android中点击系统返回键退出

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.g160628_12_widget2.AActivity">

<TextView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="这是a"
android:textSize="50dp"/>

</LinearLayout>

---------MainActivity.java

//方法@Overridepublic boolean onKeyDown(int keyCode, KeyEvent event) {//点击返回键if(keyCode==KeyEvent.KEYCODE_BACK){//声明弹出对象并初始化AlertDialog.Builder builder=new AlertDialog.Builder(this);builder.setTitle("提示：");builder.setMessage("是否退出？");//设置确定按钮builder.setNegativeButton("确定", new DialogInterface.OnClickListener() {@Overridepublic void onClick(DialogInterface dialog, int which) {finish();}});//设置取消按钮builder.setPositiveButton("取消",null);//显示弹窗builder.show();}return super.onKeyDown(keyCode, event);}