坚持这个java.lang.NullPointerException：尝试在空对象引用上调用虚方法int java.util.ArrayList.size()。
上述错误已得到解决。
但现在的问题是：
我们从MainActivity分配给它的数组列表在我们将adAdter设置到我们的Recycler视图时没有改变我们的显示，我们只是得到一个空显示，即没有为适配器分配数据集，如何纠正
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)
长期坚持下去。
LogCat中的警告部分 This is the link
提前致谢！
答案 0 :(得分：2)
崩溃正在发生，因为你在MainActivity.java中初始化了一个null arrayList：
ArrayList<News> arrayList;
然后尝试在NewsAdapter.java中的null对象上调用.size（）方法：
@Override
public int getItemCount() {
return arrayList.size();
}
所以将Adapter类的getItemCount（）函数更改为
@Override
public int getItemCount() {
return arrayList != null ? arrayList.size() : 0;
}
答案 1 :(得分：1)
在NewsAdapter.java中
@Override
public int getItemCount() {
if(arrayList != null)
return arrayList.size();
else
return 0;
}