Navigation Drawer NullPointerException

Question

I implemented a Navigation Drawer but I receive a NullPointerException after selecting an item. When I click on an item in the navigation drawer, the Activity shall show a ListView with some items ("surname, name").

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

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);
     }
 }

and

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

The ListView

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

is in pageuserslist.xml which belongs to the fragment

But in Activity you have

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

leading to NullPointerException.

So you need to initialize the listview in fragments onCreateView

Answer 2

The PrintUtentList() method belongs to the PageOperationsHistory fragment, not to your activity. You should call it in the fragment onActivityCreated() method.

By the way, method and variable names usually starts with a lowercase in Java