Android 按手机返回键退出时出现退出提示框

    private void exitDialog() {
        Dialog dialog = new AlertDialog.Builder(this).setIcon(R.drawable.pic_m)
                .setTitle("程序退出？ ").setMessage("您确定要退出本程序吗？")

                .setNegativeButton("取消", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                    }
                })
                .setPositiveButton("确定", new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        MainActivity.this.finish();
                    }
                }).create();

        dialog.show();
    }

    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            this.exitDialog();
        }
        return false;
    }