我正在尝试从 openlibrary 获取 API 请求。我使用网址中的查询:https://openlibrary.org/search.json?author=Plato 。这给了我一个结果，但是使用我编写的代码和 Postman，我得到一个空的 Json。我总是在 getItemCount() 处的文档列表中得到 nullpointerException。有趣的是，应用程序运行了一段时间，比如 20 秒，然后在 public int getItemCount() 上给了我空指针异常。 查了很多以前的答案，但没有帮助。 ( Android - RecyclerView NullPointerException getItemCount? ; What is a NullPointerException, and how do I fix it? )
我的界面:
package com.example.mark.retrofitpractice_01.service;
import com.example.mark.retrofitpractice_01.model.WorksByAuthor;
import com.example.mark.retrofitpractice_01.model.WorksBySubject;
import retrofit2.Call;
import retrofit2.http.GET;
import retrofit2.http.Headers;
import retrofit2.http.Path;
import retrofit2.http.Query;
public interface ApiInterface {
//@Headers({"Content-Type: application/json", "Accept: application/json"})
@GET("subjects/{subject}.json")
Call<WorksBySubject> getContacts(@Path("subject") String subject);
@Headers({"Content-Type: application/json", "Accept: application/json"})
@GET("/search.json")
Call<WorksByAuthor> getAuthor(@Query("author") String author);
}
我的适配器类:
package com.example.mark.retrofitpractice_01.service;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.example.mark.retrofitpractice_01.R;
import com.example.mark.retrofitpractice_01.model.Docs;
import com.example.mark.retrofitpractice_01.model.Work;
import com.example.mark.retrofitpractice_01.model.WorksByAuthor;
import java.util.ArrayList;
import java.util.List;
public class AuthorAdapter extends RecyclerView.Adapter<AuthorAdapter.MyViewHolder> {
private WorksByAuthor worksByAuthor;
//private List<Docs> docsList;
public AuthorAdapter() {
worksByAuthor = new WorksByAuthor();
}
public AuthorAdapter(WorksByAuthor worksByAuthor) {
this.worksByAuthor = worksByAuthor;
}
@NonNull
@Override
public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.author_row, parent, false);
return new MyViewHolder(view);
}
@Override
public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
//Docs docs = worksByAuthor.getDocsList().get(position);
//holder.bookTitle.setText(docs.getTitle_suggest());
//holder.bookAuthor.setText(docs.getAuthor_name());
holder.bookTitle.setText(worksByAuthor.getDocsList().get(position).getTitle_suggest());
}
@Override
public int getItemCount() {
return worksByAuthor.getDocsList().size();
}`enter code here`
public static class MyViewHolder extends RecyclerView.ViewHolder {
private TextView bookTitle;
//private TextView bookAuthor;
public MyViewHolder(View itemView) {
super(itemView);
bookTitle = itemView.findViewById(R.id.bookTitle);
//bookAuthor = itemView.findViewById(R.id.authorName);
}
}
}
我的结果 Activity 类别:
package com.example.mark.retrofitpractice_01;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import com.example.mark.retrofitpractice_01.model.ApiClient;
import com.example.mark.retrofitpractice_01.model.Docs;
import com.example.mark.retrofitpractice_01.model.WorksByAuthor;
import com.example.mark.retrofitpractice_01.service.ApiInterface;
import com.example.mark.retrofitpractice_01.service.AuthorAdapter;
import java.util.List;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
public class AuthorResutlActivity extends AppCompatActivity {
private RecyclerView recyclerView;
private RecyclerView.LayoutManager layoutManager;
private AuthorAdapter authorAdapter;
private WorksByAuthor worksByAuthor;
private List<Docs> docsList;
private ApiInterface apiInterface;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_author_result);
Retrofit.Builder builder = new Retrofit.Builder()
.baseUrl("https://openlibrary.org/")
.addConverterFactory(GsonConverterFactory.create());
Retrofit retrofit = builder.build();
recyclerView = findViewById(R.id.recyclerViewAuthor);
layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
recyclerView.setHasFixedSize(true);
apiInterface = retrofit.create(ApiInterface.class);
Intent intent = getIntent();
String author = intent.getStringExtra("author");
Call<WorksByAuthor> call = apiInterface.getAuthor(author);
call.enqueue(new Callback<WorksByAuthor>() {
@Override
public void onResponse(Call<WorksByAuthor> call, Response<WorksByAuthor> response) {
WorksByAuthor worksByAuthor = response.body();
authorAdapter = new AuthorAdapter(worksByAuthor);
recyclerView.setAdapter(authorAdapter);
}
@Override
public void onFailure(Call<WorksByAuthor> call, Throwable t) {
}
});
}
}
堆栈跟踪:
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.mark.retrofitpractice_01, PID: 3808
java.lang.NullPointerException
at com.example.mark.retrofitpractice_01.service.AuthorAdapter.getItemCount(AuthorAdapter.java:48)
at android.support.v7.widget.RecyclerView.dispatchLayoutStep1(RecyclerView.java:3493)
at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:3019)
at android.view.View.measure(View.java:16525)
at android.widget.RelativeLayout.measureChild(RelativeLayout.java:689)
at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:473)
at android.view.View.measure(View.java:16525)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5127)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:141)
at android.view.View.measure(View.java:16525)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5127)
at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:400)
at android.view.View.measure(View.java:16525)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5127)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at android.view.View.measure(View.java:16525)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5127)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
at android.view.View.measure(View.java:16525)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5127)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2358)
at android.view.View.measure(View.java:16525)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1962)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1159)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1341)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1046)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5663)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
at android.view.Choreographer.doCallbacks(Choreographer.java:574)
at android.view.Choreographer.doFrame(Choreographer.java:544)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5113)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:796)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:612)
at dalvik.system.NativeStart.main(Native Method)
I/Process: Sending signal. PID: 3808 SIG: 9
Application terminated.
最佳答案
您可以使用我创建的适配器类。在这个类中你永远不会得到 NullPointerException。可能您的列表为空，并且您正在像 Levi 所说的那样在 onItemCount 方法中获取其大小。
我建议您不要在适配器类中传递WorksByAuthor，而仅传递arraylist。因为你的 Adapter 类应该是通用的。
通过下面的适配器类，您需要调用adapter.insertItemsInList(pass_your_list)。适配器类本身将检查是否为 null。
public class AdapterDemo extends RecyclerView.Adapter<AdapterDemo.MyViewHolder> {
private ArrayList<String> list = new ArrayList<>();
private Context context;
class MyViewHolder extends RecyclerView.ViewHolder {
MyViewHolder(View view) {
super(view);
// TODO: 4/29/2018 add your code here
}
}
public void insertItemInList(String beanChat) {
if (list == null) list = new ArrayList<>();
list.add(beanChat);
notifyItemInserted(list.size() - 1);
}
public void insertItemsInList(ArrayList<String> myList) {
if (list == null) list = new ArrayList<>();
for (String beanChat : myList) {
insertItemInList(beanChat);
}
}
public ArrayList<String> getList() {
if (list == null) list = new ArrayList<>();
return list;
}
public void clearList() {
if (list == null) return;
list.clear();
notifyDataSetChanged();
}
public AdapterDemo(Context context) {
this.context = context;
}
@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View itemView = LayoutInflater.from(parent.getContext())
.inflate(R.layout.layout_nothing_to_display, parent, false);
return new MyViewHolder(itemView);
}
@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
String s = list.get(position);
// TODO: 4/29/2018 your code here
}
@Override
public int getItemCount() {
if (list == null) list = new ArrayList<>();
return list.size();
}
}
关于java - URL 查询给出的结果与 postman 不同，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50079746/