我想在我的片段中尝试自定义列表视图但是得到一个空指针错误，我该如何解决这个问题..我是初学者。
listmain.xml：
<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:layout_marginLeft="5dp"
android:layout_marginRight="5dp"
android:layout_marginTop="5dp"
tools:context=".MainActivity" >
<ListView
android:id="@+id/list"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:divider="@null"
android:dividerHeight="5dp"
android:listSelector="@drawable/list_row_selector" />
</RelativeLayout>
activity_main.xml中：
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent">
<!-- Framelayout to display Fragments -->
<FrameLayout
android:id="@+id/frame_container"
android:layout_width="match_parent"
android:layout_height="match_parent" >
</FrameLayout>
<!-- Listview to display slider menu -->
<ListView
android:id="@+id/list_slidermenu"
android:layout_width="240dp"
android:layout_height="match_parent"
android:layout_gravity="start"
android:choiceMode="singleChoice"
android:divider="@color/list_divider"
android:dividerHeight="1dp"
android:listSelector="@drawable/list_selector"
android:background="@color/list_background"/>
</android.support.v4.widget.DrawerLayout>
我的主要活动是导航抽屉：
public class MainActivity extends Activity {
private DrawerLayout mDrawerLayout;
private ListView mDrawerList;
private ActionBarDrawerToggle mDrawerToggle;
// nav drawer title
private CharSequence mDrawerTitle;
// used to store app title
private CharSequence mTitle;
// slide menu items
private String[] navMenuTitles;
private TypedArray navMenuIcons;
private ArrayList<NavDrawerItem> navDrawerItems;
private NavDrawerListAdapter adapter;
@SuppressLint("NewApi")
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTitle = mDrawerTitle = getTitle();
// load slide menu items
navMenuTitles = getResources().getStringArray(R.array.nav_drawer_items);
// nav drawer icons from resources
navMenuIcons = getResources()
.obtainTypedArray(R.array.nav_drawer_icons);
mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
mDrawerList = (ListView) findViewById(R.id.list_slidermenu);
navDrawerItems = new ArrayList<NavDrawerItem>();
// adding nav drawer items to array
// Home
navDrawerItems.add(new NavDrawerItem(navMenuTitles[0], navMenuIcons.getResourceId(0, -1)));
// Find People
navDrawerItems.add(new NavDrawerItem(navMenuTitles[1], navMenuIcons.getResourceId(1, -1)));
// Photos
navDrawerItems.add(new NavDrawerItem(navMenuTitles[2], navMenuIcons.getResourceId(2, -1)));
// Communities, Will add a counter here
navDrawerItems.add(new NavDrawerItem(navMenuTitles[3], navMenuIcons.getResourceId(3, -1)));
// Pages
navDrawerItems.add(new NavDrawerItem(navMenuTitles[4], navMenuIcons.getResourceId(4, -1)));
// What's hot, We will add a counter here
navDrawerItems.add(new NavDrawerItem(navMenuTitles[5], navMenuIcons.getResourceId(5, -1)));
// Recycle the typed array
navMenuIcons.recycle();
mDrawerList.setOnItemClickListener(new SlideMenuClickListener());
// setting the nav drawer list adapter
adapter = new NavDrawerListAdapter(getApplicationContext(),
navDrawerItems);
mDrawerList.setAdapter(adapter);
// enabling action bar app icon and behaving it as toggle button
getActionBar().setDisplayHomeAsUpEnabled(true);
getActionBar().setHomeButtonEnabled(true);
getActionBar().setBackgroundDrawable(
new ColorDrawable(Color.parseColor("#009933")));
//getActionBar().setIcon(
// new ColorDrawable(getResources().getColor(android.R.color.transparent)));
mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
R.drawable.ic_drawer, //nav menu toggle icon
R.string.app_name, // nav drawer open - description for accessibility
R.string.app_name // nav drawer close - description for accessibility
) {
public void onDrawerClosed(View view) {
getActionBar().setTitle(mTitle);
// calling onPrepareOptionsMenu() to show action bar icons
invalidateOptionsMenu();
}
public void onDrawerOpened(View drawerView) {
getActionBar().setTitle(mDrawerTitle);
// calling onPrepareOptionsMenu() to hide action bar icons
invalidateOptionsMenu();
}
};
mDrawerLayout.setDrawerListener(mDrawerToggle);
if (savedInstanceState == null) {
// on first time display view for first nav item
displayView(0);
}
}
/**
* Slide menu item click listener
* */
private class SlideMenuClickListener implements
ListView.OnItemClickListener {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position,
long id) {
// display view for selected nav drawer item
displayView(position);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// toggle nav drawer on selecting action bar app icon/title
if (mDrawerToggle.onOptionsItemSelected(item)) {
return true;
}
// Handle action bar actions click
switch (item.getItemId()) {
case R.id.action_settings:
return true;
default:
return super.onOptionsItemSelected(item);
}
}
/* *
* Called when invalidateOptionsMenu() is triggered
*/
@Override
public boolean onPrepareOptionsMenu(Menu menu) {
// if nav drawer is opened, hide the action items
boolean drawerOpen = mDrawerLayout.isDrawerOpen(mDrawerList);
menu.findItem(R.id.action_settings).setVisible(!drawerOpen);
return super.onPrepareOptionsMenu(menu);
}
/**
* Diplaying fragment view for selected nav drawer list item
* */
private void displayView(int position) {
// update the main content by replacing fragments
Fragment fragment = null;
switch (position) {
case 0:
fragment = new HomeFragment(this);
break;
case 1:
fragment = new FindPeopleFragment();
break;
case 2:
fragment = new PhotosFragment();
break;
case 3:
fragment = new CommunityFragment();
break;
case 4:
fragment = new PagesFragment();
break;
case 5:
fragment = new WhatsHotFragment();
break;
default:
break;
}
if (fragment != null) {
FragmentManager fragmentManager = getFragmentManager();
fragmentManager.beginTransaction()
.replace(R.id.frame_container, fragment).commit();
// update selected item and title, then close the drawer
mDrawerList.setItemChecked(position, true);
mDrawerList.setSelection(position);
setTitle(navMenuTitles[position]);
mDrawerLayout.closeDrawer(mDrawerList);
} else {
// error in creating fragment
Log.e("MainActivity", "Error in creating fragment");
}
}
@Override
public void setTitle(CharSequence title) {
mTitle = title;
getActionBar().setTitle(mTitle);
}
/**
* When using the ActionBarDrawerToggle, you must call it during
* onPostCreate() and onConfigurationChanged()...
*/
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
// Sync the toggle state after onRestoreInstanceState has occurred.
mDrawerToggle.syncState();
}
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
// Pass any configuration change to the drawer toggls
mDrawerToggle.onConfigurationChanged(newConfig);
}
}
这是我的片段：
public class HomeFragment extends ListFragment {
private static final String TAG = MainActivity.class.getSimpleName();
private Activity activity;
// Movies json url
private static final String url = "http://api.androidhive.info/json/movies.json";
private ProgressDialog pDialog;
private List<Movie> movieList = new ArrayList<Movie>();
private ListView listView;
private CustomListAdapter adapter;
public View rootView;
@SuppressLint("NewApi")
public HomeFragment(Activity acticity){
this.activity=activity;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
rootView = inflater.inflate(R.layout.listmain, container, false);
listView = (ListView) rootView.findViewById(R.id.list);
adapter = new CustomListAdapter(getActivity(), movieList);
listView.setAdapter(adapter);
pDialog = new ProgressDialog(getActivity());
// Showing progress dialog before making http request
pDialog.setMessage("Loading...");
pDialog.show();
// changing action bar color
// Creating volley request obj
JsonArrayRequest movieReq = new JsonArrayRequest(url, new Response.Listener<JSONArray>() {
@Override
public void onResponse(JSONArray response) {
Log.d(TAG, response.toString());
hidePDialog();
// Parsing json
for (int i = 0; i < response.length(); i++) {
try {
JSONObject obj = response.getJSONObject(i);
Movie movie = new Movie();
movie.setTitle(obj.getString("title"));
movie.setThumbnailUrl(obj.getString("image"));
movie.setRating(((Number) obj.get("rating")).doubleValue());
movie.setYear(obj.getInt("releaseYear"));
// Genre is json array
JSONArray genreArry = obj.getJSONArray("genre");
ArrayList<String> genre = new ArrayList<String>();
for (int j = 0; j < genreArry.length(); j++) {
genre.add((String) genreArry.get(j));
}
movie.setGenre(genre);
// adding movie to movies array
movieList.add(movie);
} catch (JSONException e) {
e.printStackTrace();
}
}
// notifying list adapter about data changes
// so that it renders the list view with updated data
adapter.notifyDataSetChanged();
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
VolleyLog.d(TAG, "Error: " + error.getMessage());
hidePDialog();
}
});
// Adding request to request queue
AppController.getInstance().addToRequestQueue(movieReq);
return rootView;
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {}
@Override
public void onDestroy() {
super.onDestroy();
hidePDialog();
}
private void hidePDialog() {
if (pDialog != null) {
pDialog.dismiss();
pDialog = null;
}
}
}
这是我的惯例：
public class CustomListAdapter extends BaseAdapter {
private Activity activity;
//public Context context;
private LayoutInflater inflater;
private List<Movie> movieItems;
ImageLoader imageLoader = AppController.getInstance().getImageLoader();
public CustomListAdapter( Activity activity, List<Movie> movieItems) {
this.activity = activity;
this.movieItems = movieItems;
//this.context = context;
}
@Override
public int getCount() {
return movieItems.size();
}
@Override
public Object getItem(int location) {
return movieItems.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) activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (convertView == null)
convertView = inflater.inflate(R.layout.list_row, null);
if (imageLoader == null)
imageLoader = AppController.getInstance().getImageLoader();
NetworkImageView thumbNail = (NetworkImageView) convertView.findViewById(R.id.thumbnail);
TextView title = (TextView) convertView.findViewById(R.id.title);
TextView rating = (TextView) convertView.findViewById(R.id.rating);
TextView genre = (TextView) convertView.findViewById(R.id.genre);
TextView year = (TextView) convertView.findViewById(R.id.releaseYear);
// getting movie data for the row
Movie m = movieItems.get(position);
// thumbnail image
thumbNail.setImageUrl(m.getThumbnailUrl(), imageLoader);
// title
title.setText(m.getTitle());
// rating
rating.setText("Rating: " + String.valueOf(m.getRating()));
// genre
String genreStr = "";
for (String str : m.getGenre()) {
genreStr += str + ", ";
}
genreStr = genreStr.length() > 0 ? genreStr.substring(0, genreStr.length() - 2) : genreStr;
genre.setText(genreStr);
// release year
year.setText(String.valueOf(m.getYear()));
return convertView;
}
}
我的日志：
11-25 06:41:20.813: W/dalvikvm(1135): threadid=1: thread exiting with uncaught exception (group=0xa4c18648)
11-25 06:41:20.821: E/AndroidRuntime(1135): FATAL EXCEPTION: main
11-25 06:41:20.821: E/AndroidRuntime(1135): java.lang.RuntimeException: Unable to start activity ComponentInfo{info.androidhive.slidingmenu/info.androidhive.slidingmenu.MainActivity}: java.lang.NullPointerException
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.ActivityThread.access$600(ActivityThread.java:141)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.os.Handler.dispatchMessage(Handler.java:99)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.os.Looper.loop(Looper.java:137)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.ActivityThread.main(ActivityThread.java:5103)
11-25 06:41:20.821: E/AndroidRuntime(1135): at java.lang.reflect.Method.invokeNative(Native Method)
11-25 06:41:20.821: E/AndroidRuntime(1135): at java.lang.reflect.Method.invoke(Method.java:525)
11-25 06:41:20.821: E/AndroidRuntime(1135): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
11-25 06:41:20.821: E/AndroidRuntime(1135): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-25 06:41:20.821: E/AndroidRuntime(1135): at dalvik.system.NativeStart.main(Native Method)
11-25 06:41:20.821: E/AndroidRuntime(1135): Caused by: java.lang.NullPointerException
11-25 06:41:20.821: E/AndroidRuntime(1135): at info.androidhive.slidingmenu.listadapter.CustomListAdapter.<init>(CustomListAdapter.java:28)
11-25 06:41:20.821: E/AndroidRuntime(1135): at info.androidhive.slidingmenu.HomeFragment.onCreateView(HomeFragment.java:57)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.Fragment.performCreateView(Fragment.java:1695)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:885)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1057)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.BackStackRecord.run(BackStackRecord.java:682)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1435)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.Activity.performStart(Activity.java:5142)
11-25 06:41:20.821: E/AndroidRuntime(1135): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
11-25 06:41:20.821: E/AndroidRuntime(1135): ... 11 more
答案 0 :(得分：0)
在JSON处理发生之前，您正在调用customListAdapter。因此，传递给customListAdapter的movielist没有任何值。这就是为什么你得到一个空指针异常。
解决方案：在JSON处理完成后执行以下代码行
listView.setAdapter（适配器）;
您正在尝试的最佳方法是使用AsyncTask从Server获取数据，并在AsyncTask的onPostExecute方法中执行该行
listView.setAdapter（适配器）;
答案 1 :(得分：0)
最初，您的movieList为空，没有任何数据。
adapter = new CustomListAdapter(getActivity(), movieList);
listView.setAdapter(adapter);
您只需要使用onResponse()方法而非onCreateView()方法移动上述代码。所以看起来像
@Override
public void onResponse(JSONArray response) {.....
adapter = new CustomListAdapter(getActivity(), movieList);
listView.setAdapter(adapter);
adapter.notifyDataSetChanged();
}.......
在此方法中，您的数据和用户界面将会更新。