Question

抛出以下异常：

java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
                                                                                      at android.widget.SimpleAdapter.getCount(SimpleAdapter.java:100)
                                                                                      at android.widget.ListView.setAdapter(ListView.java:491)
                                                                                      at android.app.ListActivity.setListAdapter(ListActivity.java:265)
                                                                                      at com.example.example.feed.ActivityFeedActivity$GetStudents.onPostExecute(ActivityFeedActivity.java:93)
                                                                                      at com.example.example.feed.ActivityFeedActivity$GetStudents.onPostExecute(ActivityFeedActivity.java:47)
                                                                                      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)
                                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

代码：

import android.app.ListActivity;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListAdapter;
import android.widget.SimpleAdapter;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;


public class ActivityFeedActivity extends ListActivity {

    // URL to get contacts JSON
    private static String url = "https://raw.githubusercontent.com/mobilesiri/JSON-Parsing-in-Android/master/index.html";

    // JSON Node names
    private static final String TAG_STUDENTINFO = "studentsinfo";
    private static final String TAG_ID = "id";
    private static final String TAG_NAME = "name";
    private static final String TAG_EMAIL = "email";
    private static final String TAG_ADDRESS = "address";
    private static final String TAG_GENDER = "gender";
    private static final String TAG_PHONE = "phone";
    private static final String TAG_PHONE_MOBILE = "mobile";
    private static final String TAG_PHONE_HOME = "home";

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

        // Calling async task to get json
        new GetStudents().execute();
    }

    /**
     * Async task class to get json by making HTTP call
     */
    private class GetStudents extends AsyncTask<Void, Void, Void> {

        // Hashmap for ListView
        ArrayList<HashMap<String, String>> studentList = new ArrayList<HashMap<String, String>>();;
        ProgressDialog pDialog;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Showing progress dialog
            pDialog = new ProgressDialog(ActivityFeedActivity.this);
            pDialog.setMessage("Please wait...");
            pDialog.setCancelable(false);
            pDialog.show();
        }

        @Override
        protected Void doInBackground(Void... arg0) {
            // Creating service handler class instance
            WebRequest webreq = new WebRequest();

            // Making a request to url and getting response
            String jsonStr = webreq.makeWebServiceCall(url, WebRequest.GET);

            Log.d("Response: ", "> " + jsonStr);

            studentList = ParseJSON(jsonStr);

            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);
            // Dismiss the progress dialog
            if (pDialog.isShowing())
                pDialog.dismiss();
            /**
             * Updating parsed JSON data into ListView
             * */
            ListAdapter adapter = new SimpleAdapter(
                    ActivityFeedActivity.this, studentList,
                    R.layout.feed_item, new String[]{TAG_NAME, TAG_EMAIL,
                    TAG_PHONE_MOBILE}, new int[]{R.id.name,
                    R.id.email, R.id.mobile});

            setListAdapter(adapter);
        }

    }

    private ArrayList<HashMap<String, String>> ParseJSON(String json) {
        if (json != null) {
            try {
                // Hashmap for ListView
                ArrayList<HashMap<String, String>> studentList = new ArrayList<HashMap<String, String>>();

                JSONObject jsonObj = new JSONObject(json);

                // Getting JSON Array node
                JSONArray students = jsonObj.getJSONArray(TAG_STUDENTINFO);

                // looping through All Students
                for (int i = 0; i < students.length(); i++) {
                    JSONObject c = students.getJSONObject(i);

                    String id = c.getString(TAG_ID);
                    String name = c.getString(TAG_NAME);
                    String email = c.getString(TAG_EMAIL);
                    String address = c.getString(TAG_ADDRESS);
                    String gender = c.getString(TAG_GENDER);

                    // Phone node is JSON Object
                    JSONObject phone = c.getJSONObject(TAG_PHONE);
                    String mobile = phone.getString(TAG_PHONE_MOBILE);
                    String home = phone.getString(TAG_PHONE_HOME);

                    // tmp hashmap for single student
                    HashMap<String, String> student = new HashMap<String, String>();

                    // adding each child node to HashMap key => value
                    student.put(TAG_ID, id);
                    student.put(TAG_NAME, name);
                    student.put(TAG_EMAIL, email);
                    student.put(TAG_PHONE_MOBILE, mobile);

                    // adding student to students list
                    studentList.add(student);
                }
                return studentList;
            } catch (JSONException e) {
                e.printStackTrace();
                return null;
            }
        } else {
            Log.e("ServiceHandler", "Couldn't get any data from the url");
            return null;
        }
    }

}

如果我理解正确的话，当OnPostExecute（）方法执行时，应该从url解析studentList中的数据，那么我错过了什么导致异常抛出呢？

Answer 1

问题可能在于您的JSON解析实现，因为如果引发<html lang="en" > <head> <meta charset="utf-8">  <script type="text/javascript" src="apps.js"></script> <script src="name_template.js"></script> <scr <center><h1>JavaScript Variables</h1></center> <script type="text/javascript" src="vra_config.js"></script> <center><p>In this example, x, y, and z are variables</p></center>   <body onload="initialize('name');"> <script> var script = document.createElement('script'); script.type = 'text/javascript'; script.src = port_list_var; document.body.appendChild(script); </script> <p id="port_list_var"></p>  <p> <center> Check for Table </center> </p> <table height="100px" width="840px" align="center" cellspacing=0 cellpadding=0 border=2> <tr><td colspan="3"><span id="name_table_body"></span></td></tr> <br> </table> <center> <button style="width:180px;margin-top:5px;margin-bottom:20px;" onClick="submitForm();">Submit Results</button> </body> </html>则返回null或实际请求，因为在JSON不是的情况下您也返回JSONException成功获取。在崩溃之前LogCat中是否还有另一个例外？

由于未知原因引发的异常java.util.List.size（）

1 个答案: