Question

我正在尝试从url解析JSON，但它无法正常工作..

错误在第72行第42行

MainActivity$ReadJSON.onPostExecute(MainActivity.java:71)
   at com.ac.app.MainActivity$ReadJSON.onPostExecute(MainActivity.java:42)

这是代码

public class MainActivity extends AppCompatActivity {

    ArrayList<Video> arrayList;
    ListView lv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        arrayList = new ArrayList<>();
        lv = (ListView) findViewById(R.id.listView);
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                new ReadJSON().execute("http://example.com/api/data");
            }
        });
    }



    class  ReadJSON extends AsyncTask<String, String, String>//line no 42
    {
        @Override
        protected String doInBackground(String... params) {
            return readURL(params[0]);
        }

        @Override
        protected void onPostExecute(String content) {
            try {
                JSONObject jsonObject = new JSONObject(content);
                JSONArray jsonArray = jsonObject.getJSONArray("videos");
                for(int i=0;i<jsonArray.length();i++)
                {
                    JSONObject videoObject = jsonArray.getJSONObject(i);
                    arrayList.add(new Video(
                            videoObject.getString("picture"),
                            videoObject.getString("title"),
                            videoObject.getString("pageid")
                    ));
                }

            } catch (JSONException e) {
                e.printStackTrace();
            }
            CustomListAdapter adapter = new CustomListAdapter(
                    getApplicationContext(), R.layout.custom_list_layout, arrayList);
            lv = (ListView) findViewById(R.id.listView);
            lv.setAdapter(adapter); //line no 71

        }
    }

java.lang.NullPointerException：尝试调用虚方法＆＃39; void android.widget.ListView.setAdapter（android.widget.ListAdapter）＆＃39;在...上 null对象引用

XML

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.ac.app.MainActivity">


<ListView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/listView"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />
</RelativeLayout>

Answer 1

这意味着它无法从您的xml文件中找到listview，从而抛出nullpointer异常。检查你的xml文件

Answer 2

对于此错误，将有两种可能性

1）列表没有在xml中正确定义（可能是listview id如android：id =“@ android：id / list”表示不是android：id =“@ + id / listView”）

2）arrayList将为null，因为可能未在arrayList中添加Video。

所以在onPostExecute：

的开头放在下面的行

arrayList = new ArrayList<Video>();

Answer 3

我试试这个：

public class MainActivity extends AppCompatActivity {

ArrayList<Video> arrayList;
ListView lv;
CustomListAdapter adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    arrayList = new ArrayList<>();
    lv = (ListView) findViewById(R.id.listView);

    new ReadJSON().execute("http://example.com/api/data");
}



class  ReadJSON extends AsyncTask<String, String, String>
{
    @Override
    protected String doInBackground(String... params) {
        return readURL(params[0]);
    }

    @Override
    protected void onPostExecute(String content) {
        try {
            JSONObject jsonObject = new JSONObject(content);
            JSONArray jsonArray = jsonObject.getJSONArray("videos");
            for(int i=0;i<jsonArray.length();i++)
            {
                JSONObject videoObject = jsonArray.getJSONObject(i);
                arrayList.add(new Video(
                        videoObject.getString("picture"),
                        videoObject.getString("title"),
                        videoObject.getString("pageid")
                ));
            }

        } catch (JSONException e) {
            e.printStackTrace();
        }
        adapter = new CustomListAdapter(getApplicationContext(), R.layout.custom_list_layout, arrayList);
        lv.setAdapter(adapter);

    }
}

在listview android中的空对象引用

3 个答案: