How to Creating an alertbox

package com.codingonandroid;

import android.app.Activity;

import android.app.AlertDialog;

import android.content.Intent;

import android.os.Bundle;

import android.view.Menu;

import android.view.MenuInflater;

import android.view.MenuItem;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.EditText;

public class second extends Activity implements OnClickListener{

Intent i1,i2,i3;

EditText ed1,ed2;

@Override

protected void onCreate(Bundle savedInstanceState) {

// TODO Auto-generated method stub

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

ed1=(EditText)findViewById(R.id.editText1);

ed2=(EditText)findViewById(R.id.editText2);

ed1.setOnClickListener(this);  

 

 

 

}

@Override

public boolean onCreateOptionsMenu(Menu menu) {

// TODO Auto-generated method stub

MenuInflater mi=getMenuInflater();

mi.inflate(R.menu.main, menu);

return super.onCreateOptionsMenu(menu);

}

@Override

public boolean onOptionsItemSelected(MenuItem item) {

// TODO Auto-generated method stub

switch (item.getItemId()) {

case R.id.item1:

i1=new Intent(this,MainActivity.class);

startActivity(i1);

break;

case R.id.item2:

i2=new Intent(this,search.class);

startActivity(i2);

break;

case R.id.item3:

i3=new Intent(this,contact.class);

startActivity(i3);

default:

break;

}

return super.onOptionsItemSelected(item);

}

@Override

public void onClick(View arg0) {

// TODO Auto-generated method stub

AlertDialog.Builder ad=new AlertDialog.Builder(this);

                ad.setTitle("alert box");

                ad.setMessage("enter valid username");

                ad.setPositiveButton("ok",null);

                ad.setNegativeButton("cancle",null);

                ad.show();

}

}