适配器在为其分配数据集时未更新

Question

卡在这个 java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法 int java.util.ArrayList.size()。 上述错误已解决。 但现在的问题是： 当我们将适配器设置为回收器视图时，我们从 MainActivity 分配给它的数组列表不会改变我们的显示，我们只是得到一个空显示，即没有数据集分配给适配器，如何纠正它

MainActivity.java

public class MainActivity extends AppCompatActivity {
ArrayList<News> arrayList;
RecyclerView recyclerView;
String abcnews="https://newsapi.org/v2/top-headlines?country=us&category=business&apiKey=e0322279ec2b4440b84cf4b9c8d2d348";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    recyclerView=findViewById(R.id.rv);
    LinearLayoutManager linearLayoutManager=new LinearLayoutManager(this,LinearLayoutManager.VERTICAL,false);
    recyclerView.setLayoutManager(linearLayoutManager);
    OkHttpClient okHttpClient=new OkHttpClient();
    final Request request=new Request.Builder()
            .url(abcnews)
            .build();
    okHttpClient.newCall(request).enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {

        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {
            String string=response.body().string();
            try {
                Log.e("test","yolo");
                arrayList=jsonParse(string);
            } catch (JSONException e) {
                e.printStackTrace();
            }
            final NewsAdapter newsAdapter=new NewsAdapter(MainActivity.this,arrayList);
            recyclerView.post(new Runnable() {
                @Override
                public void run() {
                    recyclerView.setAdapter(newsAdapter);

                }
            });
        }
    });
}
private ArrayList<News> jsonParse(String s)throws JSONException{
    ArrayList<News> list=new ArrayList<>();
    JSONObject jsonObject=new JSONObject(s);
    JSONArray jsonArray=jsonObject.getJSONArray("articles");
    for(int i=0;i<jsonArray.length();i++){
        JSONObject current=jsonArray.getJSONObject(i);
        String title=current.getString("title");
        String descrtiption=current.getString("description");
        String photo=current.getString("urlToImage");
        String more=current.getString("urt");
        News news=new News(title,descrtiption,photo,more);
        arrayList.add(news);
    }
    return list;
}

}

NewsAdapter.java

public class NewsAdapter extends RecyclerView.Adapter<NewsAdapter.NewsHolder> {
ArrayList<News> arrayList=null;
Context context;
NewsAdapter(Context c,ArrayList<News> list){
    this.context=c;
    this.arrayList=list;
}
class NewsHolder extends RecyclerView.ViewHolder {
    ImageView imageView;
    TextView title,description;
    public NewsHolder(View itemView) {
        super(itemView);
        title=itemView.findViewById(R.id.title);
        description=itemView.findViewById(R.id.description);
        imageView=itemView.findViewById(R.id.image);
    }
}

@Override
public NewsAdapter.NewsHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    return new NewsHolder(LayoutInflater.from(parent.getContext()).inflate(R.layout.item_row,parent,false));
}

@Override
public void onBindViewHolder(NewsAdapter.NewsHolder holder, int position) {
    News news=arrayList.get(position);
    holder.title.setText(news.getTitle());
    holder.description.setText(news.getDescription());
    Picasso.with(context)
            .load(news.getPhoto())
            .error(R.mipmap.ic_launcher_round)
            .placeholder(R.mipmap.ic_launcher_round)
            .into(holder.imageView);
}

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

}

News.kt

data class News(val title:String,val description:String,val photo:String,val more:String)     

LogCat 中的错误部分

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
                                                 at com.example.tanis.newsus.NewsAdapter.getItemCount(NewsAdapter.java:57)
                                                 at android.support.v7.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:3603)
                                                 at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:3408)
                                                 at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:3962)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1743)
                                                 at android.widget.LinearLayout.layoutHorizontal(LinearLayout.java:1732)
                                                 at android.widget.LinearLayout.onLayout(LinearLayout.java:1497)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
                                                 at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:443)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
                                                 at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1743)
                                                 at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1586)
                                                 at android.widget.LinearLayout.onLayout(LinearLayout.java:1495)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.widget.FrameLayout.layoutChildren(FrameLayout.java:336)
                                                 at android.widget.FrameLayout.onLayout(FrameLayout.java:273)
                                                 at com.android.internal.policy.PhoneWindow$DecorView.onLayout(PhoneWindow.java:2680)
                                                 at android.view.View.layout(View.java:16833)
                                                 at android.view.ViewGroup.layout(ViewGroup.java:5438)
                                                 at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2469)
                                                 at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2126)
                                                 at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1228)
                                                 at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6523)
                                                 at android.view.Choreographer$CallbackRecord.run(Choreographer.java:919)
                                                 at android.view.Choreographer.doCallbacks(Choreographer.java:710)
                                                 at android.view.Choreographer.doFrame(Choreographer.java:645)
                                                 at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:905)
                                                 at android.os.Handler.handleCallback(Handler.java:739)
                                                 at android.os.Handler.dispatchMessage(Handler.java:95)
                                                 at android.os.Looper.loop(Looper.java:148)
                                                 at android.app.ActivityThread.main(ActivityThread.java:5519)
                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

坚持了很久。

LogCat 中的警告部分 This is the link

提前致谢！

Answer 1

发生崩溃是因为您在 MainActivity.java 中初始化了一个空数组列表：

ArrayList<News> arrayList;

然后尝试在 NewsAdapter.java 中的空对象上调用 .size() 方法：

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

因此，将 Adapter 类的 getItemCount() 函数更改为

@Override
public int getItemCount() {
    return arrayList != null ? arrayList.size() : 0;
}

Answer 2

在您的 NewsAdapter.java 中

@Override
public int getItemCount() {
    if(arrayList != null)
       return arrayList.size();
    else
       return 0;
}