我正在尝试将以前基于活动的教程转换为Fragment，但我继续在我的适配器上遇到NullPointerException错误。
本教程基于this，我已经缩减了适配器的构造函数，因为它曾经调用了一个Activity。
有一种可能的解决方案here，但没有人知道如何处理同样的问题。
我想将所有内容转换为有效的片段。如果您需要我提供更多信息，请与我们联系。
主要转换类：
public class mainViewController2 extends Fragment {
private static final String TAG = mainViewController2.class.getSimpleName();
private ListView listView;
private FeedListAdapter listAdapter;
private List<FeedItem> feedItems;
private String URL_FEED = "http://api.androidhive.info/feed/feed.json";
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.mainviewcontroller2_fragment, container, false);
listView = (ListView) v.findViewById(R.id.list);
feedItems = new ArrayList<FeedItem>();
//where the error shows up?
listAdapter = new FeedListAdapter(feedItems);
listView.setAdapter(listAdapter);
// We first check for cached request
Cache cache = AppController.getInstance().getRequestQueue().getCache();
Entry entry = cache.get(URL_FEED);
if (entry != null) {
// fetch the data from cache
try {
String data = new String(entry.data, "UTF-8");
try {
parseJsonFeed(new JSONObject(data));
} catch (JSONException e) {
e.printStackTrace();
}
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
} else {
// making fresh volley request and getting json
JsonObjectRequest jsonReq = new JsonObjectRequest(Method.GET,
URL_FEED, null, new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
VolleyLog.d(TAG, "Response: " + response.toString());
if (response != null) {
parseJsonFeed(response);
}
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
VolleyLog.d(TAG, "Error: " + error.getMessage());
}
});
// Adding request to volley request queue
AppController.getInstance().addToRequestQueue(jsonReq);
}
return v;
}
/**
* Parsing json reponse and passing the data to feed view list adapter
* */
private void parseJsonFeed(JSONObject response) {
try {
JSONArray feedArray = response.getJSONArray("feed");
for (int i = 0; i < feedArray.length(); i++) {
JSONObject feedObj = (JSONObject) feedArray.get(i);
FeedItem item = new FeedItem();
item.setId(feedObj.getInt("id"));
item.setName(feedObj.getString("name"));
// Image might be null sometimes
String image = feedObj.isNull("image") ? null : feedObj
.getString("image");
item.setImge(image);
item.setStatus(feedObj.getString("status"));
item.setProfilePic(feedObj.getString("profilePic"));
item.setTimeStamp(feedObj.getString("timeStamp"));
// url might be null sometimes
String feedUrl = feedObj.isNull("url") ? null : feedObj
.getString("url");
item.setUrl(feedUrl);
feedItems.add(item);
}
// notify data changes to list adapater
listAdapter.notifyDataSetChanged();
} catch (JSONException e) {
e.printStackTrace();
}
}
}
FeedListAdapter：
public class FeedListAdapter extends BaseAdapter {
private Activity activity;
private LayoutInflater inflater;
private List<FeedItem> feedItems;
Context context;
ImageLoader imageLoader;
public FeedListAdapter(Context ctx,List<FeedItem> feedItems) {
this.context= ctx;
this.feedItems = feedItems;
imageLoader = AppController.getInstance().getImageLoader();
inflater = LayoutInflater.from(context);
}
@Override
public int getCount() {
return feedItems.size();
}
@Override
public Object getItem(int location) {
return feedItems.get(location);
}
@Override
public long getItemId(int position) {
return position;
}
@SuppressLint("InflateParams")
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (inflater == null)
inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (convertView == null)
convertView = inflater.inflate(R.layout.feed_item, null);
if (imageLoader == null)
imageLoader = AppController.getInstance().getImageLoader();
TextView name = (TextView) convertView.findViewById(R.id.name);
TextView timestamp = (TextView) convertView
.findViewById(R.id.timestamp);
TextView statusMsg = (TextView) convertView
.findViewById(R.id.txtStatusMsg);
TextView url = (TextView) convertView.findViewById(R.id.txtUrl);
NetworkImageView profilePic = (NetworkImageView) convertView
.findViewById(R.id.profilePic);
FeedImageView feedImageView = (FeedImageView) convertView
.findViewById(R.id.feedImage1);
FeedItem item = feedItems.get(position);
name.setText(item.getName());
// Converting timestamp into x ago format
CharSequence timeAgo = DateUtils.getRelativeTimeSpanString(
Long.parseLong(item.getTimeStamp()),
System.currentTimeMillis(), DateUtils.SECOND_IN_MILLIS);
timestamp.setText(timeAgo);
// Chcek for empty status message
if (!TextUtils.isEmpty(item.getStatus())) {
statusMsg.setText(item.getStatus());
statusMsg.setVisibility(View.VISIBLE);
} else {
// status is empty, remove from view
statusMsg.setVisibility(View.GONE);
}
// Checking for null feed url
if (item.getUrl() != null) {
url.setText(Html.fromHtml("<a href=\"" + item.getUrl() + "\">"
+ item.getUrl() + "</a> "));
// Making url clickable
url.setMovementMethod(LinkMovementMethod.getInstance());
url.setVisibility(View.VISIBLE);
} else {
// url is null, remove from the view
url.setVisibility(View.GONE);
}
// user profile pic
profilePic.setImageUrl(item.getProfilePic(), imageLoader);
// Feed image
if (item.getImge() != null) {
feedImageView.setImageUrl(item.getImge(), imageLoader);
feedImageView.setVisibility(View.VISIBLE);
feedImageView
.setResponseObserver(new FeedImageView.ResponseObserver() {
@Override
public void onError() {
}
@Override
public void onSuccess() {
}
});
} else {
feedImageView.setVisibility(View.GONE);
}
return convertView;
}
}
LogCat错误：
09-06 02:51:55.823: E/AndroidRuntime(8279): FATAL EXCEPTION: main
09-06 02:51:55.823: E/AndroidRuntime(8279): Process: com.rynovation.kline, PID: 8279
09-06 02:51:55.823: E/AndroidRuntime(8279): java.lang.NullPointerException
09-06 02:51:55.823: E/AndroidRuntime(8279): at androidFeedClasses.FeedListAdapter.<init>(FeedListAdapter.java:35)
09-06 02:51:55.823: E/AndroidRuntime(8279): at com.rynovation.kline.mainViewController2.onCreateView(mainViewController2.java:51)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.Fragment.performCreateView(Fragment.java:1700)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:890)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1062)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.BackStackRecord.run(BackStackRecord.java:684)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1453)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.FragmentManagerImpl$1.run(FragmentManager.java:443)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.os.Handler.handleCallback(Handler.java:733)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.os.Handler.dispatchMessage(Handler.java:95)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.os.Looper.loop(Looper.java:146)
09-06 02:51:55.823: E/AndroidRuntime(8279): at android.app.ActivityThread.main(ActivityThread.java:5487)
09-06 02:51:55.823: E/AndroidRuntime(8279): at java.lang.reflect.Method.invokeNative(Native Method)
09-06 02:51:55.823: E/AndroidRuntime(8279): at java.lang.reflect.Method.invoke(Method.java:515)
09-06 02:51:55.823: E/AndroidRuntime(8279): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
09-06 02:51:55.823: E/AndroidRuntime(8279): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
09-06 02:51:55.823: E/AndroidRuntime(8279): at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：27)
我猜你错过了清单文件中的简单代码。您的AppController包扩展了Application，因此您的清单中的应用标记正在搜索AppController，但无法找到。
只需在AndroidManifest.xml
android:name = your.package.AppController
答案 1 :(得分：11)
可能有点晚了，但对user3902144的答案有代表性的答案：
此问题的类似答案：Link
你可能错过了你的清单：
<application
android:name="<package-name>.app.AppController" // <-- this one
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme">
这就是为什么onCreate从未调用过，mInstance为空。
希望这会有所帮助：）
答案 2 :(得分：1)
查看错误，您似乎在getActivity()内调用了onCreateView()。你应该在onActivityCreated中获取活动，而不是它应该为空。
答案 3 :(得分：1)
从
更改此内容listView = (ListView) getActivity().findViewById(R.id.list);
到
listView = (ListView)v.findViewById(R.id.list);
这是因为getActivity()通常用作Fragment中的Context。当您在Fragment中找到UI元素的ID时，您必须提供View's对象的引用，该对象将以onCreateView()方法返回。
更新
更改您的适配器，
listAdapter = new FeedListAdapter(getActivity(),feedItems);
现在在其构造函数
中 ImageLoader imageLoader;
public FeedListAdapter(Context ctx,List<FeedItem> feedItems) {
this.context= ctx;
this.feedItems = feedItems;
imageLoader = AppController.getInstance().getImageLoader()
}
现在，在适配器类中，使用context变量作为Context。
答案 4 :(得分：0)
请仔细检查所有参数，确保不要在参数中发送任何空值，如果使用一些存储的值，则使用如下条件：
@Override
protected Map<String, String> getParams() {
Map<String, String> params = new HashMap<String, String>();
if(paramValue!= null)
params.put("paramKey", paramValue);
else
params.put("paramKey", "anyOtherDefaultValue");
return params;
}
答案 5 :(得分：0)
//我在这个错误中被堆栈，我只是通过在你的 AppController 类中写这个来解决这个问题。
@Override
public void onCreate() {
super.onCreate();
instance = this;
}
}