如何根据列表是否为空在Main Activity中显示片段？

Question

我正在为即将到来的生日开发一个Android应用程序。我有一种方法可以检查SQLite数据库中的日期是否在两天内即将到来的生日并加载到列表中。还有两个片段：MainFragment和BirthdayFragment。启动应用程序时，我希望BirthdayFragment仅在列表中包含某些内容时显示，而在没有列表时应显示MainFragment。但是，当我使用下面的代码启动该应用程序时，它显示为空活动。当列表为空或不为空时，我尝试了此操作。

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //check for upcoming birthday here...
    //if we have birthday, display Birthday Fragment
    DatabaseOperations dbo = new DatabaseOperations(getApplicationContext());
    List<Student> studentList = dbo.getAllStudentsToList();
    if (studentList.isEmpty())
        //if we don't have any birthday, display main fragment
        if (savedInstanceState == null) {
            MainFragment fragment = new MainFragment();
            replaceFragment(fragment);
        }
    else {//if we have any birthday, display birthday fragment
        BirthdayFragment birthdayFragment = new BirthdayFragment();
        replaceFragment(birthdayFragment);
    }
}

但是，如果删除代码以检查即将到来的生日，它将很好地显示MainFragment。代码是：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        MainFragment fragment = new MainFragment();
        replaceFragment(fragment);
    }
}

下面是替换片段的方法

public void replaceFragment(Fragment fragment){
    FragmentManager fragmentManager = getFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
    fragmentTransaction.replace(R.id.fragment_container, fragment);
    fragmentTransaction.addToBackStack(null);
    fragmentTransaction.commit();
}

请帮助我。