NullPointer异常在setAdapter中的ListView实现

Question

当wifi开始工作时，但当它关闭时，我会得到nullPointer异常。

package ua.khuta.myskidka;

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

import android.database.Cursor;
import android.graphics.Color;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.Fragment;

import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ListView;

public class FragmentNews extends Fragment {
private View v;
ListView lvNews;
Database db;
mySCadapter scAdapter;
public String DEBUG_TAG = "myLogs";
String[] from = new String[] { Database.NEWS_TITLE, Database.NEWS_IMAGE_URL,
        Database.NEWS_SHORT_DESCRIPTION };
int [] to = new int[] { R.id.item_news_tv_title, R.id.item_news_iv_image,
        R.id.item_news_tv_short_description };



@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    v = inflater.inflate(R.layout.fragment_news, null);
    lvNews = (ListView) v.findViewById(R.id.lv_new);
    lvNews.setCacheColorHint(Color.TRANSPARENT);

    return v;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    db = Database.getInstance(getActivity());

    ConnectivityManager connManager = (ConnectivityManager) getActivity()
            .getSystemService(getActivity().CONNECTIVITY_SERVICE);
    NetworkInfo mWifi = connManager
            .getNetworkInfo(ConnectivityManager.TYPE_WIFI);

    if (mWifi.isConnected()) {
        Log.d(DEBUG_TAG, "wi-fi is on");
        db.cleanDB();
        GetJSON getJSON = new GetJSON();
        getJSON.execute();
    } else {

        Log.d(DEBUG_TAG, "wi-fi is off");

        Cursor c = db.getNewsFromDb();
        Log.d(DEBUG_TAG, String.valueOf(c.getCount()));
         mySCadapter scAdapter = new mySCadapter(getActivity(), R.layout.item_news, c,
                from, to);

        lvNews.setAdapter(scAdapter);
    }

}



class GetJSON extends AsyncTask<Void, Void, Void> {

    @Override
    protected Void doInBackground(Void... params) {
        // Creating JSON Parser instance

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {

        Cursor c = db.getNewsFromDb();
        scAdapter = new mySCadapter(getActivity(), R.layout.item_news, c,
                from, to);

        lvNews.setAdapter(scAdapter);

        super.onPostExecute(result);
    }

}

}

ErrorLog

    10-10 09:12:53.275: E/AndroidRuntime(28582): FATAL EXCEPTION: main
10-10 09:12:53.275: E/AndroidRuntime(28582): java.lang.RuntimeException: Unable to start activity ComponentInfo{ua.khuta.myskidka/ua.khuta.myskidka.MainActivity}: java.lang.NullPointerException
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.ActivityThread.access$600(ActivityThread.java:141)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.os.Handler.dispatchMessage(Handler.java:99)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.os.Looper.loop(Looper.java:137)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.ActivityThread.main(ActivityThread.java:5103)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at java.lang.reflect.Method.invokeNative(Native Method)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at java.lang.reflect.Method.invoke(Method.java:525)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at dalvik.system.NativeStart.main(Native Method)
10-10 09:12:53.275: E/AndroidRuntime(28582): Caused by: java.lang.NullPointerException
10-10 09:12:53.275: E/AndroidRuntime(28582):    at ua.khuta.myskidka.FragmentNews.onCreate(FragmentNews.java:82)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.support.v4.app.Fragment.performCreate(Fragment.java:1455)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:893)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1460)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:556)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.Activity.performStart(Activity.java:5143)
10-10 09:12:53.275: E/AndroidRuntime(28582):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
10-10 09:12:53.275: E/AndroidRuntime(28582):    ... 11 more

Answer 1

问题是，您正在onCreateView()中创建您的onCreateView()，并将其adapter设置为onCreate()。

您应该知道，onCreate()总是在onCreateView()之前被调用，到那时还没有创建listview。这就是为什么你要得到NullPointerException。

将onCreate()代码放在onCreateView()中

Answer 2

您的问题是您已经在ListView方法上创建了您的onCreateView()，并且已经在onCreate()中设置了适配器。所以onCreate()必须在onCreateView()之前被调用，这就是为什么ListViews没有初始化，并且您已经找到了空指针异常。

Answer 3

问题是，当您在同一活动.So中调用任何其他布局文件时，您正在另一个布局文件中创建您的activity.So，这与调用您的活动使用的相同布局中的activity.So place ListView的布局不匹配，或者如果您希望在另一个布局中使用ListView，则更改ArrayAdapter对象上下文，例如：将其更改为

ArrayAdapter适配器=新ArrayAdapter(this，R.layout.，R.id. )；

，下面是：

ArrayAdapter适配器=新ArrayAdapter(，R.layout.，R.id.，)；