尝试在片段中填充列表时出现 NullPointerException

Question

我一直在尝试从 Android 片段内的数据库中填充 ListView。不幸的是，在启动应用程序时，我在尝试填充和创建我的列表时收到NullPointerException。 我得到NullPointerException的代码

public class AlarmFrag extends Fragment {

    List<UserInputAlarmData> UserInputAlarmData = new ArrayList<UserInputAlarmData>();
    DatabaseHandler dbHandler;
    ListView AlarmListView;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.frag_alarm, container, false);
        AlarmListView = (ListView)rootView.findViewById(R.id.AlarmListView);
        return rootView;

    }

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        dbHandler = new DatabaseHandler(getActivity());

        List<UserInputAlarmData> addableAlarmData = dbHandler.getAllAlarmData();
        int AlarmDataCount = dbHandler.getAlarmDataCount();

        for (int i = 0; i < AlarmDataCount; i++) {
            UserInputAlarmData.add(addableAlarmData.get(i));
        }

        if (!addableAlarmData.isEmpty())
            populateList();
    }

    private void populateList() {
        ArrayAdapter<UserInputAlarmData> adapter = new AlarmListAdapter();
        AlarmListView.setAdapter(adapter);

    }

    private void AddAlarm(int id, String name, String phone, String email, String address) {
        UserInputAlarmData.add(new UserInputAlarmData(id, name, phone, email, address));
    }


    private class AlarmListAdapter extends ArrayAdapter<UserInputAlarmData> {
        public AlarmListAdapter() {
            super(getActivity(), R.layout.alarm_view_item, UserInputAlarmData);

        }
    }
}

引用的 frag_alarm.xml 文件

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

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:background="#363">

<TextView android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:gravity="center"
    android:text="AlarmScreen"
    android:textSize="20dp"
    android:layout_centerInParent="true"
    android:id="@+id/textView" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="100px"
        android:text="Add New Alarm"
        android:id="@+id/AlarmSetupButton"
        android:onClick="AlarmSetupButtonClick"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />

    <ListView
        android:layout_width="0dp"
        android:layout_height="100dp"
        android:id="@+id/AlarmListView"
        android:layout_gravity="center"/>


</RelativeLayout>

引用的 alarm_view_item.xml 文件。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:weightSum="1">

    <TextView
        android:layout_width="128dp"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Alarm Title:"
        android:id="@+id/TitleName"
        android:layout_weight="0.03" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Time:"
        android:id="@+id/TimeName" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Date:"
        android:id="@+id/DateName" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Song:"
        android:id="@+id/SongName" />

</LinearLayout>

Logcat 提供崩溃报告。

java.lang.NullPointerException
            at com.goldin.foodtektrakr.AlarmFrag.populateList(AlarmFrag.java:49)
            at com.goldin.foodtektrakr.AlarmFrag.onCreate(AlarmFrag.java:44)
            at android.support.v4.app.Fragment.performCreate(Fragment.java:1763)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:913)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:486)
            at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1073)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
            at android.view.View.measure(View.java:12752)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
            at android.view.View.measure(View.java:12752)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:812)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:553)
            at android.view.View.measure(View.java:12752)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4698)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2267)
            at android.view.View.measure(View.java:12752)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1074)
            at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2462)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4532)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
            at dalvik.system.NativeStart.main(Native Method)

非常感谢您的帮助。我可以提供解决问题所需的任何其他类或 xml 文件。

Answer 1

onCreateView() 方法在onCreate() 之后调用。当您从onCreate() 调用populateList() 方法时，您的AlarmListView 尚未初始化。因此，只需将调用移至 onCreateView() 方法即可解决您的问题。

完整的Fragment生命周期如下：

---

不幸的是，我现在遇到另一个错误：ArrayAdapter 要求资源 ID 是 TextView

将您的AlarmListAdapter 实例化为

private class AlarmListAdapter extends ArrayAdapter<UserInputAlarmData> {
    public AlarmListAdapter() {
        super(getActivity(),
              R.layout.alarm_view_item,
              R.id.TitleName, // Passing TextView id
              UserInputAlarmData);
    }
}

虽然这样可以消除错误，但不会完全达到您的预期。我看到您的行项目布局中有四个TextViews，但ArrayAdapter 只能将文本绑定到其中一个。因为，我们在那里传递了R.id.TitleName，所以adpater 只会将文本绑定到第一个TextView。

要拆分您的 adpater 项并在多个 视图 中显示数据以构成 列表项，您需要扩展 ArrayAdapter类并实现getView()。以下是在线提供的众多教程之一，展示了如何实现一个：ListView with Custom Adpater。

Answer 2

你还没有初始化你的ListView AlarmListView;所以任何引用变量的默认值都是null这就是你得到Null Pointer Exception的原因

所以当你调用这条线时

AlarmListView.setAdapter(adapter);

其实是

null.setAdapter(adapter);
 /\
 ||
 So Null Pointer Exception