Retrofit: java.lang.NullPointerException

Question

NullPointerException且未连接适配器；每次我在genymotion上测试这个应用程序时，都会出现跳过布局的情况。我读过和同一问题相关的其他问题，但没有任何帮助。

logcat

09-20 13:21:13.738 4658-4658/com.example.kiran.detailapp E/AndroidRuntime：致命异常：main流程：com.example.kiran.detailapp，PID:4658java.lang.NullPointerException：试图对null对象引用调用接口方法"int java.util.List.size()"网址：com.example.kiran.detailapp.RecyclerAdapter.getItemCount(Recyclerdapter.java:38)在android.support.v7.widget.CyclerView.dispatchLayoutStep1(RecyclerView.java：3493)在android.support.v7.widget.CyclerView.onMeasure(RecyclerView.java：3019)在android.view.view.measure(view.java：17430)在android.widget.RelativeLayout.measureChild(RelativeLayut.java:697)在android.widget.RelativeLayout.onMeasure(RelativeLayout.java：481)在android.view.view.measure(view.java：17430)在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java：5463)在android.widget.FrameLayout.onMeasure(FrameLayout.java:430)在android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)在android.view.view.measure(view.java：17430)在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java：5463)位于android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlaylayLayout.java:393)在android.view.view.measure(view.java：17430)在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java：5463)在android.widget.FrameLayout.onMeasure(FrameLayout.java:430)在android.view.view.measure(view.java：17430)在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java：5463)在android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)在android.widget.LinearLayout.measureVertical(LinearLayout.java:722)在android.widget.LinearLayout.onMeasure(LinearLayout.java:613)在android.view.view.measure(view.java：17430)在android.view.ViewGroup.measureChildWithMargins(ViewGroup.java：5463)在android.widget.FrameLayout.onMeasure(FrameLayout.java:430)网址：com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)在android.view.view.measure(view.java：17430)在android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)在android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1116)在android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)在android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)在android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)在android.view.Cchoreographer$CallbackRecord.run(Choreographer.java:767)在android.view.Cchoreographer.doCallbacks(Choreographer.java:580)在android.view.Cchoreographer.doFrame(Choreographer.java:550)在android.view.Cchoreographer$FrameDisplayEventReceiver.run(Choreographer.java：753)在android.os.Handler.handleCallback(Handler.java:739)在android.os.Handler.dispatchMessage(Handler.java:95)在android.os.Looper.loop(Looper.java:135)在android.app.ActivityThread.main(ActivityThreads.java:5221)位于java.lang.reflect.Method.ioke(本机方法)位于java.lang.reflect.Method.ioke(Method.java:372)网址：com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)网址：com.android.internal.os.ZygoteInit.main(ZygoteNit.java:694)

联系方式.java

package com.example.kiran.detailapp;
import com.google.gson.annotations.SerializedName;
public class Contact {
@SerializedName("name")
private String Name;
@SerializedName("email")
private String Email;
public String getName() {
return Name;
}
public void setName(String name) {
Name = name;
}
public String getEmail() {
return Email;
}
public void setEmail(String email) {
Email = email;
}
}

ApiClient.java

package com.example.kiran.detailapp;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;
public class ApiClient {
public static final String BASE_URL = "http://10.0.3.2/ContactApp/";
public static Retrofit retrofit = null;
public static Retrofit getApiClient()
{
retrofit = new Retrofit.Builder().baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create()).build();
return retrofit;
}
}

ApiInterface.java

package com.example.kiran.detailapp;
import java.util.List;
import retrofit2.Call;
import retrofit2.http.POST;
public interface ApiInterface {

@POST("ReadContacts.php")
Call<List<Contact>> getContacts();
}

RecyclerAdapter.java

package com.example.kiran.detailapp;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import org.w3c.dom.Text;
import java.util.List;
public class RecyclerAdapter extends RecyclerView.Adapter<RecyclerAdapter.MyViewHolder> {
private List<Contact> contacts;
public RecyclerAdapter(List<Contact> contacts)
{
this.contacts = contacts;
}
@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.row_item,parent,false);
return new MyViewHolder(view);
}
@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
holder.Name.setText(contacts.get(position).getName());
holder.Email.setText(contacts.get(position).getEmail());
}
@Override
public int getItemCount() {
return contacts.size();
}
public class MyViewHolder extends RecyclerView.ViewHolder {
TextView Name,Email;
public MyViewHolder(View itemView) {
super(itemView);
Name = (TextView) itemView.findViewById(R.id.name);
Email = (TextView) itemView.findViewById(R.id.email);
}
}
}

MainActivity.java

package com.example.kiran.detailapp;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import java.util.List;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
public class MainActivity extends AppCompatActivity {
private RecyclerView recyclerView;
private RecyclerView.LayoutManager layoutManager;
private RecyclerAdapter adapter;
private List<Contact> contacts;
private ApiInterface apiInterface;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
adapter = new RecyclerAdapter(contacts);
recyclerView = (RecyclerView) findViewById(R.id.Recycleview);
layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
recyclerView.setHasFixedSize(true);
apiInterface = ApiClient.getApiClient().create(ApiInterface.class);
Call<List<Contact>> call = apiInterface.getContacts();
call.enqueue(new Callback<List<Contact>>() {
@Override
public void onResponse(Call<List<Contact>> call, Response<List<Contact>> response) {
contacts = response.body();
recyclerView.setAdapter(adapter);
adapter.notifyDataSetChanged();
}
@Override
public void onFailure(Call<List<Contact>> call, Throwable t) {
}
});
}
}

Answer 1

您忘记初始化数组列表

private List<Contact> contacts;

那么在onCreate()中执行这个

contacts= new ArrayList<>();

Answer 2

我在代码中看到了一些问题。

1) 您的contacts变量将在适配器内未初始化。

2) 您已经在后台线程中运行的onResponse方法中设置了recyclerview。

解决方案：

您需要初始化联系人，然后在onResponse方法中为其赋值，并在activity'srunOnUiThread方法中设置适配器。

希望这个代码能有所帮助。

1) 初始化联系人

private List<Contact> contacts = new ArrayList<>();

2) 在onResponse方法中给出期望值，并在那里初始化适配器，并从onCreate方法中删除初始化。

call.enqueue(new Callback<List<Contact>>() {
@Override
public void onResponse(Call<List<Contact>> call, Response<List<Contact>> response) {
contacts = response.body();
adapter = new RecyclerAdapter(contacts);
runOnUiThread(() -> {
recyclerView.setAdapter(adapter);
adapter.notifyDataSetChanged();
});
}
@Override
public void onFailure(Call<List<Contact>> call, Throwable t) {
}