带有RecycleView+CardView的片段（致命异常）

Question

我是编程新手，我很难尝试设置这个recycleview。请帮帮我！如果你可以的话。我相信主要的问题出在主要的activity上。

错误原因：java.lang.NullPointerException：尝试对com.example.application.fragments.home.HomeFragment.onCreate（HomeFragment.java:32)上的空对象引用调用虚拟方法“void androidx.recyclerview.widget.recyclerview.setLayoutManager（androidx.recyclerview.widget.recyclerview$LayoutManager）”

这是主要活动

    private BottomNavigationView bottomNavigationView;
    private RecyclerView recyclerView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        bottomNavigationView = findViewById(R.id.bottonNavigationView);

        loadFragment(new HomeFragment());
        setBottomNavigationView();

    }

    private void loadFragment(Fragment fragment) {
        // create a FragmentManager
        FragmentManager fragmentManager = getSupportFragmentManager();
        // create a FragmentTransaction to begin the transaction and replace the Fragment
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        // replace the FrameLayout with new Fragment
        fragmentTransaction.replace(R.id.idMainFrameLayout, fragment);
        fragmentTransaction.commit(); // save the changes
        //crédits abhiandroid
    }

    @SuppressLint("NonConstantResourceId")
    public void setBottomNavigationView() {
        bottomNavigationView.setOnNavigationItemSelectedListener(item -> {

            switch (item.getItemId()) {
                case R.id.btnSearch:
                    loadFragment(new SearchFragment());
                    break;
                case R.id.btnFeed:
                    loadFragment(new FeedFragment());
                    break;
                case R.id.btnShedule:
                    loadFragment(new SheduleFragment());
                    break;
                case R.id.btnProfile:
                    loadFragment(new ProfileFragment());
                    break;
                default:
                    loadFragment(new HomeFragment());
            }
            return true;
        });
    }
}```

AND THE FRAGMENT

```class HomeFragment extends Fragment {

    private RecyclerView recyclerView = getView().findViewById(R.id.recycleViewHome);

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
        MyAdapter myAdapter = new MyAdapter(getContext(), getMyList());
        recyclerView.setAdapter(myAdapter);
    }

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

        View view = inflater.inflate(R.layout.fragment_home, container, false);
        RecyclerView recyclerView = getView().findViewById(R.id.recycleViewHome);
        return view;
    }

    private ArrayList<Store> getMyList() {

        Drawable drawable = getResources().getDrawable(R.drawable.ic_google_svg, null);
        ImageView imageView = null;

        TextView mTitle = null, mSubTitle = null;

        imageView.setImageDrawable(drawable);
        mTitle.setText("GOOGLE");
        mSubTitle.setText("Description");

        ArrayList<Store> storeArrayList = new ArrayList<>();

        Store store = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store1 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store2 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store3 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store4 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mSubTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store5 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store6 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store7 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        Store store8 = new Store();
        store.setPhotoId(imageView);
        store.setmTitle(mTitle);
        store.setSubTitle(mSubTitle);
        storeArrayList.add(store);

        return storeArrayList;
    }
}```

<<<<<<<<RECYCLEVIEW>>>>>>>>>>>

```class MyHolder extends RecyclerView.ViewHolder {

    private ImageView mImageView;
    private TextView mTitle, mSubTitle;

    public MyHolder(@NonNull View itemView) {
        super(itemView);
        this.mImageView = itemView.findViewById(R.id.btnImageStore);
        this.mTitle = itemView.findViewById(R.id.txtTitleStore);
        this.mSubTitle = itemView.findViewById(R.id.txtsubTitle);
    }

    public ImageView getmImageView() {
        return mImageView;
    }

    public void setmImageView(ImageView mImageView) {
        this.mImageView = mImageView;
    }

    public TextView getmTitle() {
        return mTitle;
    }

    public void setmTitle(TextView mTitle) {
        this.mTitle = mTitle;
    }

    public TextView getmSubTitle() {
        return mSubTitle;
    }

    public void setmSubTitle(TextView mSubTitle) {
        this.mSubTitle = mSubTitle;
    }
}```

```class MyAdapter extends RecyclerView.Adapter<MyHolder> {

    private Context context;
    ArrayList<Store> storeArrayList;

    public MyAdapter(Context context, ArrayList<Store> storeArrayList) {
        this.context = context;
        this.storeArrayList = storeArrayList;
    }

    @NonNull
    @Override
    public MyHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.card_layout_stores, null, false);
        return new MyHolder(view);

    }

    @Override
    public void onBindViewHolder(@NonNull MyHolder myholder, int position) {

        myholder.setmImageView(storeArrayList.get(position).getPhotoId());
        myholder.setmTitle(storeArrayList.get(position).getmTitle());
        myholder.setmSubTitle(storeArrayList.get(position).getSubTitle());
    }

    @Override
    public int getItemCount() {
        return storeArrayList.size();
    }
}```

Answer 1

出现此错误是因为onCreateView()方法是在onCreateView()方法之前调用的，因此您尝试在初始化RecyclerView之前设置适配器和LayoutManager，因此需要在onCreateView()方法中添加适配器和LayoutManager。请在此处检查片段生命周期:https://developer.android.com/guide/fragments/lifecycle#states

此外，您还创建了两个RecyclerView实例，一个在片段上，另一个在onCreateView()中，因此您需要将所有代码从onCreateView()移动到片段中的onCreateView()中