ผมพึ่งหัดเขียน android ช่วยหา error ให้ผมทีไม่รู้ว่า error ที่ไหนคับ

private Button bt1,bt2,bt3,bt4;        
        bt1= (Button) findViewById(R.id.btxxx);

package com.myapp;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.app.AlertDialog;

public class MainActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        final AlertDialog.Builder adb = new AlertDialog.Builder(this);
        
        final Button btn1 = (Button) findViewById(R.id.button1);
        // Perform action on click
        btn1.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                AlertDialog ad = adb.create();
                ad.setMessage("Button 2 Event");
                ad.show();
            }
        });
        
        final Button btn2 = (Button) findViewById(R.id.button2);
		// Perform action on click
        btn2.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                AlertDialog ad = adb.create();
                ad.setMessage("Button 2 Event");
                ad.show();
            }
        });
        
    }
    
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
	
}