[ Android Studio ] 返回鍵顯示為退出確認顯示的方法

//返回鍵
public boolean onKeyDown(int keyCode, KeyEvent event) {//捕捉返回鍵
   if ((keyCode == KeyEvent.KEYCODE_BACK)) {
     ConfirmExit();//按返回鍵，則執行退出確認
     return true;
   }
   return super.onKeyDown(keyCode, event);
}

//語法
public void ConfirmExit(){//退出確認
   AlertDialog.Builder ad=new AlertDialog.Builder(MainActivity.this);
   ad.setTitle("離開");
   ad.setMessage("確定要離開此程式嗎?");
   ad.setPositiveButton("是", new DialogInterface.OnClickListener() {//退出按鈕
     public void onClick(DialogInterface dialog, int i) {
       Intent intent = new Intent(context, A1.class);
       stopService(intent);
       // TODO Auto-generated method stub
       MainActivity.this.finish();//關閉activity
     }
   });
   ad.setNegativeButton("否",new DialogInterface.OnClickListener() {
     public void onClick(DialogInterface dialog, int i) {
       //不退出不用執行任何操作
     }
   });
   ad.show();//顯示對話框
}