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显示错误：尝试在空对象引用上调用接口方法'int java.util.List.size（）'。

android
8
我正在创建一个显示联系人列表的应用程序，使用listview。但是当我运行我的应用程序时，遇到了以下错误，而我很难解决它：
04-05 13:41:48.868 2488-2488/? E/AndroidRuntime: FATAL EXCEPTION: main
                                             Process: com.kksworld.jsonparsing, PID: 2488
                                             java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                                                 at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:337)
                                                 at android.widget.ListView.setAdapter(ListView.java:491)
                                                 at com.kksworld.jsonparsing.MainActivity$JSONTask.onPostExecute(MainActivity.java:120)
                                                 at com.kksworld.jsonparsing.MainActivity$JSONTask.onPostExecute(MainActivity.java:48)
                                                 at android.os.AsyncTask.finish(AsyncTask.java:651)
                                                 at android.os.AsyncTask.-wrap1(AsyncTask.java)
                                                 at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
                                                 at android.os.Handler.dispatchMessage(Handler.java:102)
                                                 at android.os.Looper.loop(Looper.java:148)
                                                 at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                 at java.lang.reflect.Method.invoke(Native Method)
                                                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)

以下是我的代码:

MainActivity.java

package com.kksworld.jsonparsing;
import android.content.Context;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import com.kksworld.jsonparsing.models.ContactModel;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

private ListView listView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    listView = (ListView)findViewById(R.id.lvContacts)
    }


public class JSONTask extends AsyncTask<String,String,List<ContactModel>>{

    @Override
    protected List<ContactModel> doInBackground(String... params) {

        HttpURLConnection httpURLConnection = null;
        BufferedReader bufferedReader = null;

        try {
            URL url = new URL(params[0]);
            httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.connect();
            InputStream inputStream = httpURLConnection.getInputStream();
            // Through this we can read the data line by line
            bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
            StringBuffer stringBuffer = new StringBuffer();
            String line = "";
            while ((line = bufferedReader.readLine()) != null) {
                stringBuffer.append(line);
            }

            String finalJSON = stringBuffer.toString();
            JSONArray mainArray = new JSONArray(finalJSON);
            JSONObject parentObject = mainArray.getJSONObject(0);
            JSONArray parentArray = parentObject.getJSONArray("contacts");

            List<ContactModel> contactModelList = new ArrayList<>();

            // For parsing a list of items you have to use for loop
            for(int i=0;i<11;i++) {
                JSONObject finalObject = parentArray.getJSONObject(i);
                ContactModel contactModel = new ContactModel();
                contactModel.setName(finalObject.getString("name"));
                contactModel.setPhone(finalObject.getString("phone"));
                contactModel.setEmail(finalObject.getString(("email")));
                contactModel.setOfficePhone(finalObject.getString(("officePhone")));
                contactModel.setLatitude((float) finalObject.getDouble("latitude"));
                contactModel.setLongitude((float) finalObject.getDouble("longitude"));
                // Adding the finalObject in the List
                contactModelList.add(contactModel);

            }
            return contactModelList;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (JSONException e) {
            e.printStackTrace();
        } finally {
            if (httpURLConnection != null) {
                httpURLConnection.disconnect();
                try {
                    if (bufferedReader != null) {
                        bufferedReader.close();
                    }
                }catch(IOException e){
                    e.printStackTrace();
                }
            }
        }
        return null;
    }

    // Value returned by doInBackGround will be passed here...

    @Override
    protected void onPostExecute(List<ContactModel> result) {
        super.onPostExecute(result);

        ContactAdapter contactAdapter = new ContactAdapter(getApplicationContext(),R.layout.row,result);
        listView.setAdapter(contactAdapter);
    }
}

public class ContactAdapter extends ArrayAdapter{

    private List<ContactModel> contactModelList;
    private int resource;
    private LayoutInflater inflater;

    public ContactAdapter(Context context, int resource, List<ContactModel> objects) {
        super(context, resource, objects);
        contactModelList=objects;
        this.resource=resource;
        inflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        if(convertView == null)
        {
            convertView = inflater.inflate(resource,null);
        }
        TextView textViewName;
        TextView textViewEmail;
        TextView textViewPhone;
        TextView textViewOfficePhone;

        textViewName = (TextView)convertView.findViewById(R.id.textViewInsertName);
        textViewEmail= (TextView)convertView.findViewById(R.id.textViewInsertEmail);
        textViewPhone= (TextView)convertView.findViewById(R.id.textViewInsertPhone);
        textViewOfficePhone = (TextView)convertView.findViewById(R.id.textViewInsertOfficeNumber);

        textViewName.setText(contactModelList.get(position).getName());
        textViewEmail.setText(contactModelList.get(position).getEmail());
        textViewPhone.setText(contactModelList.get(position).getPhone());
        textViewOfficePhone.setText(contactModelList.get(position).getOfficePhone());

        return convertView;
    }
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_refresh) {
        new JSONTask().execute("some_url");
        return true;
    }

    return super.onOptionsItemSelected(item);
}

我需要帮助。

有人可以帮忙吗？

我的猜测是你的HTTP请求返回的响应未解析为JSON，可能在AsyncTask中遇到了JSONException，导致任务返回null...然后你将其提供给ContactAdapter并触发了NullPointerException。 - Robert Nekic
在 doInBackground() 到 onPostExecute() 的传递中，contactModelList 有可能是 null 吗？ - ben_joseph
你可以在这里找到有关如何在ArrayAdapter.getCount()中获得NullPointerException的详细解释。http://stackoverflow.com/questions/36212453/my-object-is-not-null-but-im-getting-a-null-pointer-exception/36212723#36212723 - Shadab Ansari
4个回答
25
Extract this line
List<ContactModel> contactModelList = new ArrayList<>();

在 try 块之前，使用 return contactModelList; 替换 return null;

看起来你的解析失败了，并且将一个 null 对象传递给了 onPostExecute()。

2

你传递给适配器的List对象为空。因此，当适配器调用list.Size()时，会抛出空指针异常。

使用调试器，在com.kksworld.jsonparsing.MainActivity$JSONTask.onPostExecute(MainActivity.java:48)附近设置断点，如果无法解决问题，请从那里开始向后工作。

可以假设你的JSON解析失败了。

1

也许其他人会寻找答案。这种方法也可以作为错误预防。

public int getItemCount() {
        if(list==null) return 0;
        return list.size();
    }
0

我正在按照 YouTube 视频搭建类似的应用程序时，遇到了相同的错误，但我还看到了“不允许网站.com使用明文 HTTP 流量”的错误，所以我在这里找到了解决方案android:usesCleartextTraffic="true" Android 8: Cleartext HTTP traffic not permitted，它可以解决所有问题而无需修改 MainActivity。顺便说一句，在 API 16 上没有显示错误。

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