Question

我最近将代码从活动转移到片段，但是在尝试在片段内调用回收器视图适配器的公共方法“ attachDatabaseReadListener（）”时遇到此错误：

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.digicube.RecyclerViewAdapter.attachDatabaseReadListener()' on a null object reference

在转到片段之前，我可以通过调用adapter.publicMethod()之类的适配器来调用适配器的公共方法，但是它现在在片段中不起作用。

这是我的RecyclerView适配器的代码：

import ...

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder> {

    ...

    public RecyclerViewAdapter(Callback callback, Context context) {
        mCallback = callback;
        this.mContext = context;

        // ArrayList to hold task objects from Firebase database for recyclerViewAdapter
        mBrands = new ArrayList<>();

        // Initialize Firebase Components
        mFirebaseDatabase = FirebaseDatabase.getInstance();
        mBrandsDatabaseReference = mFirebaseDatabase.getReference().child("brands");
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.brand_layout, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, final int position) {
        final TaskStructure taskStructure = mBrands.get(position);
        holder.mBrandNameTextView.setText(taskStructure.getBrandName());
        holder.mCategoryTextView.setText(taskStructure.getCategory());
        holder.mAvgUserRatingTextView.setText(String.valueOf(taskStructure.getAvgUserRating()));
        holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ...
            }
        });
    }

    @Override
    public int getItemCount() {
        Collections.sort(mBrands, TaskStructure.AverageUserRatingComparator);
        return mBrands.size();
    }

    class ViewHolder extends RecyclerView.ViewHolder {
        private TextView mBrandNameTextView;
        private TextView mCategoryTextView;
        private TextView mAvgUserRatingTextView;
        private ImageView mBrandImageView;

        public ViewHolder(final View itemView) {
            super(itemView);
            mBrandNameTextView = itemView.findViewById(R.id.brand_name);
            mCategoryTextView = itemView.findViewById(R.id.category);
            mAvgUserRatingTextView = itemView.findViewById(R.id.avg_user_rating);
            mBrandImageView = itemView.findViewById(R.id.brand_image);
        }
    }

    public void attachDatabaseReadListener() {
        if (mChildEventListener == null) {
            mChildEventListener = new ChildEventListener() {
               ...
            };
            mBrandsDatabaseReference.addChildEventListener(mChildEventListener);
        }
    }

}

我正在片段的onViewCreated方法上启动它：

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        recyclerViewBrands = view.findViewById(R.id.recycler_view_brands);
        recyclerViewBrands.setLayoutManager(new LinearLayoutManager(this.getActivity()));

        brandsAdapter = new com.example.digicube.RecyclerViewAdapter(this, getActivity());
        recyclerViewBrands.setAdapter(brandsAdapter);
    }

    // Method call by activity on signed in
    public void attachListener(){
        brandsAdapter.attachDatabaseReadListener();
    }

Answer 1

通过在片段的onViewCreated（）方法末尾调用attachDatabaseReadListener()方法来使其工作。以前，我是在公共方法attachListener()上调用它的，该方法在onViewCreated（）之前由活动执行，导致错误！

感谢Mike M.的评论，这有助于我理解问题。

在片段内调用RecyclerView适配器方法

1 个答案: