Question

我实现了导航抽屉但是在选择项目后我收到了 NullPointerException 。当我单击导航抽屉中的某个项目时，Activity将显示一个带有一些项目的ListView（“姓氏，名称”）。

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

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

位于pageuserslist.xml，属于片段

但在活动中你有

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

导致NullPointerException。

因此，您需要在片段onCreateView

中初始化listview

Answer 2

PrintUtentList()方法属于PageOperationsHistory片段，而不属于您的活动。您应该在片段onActivityCreated()方法中调用它。

顺便说一句，方法和变量名称通常以Java中的小写开头

导航抽屉NullPointerException

2 个答案: