Android : preferencescreen in android java

i have preference activity and added PreferenceScreen to show alertdialog for about in the app

my prefs.xml

<PreferenceScreen android:title="@string/aboutmenu" android:key="about" android:summary="">
        <intent
        android:action="android.intent.action.DEFAULT"
        android:targetPackage="com.prefs.learn"
        android:targetClass="com.prefs.learn.About"
        />
</PreferenceScreen>

and here is my About.java class

package com.prefs.learn;

import android.app.Activity;
import android.app.AlertDialog;
import android.os.Bundle;

public class About extends Activity
{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        pp();

    }

public void pp(){ 

        final AlertDialog d = new AlertDialog.Builder(this)
        .setIcon(R.drawable.ic_launcher)
        .setCancelable(false) 
        .setTitle("About") 
        .setPositiveButton(android.R.string.ok, null)
        .setMessage("About stuff bla bla bla")
        .create();
       d.show();

    }
}

and i have also added About in android manifest file

 <activity android:name=".About" >
        </activity>

now i want when user click About in the preferences it shows alertdialog and yes it does shows alertdialog but when i clock ok it shows blank activity as i know i extended it as "extends Activity"

but if i dont do like this alertdialog builder cant be used throw error anybody can help me