尝试从自定义适配器调用 onClick 方法的空指针异常答案

【问题标题】：Null pointer exception trying to invoke onClick method from custom Adapter尝试从自定义适配器调用 onClick 方法的空指针异常
【发布时间】：2020-05-29 12:15:11
【问题描述】：

我试图在 Android Studio 中编写自己的适配器类，看起来像这样：

public class Adapter extends RecyclerView.Adapter<Adapter.MyViewHolder>{

    private List<Article> articles;
    private Context context;
    private OnItemClickListener onItemClickListener;


    public Adapter(List<Article> articles, Context context) {
        this.articles = articles;
        this.context = context;
    }

    public void setOnItemClickListener(OnItemClickListener onItemClickListener){
        this.onItemClickListener = onItemClickListener;
    }

    public interface OnItemClickListener {
        void onItemClick(View view, int position);
    }

    public class MyViewHolder extends RecyclerView.ViewHolder implements  View.OnClickListener {

        TextView title, desc, author, published_ad, source, time;
        ImageView imageView;
        ProgressBar progressBar;
        OnItemClickListener onItemClickListener;

        public MyViewHolder(View itemView, OnItemClickListener onItemClickListener) {

            super(itemView);

            itemView.setOnClickListener(this);
            title = itemView.findViewById(R.id.title);
            desc = itemView.findViewById(R.id.decs);
            author = itemView.findViewById(R.id.author);
            published_ad = itemView.findViewById(R.id.publishedAt);
            source = itemView.findViewById(R.id.source);
            time = itemView.findViewById(R.id.time);
            imageView = itemView.findViewById(R.id.img);
            progressBar = itemView.findViewById(R.id.progress_load_photo);

            this.onItemClickListener = onItemClickListener;

        }

        @Override
        public void onClick(View v) {
            onItemClickListener.onItemClick(v, getAdapterPosition());
        }
    }
}

我使用适配器的类如下所示：

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

        recyclerView = view.findViewById(R.id.recyclerView);
        layoutManager = new LinearLayoutManager(getContext());
        recyclerView.setLayoutManager(layoutManager);
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        recyclerView.setNestedScrollingEnabled(false);

        adapter = new Adapter(articles, getActivity().getApplicationContext());
        recyclerView.setAdapter(adapter);
        LoadJson();

        return view;
    }

我收到关于空指针异常的错误：“尝试在空对象引用上调用接口方法 'void com.example.app2.Adapter$OnItemClickListener.onItemClick(android.view.View, int)'”

【问题讨论】：

您忘记在Adapter 上设置OnItemClickListener。您可以将其设置在您正在执行 RecyclerView 和 Adapter 设置的其余部分的位置。

标签： java android android-fragments adapter

【解决方案1】：

在您设置之前，您的字段 Adapter.onItemClickListener 为空（您在实际代码中从未这样做过）

所以你应该在尝试通知之前测试是否有监听器。

@Override
public void onClick(View v) {
    if (onItemClickListener != null)
        onItemClickListener.onItemClick(v, getAdapterPosition());
}

我猜你会在你的 onCreateView 方法中使用它:)

adapter = new Adapter(articles, getActivity().getApplicationContext());
adapter.setOnItemClickListener(() -> { System.out.println("Click !") });
recyclerView.setAdapter(adapter);

【讨论】：