導航抽屜空指針異常

Question

我實現了一個導航抽屜但我收到一個NullPointerException選擇一個項目後。當我點擊導航抽屜中的一個項目時，活動應該顯示帶有一些項目的ListView（「姓氏，名稱」）。

PageOperations.java

import android.app.Fragment; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 

public class PageUsersList extends Fragment 
{ 
    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) 
    { 
     View rootView = inflater.inflate(R.layout.pageuserslist, container, false); 
     return rootView; 
    } 
} 

pageuserslist.xml

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:orientation="vertical" > 

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

</LinearLayout> 

activity_main.xml中

enter code here 
<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 
      android:id="@+id/content_frame" 
      android:layout_width="match_parent" 
      android:layout_height="match_parent" /> 

     <ListView 
      android:id="@+id/left_drawer" 
      android:layout_width="240dp" 
      android:layout_height="match_parent" 
      android:layout_gravity="left" 
      android:choiceMode="singleChoice" 
      android:listSelector="@drawable/list_selector" 
      android:dividerHeight="0dp" 
      android:background="#111"/> 
</android.support.v4.widget.DrawerLayout> 

MainActivity.java

public class MainActivity extends Activity 
{ 
    ... 

    private void selectItem(int position) 
    { 
     switch(position) 
     { 
      ... 

     case 2: 
      Fragment fragment2 = new PageOperationsHistory(); 
      Bundle args2 = new Bundle(); 
      fragment2.setArguments(args2); 
      FragmentManager fragmentManager2 = getFragmentManager(); 
      fragmentManager2.beginTransaction().replace(R.id.content_frame, fragment2).commit(); 
      mDrawerList.setItemChecked(position, true); 
      mDrawerLayout.closeDrawer(mDrawerList); 
      PrintUtentList(); 
      return; 
      ... 
      } 
    } 

    ... 

    private void PrintUtentList() 
    { 
     ListView ListUsers = (ListView) findViewById(R.id.ListUsers); 
     List<String> ArrayForListUsers = new ArrayList<String>(); 
     for(int i=0;i<allNames.size();i++) 
     { 
      String value = allSurnNames.get(i).toString() + ", " +allNames.get(i).toString(); // allSurnNames and Names are ArrayList<String> 
      ArrayForListUsers.add(value); // All values are like "surname, name" 
     } 
     ListAdapter AdapterForListView = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_2, ArrayForListUsers); 
     ListUsers.setAdapter(AdapterForListView); 
    } 
} 

和

04-05 16:47:36.132: E/AndroidRuntime(2597): FATAL EXCEPTION: main 
04-05 16:47:36.132: E/AndroidRuntime(2597): java.lang.NullPointerException 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at com.example.apptesina.MainActivity.PrintUtentList(MainActivity.java:192) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at com.example.apptesina.MainActivity.selectItem(MainActivity.java:174) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at com.example.apptesina.MainActivity.access$0(MainActivity.java:143) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at com.example.apptesina.MainActivity$DrawerItemClickListener.onItemClick(MainActivity.java:139) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.widget.AdapterView.performItemClick(AdapterView.java:298) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.widget.AbsListView.performItemClick(AbsListView.java:1086) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.widget.AbsListView$PerformClick.run(AbsListView.java:2859) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.widget.AbsListView$1.run(AbsListView.java:3533) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.os.Handler.handleCallback(Handler.java:615) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.os.Handler.dispatchMessage(Handler.java:92) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.os.Looper.loop(Looper.java:137) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at android.app.ActivityThread.main(ActivityThread.java:4745) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at java.lang.reflect.Method.invokeNative(Native Method) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at java.lang.reflect.Method.invoke(Method.java:511) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
04-05 16:47:36.132: E/AndroidRuntime(2597):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

ListView控件

<ListView 
    android:id="@+id/ListUsers" // does not belong to activity layout 

是pageuserslist.xml屬於片段

但在活動你有

ListView ListUsers = (ListView) findViewById(R.id.ListUsers); 

導致NullPointerException。

所以，你需要初始化列表視圖中的片段onCreateView

Answer 2

的PrintUtentList()方法屬於PageOperationsHistory片段，而不是你的活動。您應該在片段onActivityCreated()方法中調用它。

順便說一句，方法和變量名一般用在Java中的小寫