Исключение нулевого указателя

Question

Я получаю следующее сообщение об ошибке:

6-15 19:57:50.291 9743-9743/com.example.utkarsh.beatle.app E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    java.lang.NullPointerException 
      at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:392) 
      at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362) 
      at android.widget.AbsListView.obtainView(AbsListView.java:2214) 
      at android.widget.ListView.makeAndAddView(ListView.java:1852) 
      at android.widget.ListView.fillDown(ListView.java:678) 
      at android.widget.ListView.fillFromTop(ListView.java:739) 
      at android.widget.ListView.layoutChildren(ListView.java:1668) 
      at android.widget.AbsListView.onLayout(AbsListView.java:2037) 
      at android.view.View.layout(View.java:14099) 
      at android.view.ViewGroup.layout(ViewGroup.java:4464) 
      at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1021) 
      at android.view.View.layout(View.java:14099) 
      at android.view.ViewGroup.layout(ViewGroup.java:4464) 
      at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1021) 
      at android.view.View.layout(View.java:14099) 
      at android.view.ViewGroup.layout(ViewGroup.java:4464) 
      at android.widget.FrameLayout.onLayout(FrameLayout.java:448) 
      at android.view.View.layout(View.java:14099) 
      at android.view.ViewGroup.layout(ViewGroup.java:4464) 
      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1670) 
      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1528) 
      at android.widget.LinearLayout.onLayout(LinearLayout.java:1441) 
      at android.view.View.layout(View.java:14099) 
      at android.view.ViewGroup.layout(ViewGroup.java:4464) 
      at android.widget.FrameLayout.onLayout(FrameLayout.java:448) 
      at android.view.View.layout(View.java:14099) 
      at android.view.ViewGroup.layout(ViewGroup.java:4464) 
      at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2183) 
      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1947) 
      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1139) 
      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4879) 
      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:776) 
      at android.view.Choreographer.doCallbacks(Choreographer.java:579) 
      at android.view.Choreographer.doFrame(Choreographer.java:548) 
      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:762) 
      at android.os.Handler.handleCallback(Handler.java:725) 
      at android.os.Handler.dispatchMessage(Handler.java:92) 
      at android.os.Looper.loop(Looper.java:153) 
      at android.app.ActivityThread.main(ActivityThread.java:5299) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:511) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600) 
      at dalvik.system.NativeStart.main(Native Method) 

Мой код

package com.example.utkarsh.beatle.app; 

    import android.support.v4.app.Fragment; 
    import android.support.v4.app.FragmentActivity; 
    import android.os.Bundle; 
    import android.view.LayoutInflater; 
    import android.view.Menu; 
    import android.view.MenuItem; 
    import android.view.View; 
    import android.view.ViewGroup; 
    import android.widget.ArrayAdapter; 
    import android.widget.ListView; 

    import java.util.ArrayList; 
    import java.util.Arrays; 
    import java.util.List; 


public class MainActivity extends FragmentActivity { 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     if(savedInstanceState==null) 
     { 
      getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit(); 
     } 
    } 


    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 

    public static class PlaceholderFragment extends Fragment { 

     private ArrayAdapter<String> mForecastAdapter; 


     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 

      View rootView = inflater.inflate(R.layout.fragment_main, container, false); 
      String[] forecastArray= {"Today-Sunny-88/63", "Tommorrow-windy-88/94", "wed-aloo-alooo"}; 
      List<String> weekForecast = new ArrayList<String>(Arrays.asList(forecastArray)); 

      mForecastAdapter = new ArrayAdapter<String>(getActivity(), R.layout.fragment_main, R.id.list_item_forecast_textview, weekForecast); 
      ListView listView = (ListView) rootView.findViewById(R.id.list_view_forecast); 
      listView.setAdapter(mForecastAdapter); 
      return rootView; 

     } 
    } 
} 

Таковы .xml, которые я использовал, я не знаю о проблемах с макетом, пожалуйста, помогите мне в этом, я просто следую за видео на udacity, «разрабатывая приложение для Android», каждый код написан как то, что написано ими. Я не знаю, в чем вина с моей стороны.

**activity_main.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" 
tools:context="com.example.utkarsh.beatle.app.MainActivity" 
tools:ignore="MergeRootFrame" 
android:id="@+id/container"/> 

**fragment_main.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:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
    tools:context="com.example.utkarsh.beatle.app.MainActivity$PlaceholderFragment"> 

<ListView 
    android:id="@+id/list_view_forecast" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" /> 

**list_item_forecast.xml** 
    <?xml version="1.0" encoding="utf-8"?> 

    <TextView 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:minHeight="?android:attr/listPreferredItemHeight" 
    android:gravity="center_vertical" 
    android:id="@+id/list_item_forecast_textview"/> 

Answer 1

Вы используете неправильную раскладку для элементов списка. Изменение

mForecastAdapter = new ArrayAdapter<String>(getActivity(), R.layout.fragment_main, ... 

в

mForecastAdapter = new ArrayAdapter<String>(getActivity(), R.layout.list_item_forecast, ... 

NPE приходит, когда ArrayAdapter вызовы findViewById() получить указанный TextView из макета элемента списка, но вид не существует и null возвращается. Вызов setText() на этом nullTextView является NPE.

Answer 2

Вы должны использовать правильную компоновку для элемента списка.

Используйте этот

mForecastAdapter = new ArrayAdapter<String>(getActivity(), R.layout.list_item_forecast, R.id.list_item_forecast_textview, weekForecast); 

вместо:

mForecastAdapter = new ArrayAdapter<String>(getActivity(), R.layout.fragment_main, R.id.list_item_forecast_textview, weekForecast); 

Это приведет к удалению исключения нулевого указателя полностью !!