您不是在适配器中的数据项列表上进行迭代，而是设置单个数据项并从适配器中阅读该单个数据项。
适配器迭代Adapter::getSize()次。您的重写告诉它遍历NUM_PAGES数据项。当调用notifyDataSetChanged时，它在每次迭代时调用createFragment并执行该代码以创建一个片段。
问题是结构性的--您需要确保适配器观察到的数据集是可迭代的。
顺便说一下--为您添加的每个项目调用notifyDataSetChanged通常是一个糟糕的主意。适配器将在每次插入新页面时重新创建每个片段。
请注意，createFragment提供了一个参数int position。适配器（通常）接受一个List<Object>，并为该列表中的每个索引创建一个Object项。
提供的上下文太少，无法给予严格的解决方案，但理想情况下，您需要执行以下操作：
1.创建一个表示页面的类。它应该包含构造片段示例所需的所有数据的成员。

public class MyPageClass {

    // Maybe init fields here if they are required
    public MyPageClass(){} 

    private String dayTitle;
    private List<FocusGroup> focusGroups;
    private List<DayExercise> dayExercises;

    // Getters + Setters as needed, or any interface you want
    // < ... >

}

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1.修改代码以构造页面对象列表

//Get routine from intent
        userRoutine = (RoutineISO) getIntent().getSerializableExtra("routine");
        ArrayList<String> routineDays = userRoutine.getDays();

        // Define a private data member pages as an empty ArrayList--You probably want to
        // clear it here.
        this.pages = new ArrayList<>();

        for(String day: routineDays){
            // Building our page
            MyPageClass page = new MyPageClass();
            page.setDayExercises(userRoutine.getSplitDays().get(day));
            page.setFocusGroup(userRoutine.getMuscleGroups().get(day));
            page.setDayTitle(day);
            // Adding it to the list that pagerAdapter observes
            this.pages.add(page);
        }

        // Now we notify the adapter, once all of these changes have finished.
        // Consider using more specific methods, like notifyItemRangeInserted() etc
        pagerAdapter.notifyDataSetChanged();

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1.修改适配器代码以依赖于此列表。

private class DayAdapter extends FragmentStateAdapter {
       private ArrayList<Fragment> fragments;
       private ArrayList<MyPageClass> pages; // This is our list of MyPageClass instances
       private Context context;

       public DayAdapter(FragmentActivity fa, ArrayList<MyPageClass> pages) {
           super(fa);
           this.pages = pages;
       }

       @NonNull
       @Override
       public Fragment createFragment(int position) {
           // Return a NEW fragment instance in createFragment(int).
           Fragment fragment = new DayFragmentISO();

           Bundle args = new Bundle();
           args.putString("title", pages.get(position).getDayTitle());
           //Tells the fragment this is an existing day
           //so it knows to look for a focusList and dayExercises
           args.putBoolean("existing",true);
           args.putSerializable("focusList", pages.get(position).getFocusGroup()));
           args.putSerializable("dayExercises", pages.get(position).getDayExercises());
           fragment.setArguments(args);
           return fragment;
       }

       @Override
       public int getItemCount() {
           // This tells the adapter how many fragments to create.
           // We want one for each page, so we'll pass it the size of our data.
           return pages.size();
       }

   }

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