java 空指针异常

Question

目前正在使用 java 和 android studio 开发移动应用程序。（初学者）我有 3 个片段（ToDo、Doing、Done），顶部有一个 tablayout（ToDo、Ongoing、Finished）和一个带有添加按钮的工具栏。

添加按钮应该创建一个列表，允许用户修改每个list(Recyclerviewer)

的内容

获取错误：java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setAdapter(androidx.recyclerview.widget.RecyclerView$Adapter)' on a null object reference

下面是截图

代码

public class kanbanManagement extends Fragment {

    // TODO: Rename and change types of parameters
    TabLayout tabLayout;
    private ViewPager2 viewPager2;
    private MyFragment adapter;
    private RecyclerView recyclerView2;
    private com.example.myhouse.recyclerKanbanAdapter recyclerKanbanAdapter;
    ArrayList<kanbanItems>listKanban;
    Button btn_Add;
    public kanbanManagement() {
        // Required empty public constructor
    }


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view=inflater.inflate(R.layout.fragment_kanban_management, container, false);
        tabLayout=view.findViewById(R.id.tabLayout);
        viewPager2=view.findViewById(R.id.viewPager2);
        tabLayout.addTab(tabLayout.newTab().setText("To Do"));
        tabLayout.addTab(tabLayout.newTab().setText("On going"));
        tabLayout.addTab(tabLayout.newTab().setText("Finished"));
        FragmentManager fragmentManager = getParentFragmentManager();
        adapter = new MyFragment(fragmentManager , getLifecycle());
        viewPager2.setAdapter(adapter);
        tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                viewPager2.setCurrentItem(tab.getPosition());
            }

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

            }

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

            }
        });

        viewPager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                tabLayout.selectTab(tabLayout.getTabAt(position));
            }
        });
        recyclerView2=view.findViewById(R.id.taskList);
        listKanban= new ArrayList<>();
        btn_Add=view.findViewById(R.id.btnAdd);
        recyclerKanbanAdapter= new recyclerKanbanAdapter(getActivity(),listKanban);
        btn_Add.setOnClickListener(v -> {
            recyclerView2.setAdapter(recyclerKanbanAdapter);

        });

        return view;
    }
}

Answer 1

抱歉，我没有完全理解您的代码。因此，无需检查您的完整代码。我的猜测是您没有使用 LayoutManager 来显示您的 listKanban。所以再次添加 layoutManager 代码和运行。

在 onCreateView.

中添加以下代码

LinearLayoutManager myLayoutManager = new LinearLayoutManager(getContext(), LinearLayoutManager.VERTICAL, false);
        recyclerView2.setLayoutManager(myLayoutManager); // your recyclerView where you want to show your `ArrayList`.

如果问题还没有解决，请告诉我。

java 空指针异常

java NullPointer Exception

android

android-recyclerview