getContext（） 在调用 update RecyclerView 函数时使我的应用程序崩溃？

问：

当我想访问特定片段时，我的应用程序总是崩溃。错误是：

致命异常：main 进程：com.akstudios.kindergarten，PID：15680 java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法“void androidx.recyclerview.widget.RecyclerView.setLayoutManager（androidx.recyclerview.widget.RecyclerView$LayoutManager）” 在com.akstudios.kindergarten.GroupbookKrippeFragment.setRecyclerView（GroupbookKrippeFragment.java：56）

我过去看到很多人都有类似的问题，但没有想法或建议帮助我解决我的问题。

我尝试使用 getContext（）、onAttach（）、GroupbookKrippeFragment.this.getContext（） 和其他一些方式获取上下文。

更新开始 （12.11.22）

我能够进一步缩小问题范围。 该应用程序稳定且有效，直到我进行了以下更改：

1.）为了使用来自我的数据库的 recyclerview 条目更新我的 tableList，我将以下代码封装到 GroupbookKrippeFragment.java 中的新方法中：

public void setRecyclerView(List<ChildrenTable> childrenTableList) {
        recycler_view.setLayoutManager(new LinearLayoutManager(getContext()));
        childAdapter = new ChildAdapter(getContext(), childrenTableList);
//        childAdapter = new ChildAdapter(getContext());
        recycler_view.setAdapter(childAdapter);
    }

2.） 我在 AllChildren 的 onPostExecute（） AsyncTask 的 DatabaseHelper.java 类中调用了此方法：

public class DatabaseHelper {
Context context;
GroupbookKrippeFragment groupbookKrippeFragment = new GroupbookKrippeFragment();

public DatabaseHelper(Context context) {
    this.context = context;
}

public static DatabaseHelper getInstance(Context context) {
    return new DatabaseHelper(context);
}

// Insert Data
// TODO: Add all columns as Input parameter for Database
public void addNewChild(
        String child_name, String child_surname, String child_birthday,
        String child_birthday_location, String child_confession, String child_nationality) {

    class NewChild extends AsyncTask<Void, Void, ChildrenTable> {

        @Override
        protected ChildrenTable doInBackground(Void... voids) {
            ChildrenTable childrenTable = new ChildrenTable();

            // TODO: Link all columns for Database
            childrenTable.setChild_name(child_name);
            childrenTable.setChild_surname(child_surname);
            childrenTable.setChild_birthday(child_birthday);
            childrenTable.setChild_birthday_location(child_birthday_location);
            childrenTable.setChild_confession(child_confession);
            childrenTable.setChild_nationality(child_nationality);

            DatabaseClient.getInstance(context)
                    .getChildrenDatabase()
                    .childrenDAO()
                    .insertData(childrenTable);
            return childrenTable;
        }

        @Override
        protected void onPostExecute(ChildrenTable childrenTable) {
            super.onPostExecute(childrenTable);
            if (childrenTable != null) {
                Toast.makeText(
                        context,
                        childrenTable.getChild_name() + "\n" +
                        childrenTable.getChild_surname(), Toast.LENGTH_SHORT).show();
            }
        }
    }

    NewChild newChild = new NewChild();
    newChild.execute();
}

// Show all data from ChildrenTable
public void getAllChildrenData() {
    class AllChildren extends AsyncTask<Void, Void, List<ChildrenTable>> {

        @Override
        protected List<ChildrenTable> doInBackground(Void... voids) {
            List<ChildrenTable> list = DatabaseClient.getInstance(context)
                    .getChildrenDatabase()
                    .childrenDAO()
                    .selectAll();
            return list;
        }

        @Override
        protected void onPostExecute(List<ChildrenTable> childrenTable) {
            super.onPostExecute(childrenTable);
            if (childrenTable != null && childrenTable.size() > 0) {
                groupbookKrippeFragment.setRecyclerView(childrenTable);
            }

        }
    }

    AllChildren allChildren = new AllChildren();
    allChildren.execute();
}

}

每当我注释掉这个方法调用并将该方法中的行移动到 GroupbookKrippeFragment 的 onViewCreated 中时，该应用程序再次稳定，但 recyclerView 数据没有显示并更新我的数据库。

更新结束 （12.11.22）

非常感谢您的帮助 - 这是我的文件：

GroupbookKrippeFragment.java

public class GroupbookKrippeFragment extends Fragment {

Button addChildBtn;
RecyclerView recycler_view;
ChildAdapter childAdapter;
DatabaseHelper helper;

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                         @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_groupbook_krippe, container, false);

    addChildBtn = view.findViewById(R.id.btn_groupBook_krippe_1);
    addChildBtn.setOnClickListener(view1 -> {
        Intent intent = new Intent(GroupbookKrippeFragment.this.getActivity(), GroupbookAddChildFragment.class);
        startActivity(intent);
    });

    recycler_view = view.findViewById(R.id.recycler_view);

    helper = DatabaseHelper.getInstance(getContext());
    helper.getAllChildrenData();

    return view;
}

public void setRecyclerView(List<ChildrenTable> childrenTableList) {
    recycler_view.setLayoutManager(new LinearLayoutManager(getContext()));
    childAdapter = new ChildAdapter(getContext(), childrenTableList);
    recycler_view.setAdapter(childAdapter);
  }
}

GroupbookFragment.java

public class GroupbookFragment extends Fragment {

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {

    // inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_groupbook, container, false);

    // Tab layout initialization
    TabLayout tabLayout = view.findViewById(R.id.tabLayout);
    ViewPager2 viewPager = view.findViewById(R.id.viewPager_groupBook);
    GroupbookAdapter adapterGroupBook = new GroupbookAdapter(getChildFragmentManager(), getLifecycle());
    viewPager.setAdapter(adapterGroupBook);

    // set 3 Tab titles
    tabLayout.addTab(tabLayout.newTab().setText("Krippe"));
    tabLayout.addTab(tabLayout.newTab().setText("Hafen"));
    tabLayout.addTab(tabLayout.newTab().setText("Kindergarten"));

    tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            viewPager.setCurrentItem(tab.getPosition());
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });

    viewPager.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
        @Override
        public void onPageSelected(int position) {
            tabLayout.selectTab(tabLayout.getTabAt(position));
        }
    });
    return view;
  }
}