android 程序退出的对话框

package com.example.yanlei.yl;

import android.graphics.Color;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import android.text.Html;
import android.text.Html.ImageGetter;

import java.util.regex.Matcher;
import java.util.regex.Pattern;
import android.text.Editable;
import android.text.TextWatcher;
import android.widget.EditText;

import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;

import android.view.KeyEvent;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    }
    //在此方法中创建dialog
    protected void creatdialog() {
        //初始化AlertDialog构建器对象
        AlertDialog.Builder b = new Builder(MainActivity.this);
        //设置dialog的信息
        b.setMessage("确认退出吗？");
        //设置dialog的标题
        b.setTitle("提示");
        //添加确认和取消按钮
        b.setPositiveButton("确认", new OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                //如果用户点击确认退出，则对话框消失，程序关闭
                dialog.dismiss();
                MainActivity.this.finish();
            }
        });
        b.setNegativeButton("取消", new OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                //如果用户点击取消退出，则对话框消失
                dialog.dismiss();
            }
        });
        //创建对话框并且显示
        b.create().show();
    }

    //在Activity中的键盘监听回调事件
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
            creatdialog();
        }
        return false;
    }

}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >

    <!-- 定义TextView控件 -->
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:text="点击物理返回键弹出退出对话框" />

</RelativeLayout>