我正在尝试在我的 fragment 中添加一个微调器和一个监听器。 Android studio 没有向我报告任何错误，但是当我运行它时，它因这个致命异常而崩溃:
14267-14267/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: space.markwen.www.units, PID: 14267
java.lang.RuntimeException: Unable to start activity ComponentInfo{space.markwen.www.units/space.markwen.www.units.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2456)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2539)
at android.app.ActivityThread.access$900(ActivityThread.java:159)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1384)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:152)
at android.app.ActivityThread.main(ActivityThread.java:5507)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
at space.markwen.www.units.LengthActivity.onCreate(LengthActivity.java:40)
at android.app.Fragment.performCreate(Fragment.java:2198)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:942)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1148)
at android.app.BackStackRecord.run(BackStackRecord.java:793)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1535)
at android.app.FragmentController.execPendingActions(FragmentController.java:325)
at android.app.Activity.performStart(Activity.java:6320)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2419)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2539)
at android.app.ActivityThread.access$900(ActivityThread.java:159)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1384)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:152)
at android.app.ActivityThread.main(ActivityThread.java:5507)
at java.lang.reflect.Method.invoke(Native Method)
fragment 类:
public class LengthActivity extends Fragment {
View lengthView;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// Change FrameLayout to content_length
lengthView = inflater.inflate(R.layout.content_length, container, false);
// Change the title on titlebar to Length
((AppCompatActivity)getActivity()).getSupportActionBar().setTitle("Length");
return lengthView;
}
Spinner spinner;
String[] lengthUnits = {
"Kilometer", "Meter", "Centimeter", "Millimeter", "Micrometer", "Nanometer", "Mile", "Yard", "Foot", "Inch", "Nautical Mile"};
@Override
public void onCreate (Bundle savedInstanceState){
super.onCreate(savedInstanceState);
spinner = (Spinner) getView().findViewById(R.id.lengthSpinner);
spinner.setOnItemSelectedListener(new OnItemSelectedListener() {
public void onItemSelected(AdapterView<?> parent, View view,
int position, long id) {
}
@Override
public void onNothingSelected(AdapterView<?> parent) {
}
});
ArrayAdapter<String> adapter = new ArrayAdapter<String> (getActivity(), android.R.layout.simple_spinner_item, lengthUnits);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);
}
}
我的观点以防你能从中找到一些东西:
<?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:weightSum="1">
<EditText
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="numberDecimal"
android:ems="10"
android:id="@+id/editText"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:textSize="30dp"
android:layout_weight="1.01" />
<Spinner
android:layout_width="112dp"
android:layout_height="57dp"
android:id="@+id/lengthSpinner" />
<!--android:layout_below="@+id/editText"-->
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginTop="42dp" />
</LinearLayout>
谢谢
最佳答案
将所有 onCreate() 代码(从崩溃的行开始)移动到 onViewCreated() 中。在调用 onCreateView() 之前，您无法执行任何操作，并且 onCreate() 将在 onCreateView() 之前调用。 onViewCreated() 在 onCreateView() 之后调用。
关于java - fragment 中微调器的 setOnItemSelectedListener()，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/39621440/