How to create AlertBox

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class Alertbox extends Activity {
    /** Called when the activity is first created. */
   
AlertDialog.Builder builder;
AlertDialog alert;
EditText et;
Button b1;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        et=(EditText)findViewById(R.id.editText1);
        b1=(Button)findViewById(R.id.button1);
        builder=new AlertDialog.Builder(this);
        builder.setCancelable(true);
        builder.setMessage("dou you realy want to reset");
        builder.setPositiveButton("yes",new DialogInterface.OnClickListener() {

@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
et.setText("");
}
});
        builder.setNegativeButton("No",new DialogInterface.OnClickListener() {

@Override
public void onClick(DialogInterface dialog, int which) {
// TODO Auto-generated method stub
dialog.cancel();
}
});
        alert=builder.create();
       
        b1.setOnClickListener(new OnClickListener()
        {

@Override
public void onClick(View v) {
// TODO Auto-generated method stub
alert.show();

}
       
        });
       
    }
}