Android 按手机返回键退出时出现退出提示框

 1     private void exitDialog() {
 2         Dialog dialog = new AlertDialog.Builder(this).setIcon(R.drawable.pic_m)
 3                 .setTitle("程序退出？ ").setMessage("您确定要退出本程序吗？")
 4 
 5                 .setNegativeButton("取消", new DialogInterface.OnClickListener() {
 6                     public void onClick(DialogInterface dialog, int which) {
 7                     }
 8                 })
 9                 .setPositiveButton("确定", new DialogInterface.OnClickListener() {
10                     public void onClick(DialogInterface dialog, int which) {
11                         MainActivity.this.finish();
12                     }
13                 }).create();
14 
15         dialog.show();
16     }
17 
18     public boolean onKeyDown(int keyCode, KeyEvent event) {
19         if (keyCode == KeyEvent.KEYCODE_BACK) {
20             this.exitDialog();
21         }
22         return false;
23     }