我正在尝试将当前活动对象传递给自定义列表视图适配器，但是从异步类传递时会出现nullpointer异常。这是我的代码：
public class MainActivity extends Activity {
ListView list;
LazyAdapter adapter;
private ProgressDialog pDialog;
// URL to get contacts JSON
private static String url = "http://api.androidhive.info/contacts/";
// JSON Node names
static final String Events_date = "ev_date";
static final String TAG_CONTACTS = "contacts";
static final String TAG_ID = "id";
static final String TAG_NAME = "name";
static final String TAG_EMAIL = "email";
static final String TAG_ADDRESS = "address";
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";
private static final String TAG_PHONE_OFFICE = "office";
// contacts JSONArray
JSONArray contacts = null;
// Hashmap for ListView
ArrayList<HashMap<String, String>> contactList;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
contactList = new ArrayList<HashMap<String, String>>();
// Calling async task to get json
new GetContacts().execute();
}
/**
* Async task class to get json by making HTTP call
* */
private class GetContacts extends AsyncTask<Void, Void, Void> {
@Override
protected void onPreExecute() {
super.onPreExecute();
// Showing progress dialog
pDialog = new ProgressDialog(MainActivity.this);
pDialog.setMessage("Please wait...");
pDialog.setCancelable(false);
pDialog.show();
}
@Override
protected Void doInBackground(Void... arg0) {
// Creating service handler class instance
ServiceHandler sh = new ServiceHandler();
// Making a request to url and getting response
String jsonStr = sh.makeServiceCall(url, ServiceHandler.GET);
Log.d("Response: ", "> " + jsonStr);
if (jsonStr != null) {
try {
JSONObject jsonObj = new JSONObject(jsonStr);
// Getting JSON Array node
contacts = jsonObj.getJSONArray(TAG_CONTACTS);
// looping through All Contacts
for (int i = 0; i < contacts.length(); i++) {
JSONObject c = contacts.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);
String office = phone.getString(TAG_PHONE_OFFICE);
// tmp hashmap for single contact
HashMap<String, String> contact = new HashMap<String, String>();
// adding each child node to HashMap key => value
contact.put(TAG_ID, id);
contact.put(TAG_NAME, name);
contact.put(TAG_EMAIL, email);
contact.put(TAG_PHONE_MOBILE, mobile);
// adding contact to contact list
contactList.add(contact);
}
} catch (JSONException e) {
e.printStackTrace();
}
} else {
Log.e("ServiceHandler", "Couldn't get any data from the url");
}
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
* */
list=(ListView)findViewById(R.id.list);
// Getting adapter by passing xml data ArrayList
adapter=new LazyAdapter(this, contactList);
list.setAdapter(adapter);
}
}
}
这里是adapterclass主要代码片段
public class LazyAdapter extends BaseAdapter {
private Activity activity;
private ArrayList<HashMap<String, String>> data;
private static LayoutInflater inflater=null;
// public ImageLoader imageLoader;
public LazyAdapter(Activity a, ArrayList<HashMap<String, String>> d) {
activity = a;
data=d;
inflater = (LayoutInflater)activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
// imageLoader=new ImageLoader(activity.getApplicationContext());
}
public int getCount() {
return data.size();
}
public Object getItem(int position) {
return position;
}
public long getItemId(int position) {
return position;
}
public View getView(int position, View convertView, ViewGroup parent) {
View vi=convertView;
if(convertView==null)
vi = inflater.inflate(R.layout.list_row, null);
TextView title = (TextView)vi.findViewById(R.id.title); // title
TextView artist = (TextView)vi.findViewById(R.id.artist); // artist name
TextView duration = (TextView)vi.findViewById(R.id.duration); // duration
HashMap<String, String> song = new HashMap<String, String>();
song = data.get(position);
// Setting all values in listview
title.setText(song.get(CustomizedListView.KEY_TITLE));
artist.setText(song.get(CustomizedListView.KEY_ARTIST));
duration.setText(song.get(CustomizedListView.KEY_DURATION));
// imageLoader.DisplayImage(song.get(CustomizedListView.KEY_THUMB_URL), thumb_image);
return vi;
}
}
我知道我应该传递MainActivity的实例。所以，任何人都可以告诉我该怎么做，好像我从postexecute调用一个函数然后也得到nullpointer因为数据没有被插入hashmap.Kindly帮助!!!
Log cat：
01-25 23:08:29.779: D/Response:(607): "name": "Clint Eastwood",
01-25 23:08:29.779: D/Response:(607): "address": "xx-xx-xxxx,x - street, x - coun
01-25 23:08:30.029: D/AndroidRuntime(607): Shutting down VM
01-25 23:08:30.029: W/dalvikvm(607): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
01-25 23:08:30.052: E/AndroidRuntime(607): FATAL EXCEPTION: main
01-25 23:08:30.052: E/AndroidRuntime(607): at info.androidhive.jsonparsing.MainActivity$GetContacts.onPostExecute(MainActivity.java:155)
01-25 23:08:30.052: E/AndroidRuntime(607): at info.androidhive.jsonparsing.MainActivity$GetContacts.onPostExecute(MainActivity.java:1)
答案 0 :(得分：0)
使用MainActivity.this引用嵌套类（例如异步任务）中的父类this。
但是，这要求你保持asynctask嵌套类非静态，这不是一个好主意。非静态嵌套类保存对其父级的引用（在本例中为活动），并且由于活动和asynctask生命周期不同，asynctask可以保持错误的活动太长时间。
答案 1 :(得分：0)
使用adapter=new LazyAdapter(MainActivity.this, contactList);
只有this将传递asynctask类上下文。
答案 2 :(得分：0)
您可以为AsyncTask创建一个新的构造函数，接受一个Activity作为输入参数
public GetContacts(Activity mActivity){
this.mActivity = mActivity;}
在doInBackground方法中，您可以使用mActivity但要注意，除非您有对Activity的引用，否则这个不会死。
从MainActivity创建AsyncTask时，您可以将其作为输入参数传递，它应该可以正常工作。