Java 空指针异常：名称 == 空
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NullPointerException: name == null
提问by MikkoP
I'm developing an Android application that uses Fragments. I got this error.
我正在开发一个使用 Fragments 的 Android 应用程序。我收到了这个错误。
Caused by: java.lang.NullPointerException: name == null
What does name == nullmean? Here's the full log
什么name == null意思？这是完整的日志
08-04 15:02:22.422: E/AndroidRuntime(2499): FATAL EXCEPTION: main
08-04 15:02:22.422: E/AndroidRuntime(2499): java.lang.RuntimeException: Unable to start activity ComponentInfo{fi.peltoset.mikko.home/fi.peltoset.mikko.home.Koti}: android.view.InflateException: Binary XML file line #17: Error inflating class fragment
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.ActivityThread.access0(ActivityThread.java:130)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.os.Handler.dispatchMessage(Handler.java:99)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.os.Looper.loop(Looper.java:137)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.ActivityThread.main(ActivityThread.java:4745)
08-04 15:02:22.422: E/AndroidRuntime(2499): at java.lang.reflect.Method.invokeNative(Native Method)
08-04 15:02:22.422: E/AndroidRuntime(2499): at java.lang.reflect.Method.invoke(Method.java:511)
08-04 15:02:22.422: E/AndroidRuntime(2499): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
08-04 15:02:22.422: E/AndroidRuntime(2499): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-04 15:02:22.422: E/AndroidRuntime(2499): at dalvik.system.NativeStart.main(Native Method)
08-04 15:02:22.422: E/AndroidRuntime(2499): Caused by: android.view.InflateException: Binary XML file line #17: Error inflating class fragment
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:704)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.view.LayoutInflater.rInflate(LayoutInflater.java:746)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.view.LayoutInflater.inflate(LayoutInflater.java:489)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.view.LayoutInflater.inflate(LayoutInflater.java:396)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.view.LayoutInflater.inflate(LayoutInflater.java:352)
08-04 15:02:22.422: E/AndroidRuntime(2499): at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:256)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.Activity.setContentView(Activity.java:1867)
08-04 15:02:22.422: E/AndroidRuntime(2499): at fi.peltoset.mikko.home.Koti.onCreate(Koti.java:13)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.Activity.performCreate(Activity.java:5008)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
08-04 15:02:22.422: E/AndroidRuntime(2499): ... 11 more
08-04 15:02:22.422: E/AndroidRuntime(2499): Caused by: java.lang.NullPointerException: name == null
08-04 15:02:22.422: E/AndroidRuntime(2499): at java.lang.VMClassLoader.findLoadedClass(Native Method)
08-04 15:02:22.422: E/AndroidRuntime(2499): at java.lang.ClassLoader.findLoadedClass(ClassLoader.java:354)
08-04 15:02:22.422: E/AndroidRuntime(2499): at java.lang.ClassLoader.loadClass(ClassLoader.java:491)
08-04 15:02:22.422: E/AndroidRuntime(2499): at java.lang.ClassLoader.loadClass(ClassLoader.java:461)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.Fragment.instantiate(Fragment.java:574)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.Fragment.instantiate(Fragment.java:552)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.app.Activity.onCreateView(Activity.java:4656)
08-04 15:02:22.422: E/AndroidRuntime(2499): at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:680)
08-04 15:02:22.422: E/AndroidRuntime(2499): ... 21 more
This is in a class called Navigation. It extends android.app.Fragment.
这是在一个名为Navigation. 它延伸android.app.Fragment。
In it's onCreateViewmethod I inflate the layout and return it. In the onActivityCreatedmethod I check if the layout has two panes (I have different layouts for different devices) and based on that I add onClickListeners to these.
在它的onCreateView方法中，我膨胀了布局并返回它。在该onActivityCreated方法中，我检查布局是否有两个窗格（我对不同的设备有不同的布局），并基于此向这些窗格添加 onClickListeners。
My code looks like this.
我的代码看起来像这样。
public class Navigation extends Fragment {
private OnItemSelectedListener listener;
private boolean dualPane = false;
public interface OnItemSelectedListener {
public void onMenuItemSelected(int fragmentId);
}
public void onAttach(Activity activity) {
super.onAttach(activity);
if(activity instanceof OnItemSelectedListener) {
this.listener = (OnItemSelectedListener) activity;
} else {
...
}
}
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.activity_navigation, container, false);
return view;
}
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
View contentPane = getActivity().findViewById(R.id.contentFragment);
dualPane = contentPane != null && contentPane.getVisibility() == View.VISIBLE;
if(dualPane) {
final ListView menu = (ListView) getActivity().findViewById(R.id.valikko);
menu.setChoiceMode(ListView.CHOICE_MODE_SINGLE);
String[] texts = new String[] {
...
};
ArrayList<MenuItem> items = new ArrayList<MenuItem>();
items.add(new MenuItem("Home", R.drawable.koti2));
...
ImageArrayAdapter adapter = new ImageArrayAdapter(getActivity(), items, texts);
menu.setAdapter(adapter);
menu.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
String item = ((TextView) view.findViewById(R.id.label)).getText().toString();
listener.onMenuItemSelected(0); // TODO ...
}
});
} else {
ImageView btnYleiskatsaus = (ImageView) getActivity().findViewById(R.id.btnYleiskatsaus);
...
btnYleiskatsaus.setOnClickListener(onMenuClikkedListener);
...
}
}
private OnClickListener onMenuClikkedListener = new OnClickListener() {
@Override
public void onClick(View v) {
...
}
};
}
The layout file layout-sw720dp-land/activity_navigation.xmllooks this
布局文件layout-sw720dp-land/activity_navigation.xml看起来像这样
<LinearLayout
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:background="@color/pressed_koti"
android:orientation="vertical" >
<ListView
android:id="@+id/valikko"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1" >
</ListView>
</LinearLayout>
Here is the activity_koti.xmlfile.
这是activity_koti.xml文件。
<?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="horizontal" >
<fragment
android:id="@+id/menuFragment"
android:layout_width="300dp"
android:layout_height="match_parent"
class="fi.peltoset.mikko.home.Navigation" />
<fragment
android:id="@+id/contentFragment"
android:layout_width="fill_parent"
android:layout_height="match_parent" />
</LinearLayout>
I ran the application on an emulator with Android 4.1.2.
我在带有 Android 4.1.2 的模拟器上运行了该应用程序。
What causes these errors and how do I fix them?
导致这些错误的原因是什么，我该如何解决？
回答by Robadob
You should use android:nameto point to your fragments instead of class, it is also necessary to include the same attribute for your second fragment, this would explain the name==null.
您应该使用android:name指向您的片段而不是class，还需要为您的第二个片段包含相同的属性，这将解释name==null.
Edit:Apparently class may be interchangeable with android:name, but I would follow what the docs use.
编辑：显然 class 可以与 android:name 互换，但我会遵循文档使用的内容。
It should be;
它应该是;
<?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="horizontal" >
<fragment
android:id="@+id/menuFragment"
android:layout_width="300dp"
android:layout_height="match_parent"
android:name="fi.peltoset.mikko.home.Navigation" />
<fragment
android:id="@+id/contentFragment"
android:layout_width="fill_parent"
android:layout_height="match_parent"
android:name="I NEED A NAME" />
</LinearLayout>
http://developer.android.com/training/basics/fragments/creating.html#AddInLayout
http://developer.android.com/training/basics/fragments/creating.html#AddInLayout
回答by Geobits
When you add a fragment in the XML file, you need to give it a nameattribute. That's why it says "name = null", because you're not doing that.
在 XML 文件中添加片段时，需要为其赋予一个name属性。这就是为什么它说“name = null”，因为你没有这样做。
See the guide for adding Fragmentsfor more.
有关更多信息，请参阅添加 Fragments 的指南。
回答by Mena
Try adding a classto your second fragment in activity_koti.xmlif you're referencing it in your FragmentActivity.
尝试添加class到您的第二个片段中activity_koti.xml，如果你在你引用它FragmentActivity。
Or, as Robadob says, add android:name:"your.path.YourName"to both.
或者，正如 Robadob 所说，将android:name:"your.path.YourName"两者相加。
Interesting additional info on classvs android:namein XML references to Fragments here.
在有趣的附加信息classVSandroid:name在XML中引用Fragment小号这里。
