Android Notification : http://devahoy.com/2015/08/android-notification/

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/btn_notification"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="Show Notification"
        android:padding="32dp"
        android:onClick="showNotification"/>

</RelativeLayout>

activity_push.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/txt_message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"/>

</RelativeLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.devahoy.simplenotification" >

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

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

        <activity
            android:name=".PushActivity"
            android:parentActivityName=".MainActivity">
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value=".MainActivity"/>
        </activity>
    </application>

</manifest>

MainActivity.java

package com.devahoy.simplenotification;

import android.app.Notification;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.NotificationCompat;
import android.support.v4.app.TaskStackBuilder;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;

public class MainActivity extends AppCompatActivity {

    private static final String TITLE = "Devahoy News";
    private static final String MESSAGE = "สวัสดีครับ ยินดีต้อนรับเข้าสู่บทความ Android Notification :)";

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

    public void showNotification(View view) {

//        Intent intent = new Intent(Intent.ACTION_VIEW,
//                Uri.parse("http://devahoy.com/2015/08/android-notification/"));
//        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);

        Intent intent = new Intent(this, PushActivity.class);
        intent.putExtra("message", MESSAGE);
        TaskStackBuilder stackBuilder = TaskStackBuilder.create(this);
        stackBuilder.addParentStack(PushActivity.class);
        stackBuilder.addNextIntent(intent);
        PendingIntent pendingIntent =
                stackBuilder.getPendingIntent(0, PendingIntent.FLAG_UPDATE_CURRENT);

        Notification notification =
                new NotificationCompat.Builder(this)
                        .setSmallIcon(R.mipmap.ic_launcher)
                        .setContentTitle(TITLE)
                        .setContentText(MESSAGE)
                        .setAutoCancel(true)
                        .setContentIntent(pendingIntent)
                        .build();

        NotificationManager notificationManager =
                (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        notificationManager.notify(1000, notification);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

PushActivity.java

package com.devahoy.simplenotification;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;

public class PushActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_push);

        Bundle bundle = getIntent().getExtras();

        String message = bundle.getString("message");

        TextView textView = (TextView) findViewById(R.id.txt_message);
        textView.setText(message);
    }
}