" />
    <!-- GCM requires a Google account. -->
    <uses-permission android:name="android.permission.GET_ACCOUNTS" />
    <!-- Keeps the processor from sleeping when a message is received. -->
    <uses-permission android:name="android.permission.WAKE_LOCK" />

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/title_activity_main" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <receiver
            android:name="com.google.android.gcm.GCMBroadcastReceiver"
            android:permission="com.google.android.c2dm.permission.SEND" >
            <intent-filter>
                <action android:name="com.google.android.c2dm.intent.RECEIVE" />
                <action android:name="com.google.android.c2dm.intent.REGISTRATION" />

                <category android:name="com.example.gcmdemo" />
            </intent-filter>
        </receiver>

        <service android:name=".GCMIntentService" />
    </application>

</manifest>

Also you will need third party server side script as specified here.