Android JSONArray NullPointerException，而url工作正常。

Question

你好，我正在尝试使用JSON从一个网站检索一些数据。我从一个教程中得到了一些代码，但是这个教程不起作用。我认为这是因为教程中的url不起作用，但当我尝试一个有效的url时，它仍然给出同样的错误。谁能告诉我我哪里做错了。我到处读到使用JSON应该是非常简单和容易的，但我真的很头疼尝试所有的教程和阅读所有的例子。

以下是教程中名为Main.java的代码：

public class Main extends ListActivity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.listplaceholder);

    ArrayList<HashMap<String, String>> mylist = new ArrayList<HashMap<String, String>>();

//the website I use lacks some security so I don't wanne put it here.
//its still under construction.       
    JSONObject json = JSONfunctions.getJSONfromURL("http://somewebsite/jsonexample.html");

    try{

        JSONArray  student = json.getJSONArray("student");

        for(int i=0;i<student.length();i++){                        
            HashMap<String, String> map = new HashMap<String, String>();    
            JSONObject e = student.getJSONObject(i);

            map.put("id",  String.valueOf(i));
            map.put("name", "Naam: " + e.getString("name"));
            map.put("age", "Leeftijd: " +  e.getString("age"));
            mylist.add(map);            
        }       
    }catch(JSONException e)        {
         Log.e("log_tag", "Error parsing data "+e.toString());
    }

    ListAdapter adapter = new SimpleAdapter(this, mylist , R.layout.main, 
                    new String[] { "name", "age" }, 
                    new int[] { R.id.item_title, R.id.item_subtitle });

    setListAdapter(adapter);

    final ListView lv = getListView();
    lv.setTextFilterEnabled(true);  
    lv.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {              
            @SuppressWarnings("unchecked")
            HashMap<String, String> o = (HashMap<String, String>) lv.getItemAtPosition(position);                   
            Toast.makeText(Main.this, "ID '" + o.get("id") + "' was clicked.", Toast.LENGTH_SHORT).show(); 

        }
    });
}
}

以下是教程中名为JSONfunctions.java的代码：

public class JSONfunctions {

public static JSONObject getJSONfromURL(String url){
    InputStream is = null;
    String result = "";
    JSONObject jArray = null;

    //http post
    try{
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(url);
            HttpResponse response = httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            is = entity.getContent();

    }catch(Exception e){
            Log.e("log_tag", "Error in http connection "+e.toString());
    }

  //convert response to string
    try{
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                    sb.append(line + "\n");
            }
            is.close();
            result=sb.toString();
    }catch(Exception e){
            Log.e("log_tag", "Error converting result "+e.toString());
    }

    try{

        jArray = new JSONObject(result);            
    }catch(JSONException e){
            Log.e("log_tag", "Error parsing data "+e.toString());
    }

    return jArray;
}
}

json：

{ "student":{ "id“："1"，"name”："Piet"，"age“："10”}{ "id“："2"，"name”："Jaap"，"age“："6”}}

和logcat：

03-12 11:24:09.773: E/AndroidRuntime(911): FATAL EXCEPTION: main
03-12 11:24:09.773: E/AndroidRuntime(911): java.lang.RuntimeException: Unable to start              activity ComponentInfo{me.mikey.my.games.galgjex/me.mikey.my.games.galgjex.Main}:        java.lang.NullPointerException
03-12 11:24:09.773: E/AndroidRuntime(911):  at   android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
03-12 11:24:09.773: E/AndroidRuntime(911):  at   android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.app.ActivityThread.access$600(ActivityThread.java:130)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.os.Handler.dispatchMessage(Handler.java:99)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.os.Looper.loop(Looper.java:137)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.app.ActivityThread.main(ActivityThread.java:4745)
03-12 11:24:09.773: E/AndroidRuntime(911):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-12 11:24:09.773: E/AndroidRuntime(911):  at java.lang.reflect.Method.invoke(Method.java:511)
03-12 11:24:09.773: E/AndroidRuntime(911):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
03-12 11:24:09.773: E/AndroidRuntime(911):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
03-12 11:24:09.773: E/AndroidRuntime(911):  at dalvik.system.NativeStart.main(Native  Method)
03-12 11:24:09.773: E/AndroidRuntime(911): Caused by: java.lang.NullPointerException
03-12 11:24:09.773: E/AndroidRuntime(911):  at  me.mikey.my.games.galgjex.Main.onCreate(Main.java:41)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.app.Activity.performCreate(Activity.java:5008)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
03-12 11:24:09.773: E/AndroidRuntime(911):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
03-12 11:24:09.773: E/AndroidRuntime(911):  ... 11 more

修改json为：{ "student":{ "id“："1"，"name”："Piet"，"age“："10”}，{ "id“："2"，"name”："Jaap"，"age“："6”}}但它仍然提供NPE。还是我的json仍然无效？

Answer 1

将所有代码放在单独的线程中或AsyncTask中，并确保保留了权限

 <uses-permission android:name="android.permission.INTERNET"/>

在您的清单中

尝试此代码以从服务中获取数据

How one interface can be used for different background android tasks?

Answer 2

检查网站地址..

如果你的网站是正确的，如果你在模拟器中运行你的应用程序，你会得到NullpointerException.Once，检查它在你的设备上，你是否得到了JSON响应。不过，如果出现错误，请检查是否在Manifest文件中添加了权限。如果您未添加权限，请在清单文件中添加以下权限。

             <uses-permission android:name="android.permission.INTERNET"/>

看看下面的链接，这将帮助你http://www.androidhive.info/2012/01/android-json-parsing-tutorial/