Crear PopupMenu (Ventana Emergente)

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

    <item
        android:id="@+id/action_one"
        android:title="Accion1"/>

    <item
        android:id="@+id/action_two"
        android:title="Accion2"/>

    <item
        android:id="@+id/action_three"
        android:title="Accion3"/>

</menu>

<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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/btnShowPMenu"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="46dp"
        android:text="Ver Popup Menu" />

</RelativeLayout>

package com.bolivia.demopopupmenu;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.PopupMenu;
import android.widget.Toast;

public class MainActivity extends Activity {

 private Button btnShowPMenu;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  //nuevo codigo
  btnShowPMenu = ( Button )findViewById( R.id.btnShowPMenu );
  btnShowPMenu.setOnClickListener(
                new View.OnClickListener() {
                    public void onClick(View view) {
                     popupMenu();
                    }});
 }//end:onCreate

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

 /** Popup Menu */
 private void popupMenu()
 {
  //Crea instancia a PopupMenu
        PopupMenu popup = new PopupMenu(this, btnShowPMenu);
        popup.getMenuInflater().inflate(R.menu.my_popupmenu , popup.getMenu());
        //registra los eventos click para cada item del menu
        popup.setOnMenuItemClickListener(new PopupMenu.OnMenuItemClickListener() {
            public boolean onMenuItemClick(MenuItem item) {                
                if (item.getItemId() == R.id.action_one)
       {
                 Toast.makeText(MainActivity.this, 
                   "Ejecutar : " + item.getTitle() ,Toast.LENGTH_SHORT).show();
       }
                else if (item.getItemId() == R.id.action_two)
       {
                 Toast.makeText(MainActivity.this, 
                   "Ejecutar : " + item.getTitle() ,Toast.LENGTH_SHORT).show();
       }                         
                else if (item.getItemId() == R.id.action_three)
       {
                 Toast.makeText(MainActivity.this, 
                   "Ejecutar : " + item.getTitle() ,Toast.LENGTH_SHORT).show();
       }                         
                return true;
            }
        });
        popup.show();
 }

}//end:MainActivity