当我尝试在XML布局中检索 ListView 组件时，会启动NullPointerException。
ListView lv = (ListView) view.findViewById(R.id.lvAlerts);
我正在使用片段，欢迎任何帮助。
1）布局文件（tab_frag_alerts.xml）
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<Spinner
android:id="@+id/spinnerAlertCategory"
android:layout_width="fill_parent"
android:layout_height="wrap_content" />
<ListView
android:id="@+id/lvAlerts"
android:layout_width="fill_parent"
android:layout_height="fill_parent" />
</LinearLayout>
2）布局文件（alert_row.xml）
<?xml version="1.0" encoding="utf-8"?>
<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="10dip" >
<TableRow
android:id="@+id/tableRow1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="1dip" >
<TextView
android:id="@+id/alertId"
style="@style/vehicleDefaultFont.plate"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/label_text_view" />
<TextView
android:id="@+id/createdIn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/label_text_view" />
</TableRow>
</TableLayout>
3）片段Java类：
package br.com.log2br.tabs;
import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.widget.SimpleCursorAdapter;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Spinner;
import br.com.log2br.R;
import br.com.log2br.db.AlertDbAdapter;
public class AlertsTabFragment extends Fragment {
String TAG = getClass().getName();
private AlertDbAdapter dbHelper;
private SimpleCursorAdapter dataAdapter;
private Spinner spinnerAlertCategory;
View view;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
Log.i(TAG, "Loading onCreateView()");
view = inflater.inflate(R.layout.tab_frag_alerts, container, false);
dbHelper = new AlertDbAdapter(getActivity());
dbHelper.open();
dbHelper.deleteAllAlerts();
dbHelper.insertAlerts();
spinnerAlertCategory = (Spinner) view.findViewById(R.id.spinnerAlertCategory);
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(view.getContext(), R.array.alerts_array, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinnerAlertCategory.setAdapter(adapter);
spinnerAlertCategory.setOnItemSelectedListener(new OnItemSelectedListener() {
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
String itemSelected = parent.getItemAtPosition(pos).toString();
if(!itemSelected.equals(R.string.text_hint_alert_spinner)) {
Cursor cursor = dbHelper.fetchAllAlerts();
if(cursor != null && cursor.getCount() > 0) {
String[] columns = new String[] {
AlertDbAdapter.KEY_ALERT_ID,
AlertDbAdapter.KEY_CREATED_IN,
};
int[] to = new int[] {
R.id.alertId,
R.id.createdIn,
};
dataAdapter = new SimpleCursorAdapter(
view.getContext(),
R.layout.alert_row,
cursor,
columns,
to,
0);
ListView lv = (ListView) view.findViewById(R.id.lvAlerts);
lv.setAdapter(dataAdapter); //null pointer here :((
} else {
Log.i(TAG, "Found no alerts.");
}
}
}
@Override
public void onNothingSelected(AdapterView<?> arg0) {
// TODO Auto-generated method stub
}
});
return view;
}
}
4）Logcat
07-21 12:37:09.044: E/AndroidRuntime(8393): FATAL EXCEPTION: main
07-21 12:37:09.044: E/AndroidRuntime(8393): java.lang.NullPointerException
07-21 12:37:09.044: E/AndroidRuntime(8393): at br.com.log2br.tabs.AlertsTabFragment$1.onItemSelected(AlertsTabFragment.java:119)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.widget.AdapterView.fireOnSelected(AdapterView.java:892)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.widget.AdapterView.access$200(AdapterView.java:49)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:860)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.os.Handler.handleCallback(Handler.java:615)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.os.Handler.dispatchMessage(Handler.java:92)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.os.Looper.loop(Looper.java:137)
07-21 12:37:09.044: E/AndroidRuntime(8393): at android.app.ActivityThread.main(ActivityThread.java:4745)
07-21 12:37:09.044: E/AndroidRuntime(8393): at java.lang.reflect.Method.invokeNative(Native Method)
07-21 12:37:09.044: E/AndroidRuntime(8393): at java.lang.reflect.Method.invoke(Method.java:511)
07-21 12:37:09.044: E/AndroidRuntime(8393): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
07-21 12:37:09.044: E/AndroidRuntime(8393): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
07-21 12:37:09.044: E/AndroidRuntime(8393): at dalvik.system.NativeStart.main(Native Method)
07-21 12:47:18.904: E/Trace(9446): error opening trace file: No such file or directory (2)
我的光标包含_id字段。我仔细检查了我的光标，它有4行。
答案 0 :(得分：2)
这种情况正在发生，因为您使用了错误的View变量来查找ListView。
ListView lv = (ListView) view.findViewById(R.id.lvAlerts);
这里的视图变量是指：
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
您需要参考此view：
view = inflater.inflate(R.layout.tab_frag_alerts, container, false);
您应该更改声明：
View view; //change view to some other name
或者，
public void onItemSelected(AdapterView<?> parent, View someOtherView, int pos, long id) {