android 程序退出的对话框

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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>