在我添加以下代码以将选项菜单添加到我的主 Activity 之前，我的应用程序运行良好：
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.wantlist_menu, menu);
return true;
}
菜单资源如下所示：
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item android:enabled="true" android:visible="true" android:id="@+id/clearImageCashe" android:title="@string/clearImageCache"></item>
</menu>
一切都在编译，但我得到以下 NullpointerException：
03-03 19:11:09.001: ERROR/AndroidRuntime(341): java.lang.RuntimeException: Unable to start activity ComponentInfo{de.kosmowski.discogs.wants/de.kosmowski.discogs.wants.activities.wantlist}: java.lang.NullPointerException
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.ActivityThread.access$2200(ActivityThread.java:119)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.os.Handler.dispatchMessage(Handler.java:99)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.os.Looper.loop(Looper.java:123)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.ActivityThread.main(ActivityThread.java:4363)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at java.lang.reflect.Method.invokeNative(Native Method)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at java.lang.reflect.Method.invoke(Method.java:521)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at dalvik.system.NativeStart.main(Native Method)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): Caused by: java.lang.NullPointerException
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at de.kosmowski.discogs.wants.activities.wantlist.onCreate(wantlist.java:49)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
03-03 19:11:09.001: ERROR/AndroidRuntime(341): ... 11 more
它说会有一个 NullPointer 异常
Caused by: java.lang.NullPointerException
03-03 19:11:09.001: ERROR/AndroidRuntime(341): at de.kosmowski.discogs.wants.activities.wantlist.onCreate(wantlist.java:49)
但是此行的代码之前运行良好，如果我将其注释掉，下面的某些行会出现相同的错误。这对我来说毫无意义。
我只是无法弄清楚这里发生了什么。
最后但并非最不重要的完整活动类（没有导入）：
public class wantlist extends Activity implements OnClickListener {
/** Called when the activity is first created. */
ArrayList<Want> wants = new ArrayList<Want>();
@Override
public void onCreate(Bundle savedInstanceState) {
wants.add(new Want("Want1"));
wants.add(new Want("Want2"));
wants.add(new Want("Want3"));
wants.add(new Want("Want4"));
wants.add(new Want("Want5"));
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
ListView lv = (ListView) findViewById(R.id.WantList);
ImageButton b = (ImageButton) findViewById(R.id.searchbutton);
b.setOnClickListener(this); //NullPointerException here
lv.setAdapter(new WantAdapter(this,
R.layout.lplistitem, wants));
lv.setTextFilterEnabled(true);
}
public void onClick(View v) {
onSearchRequested();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.wantlist_menu, menu);
return true;
}
}
只是为了确定这是 main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
>
<RelativeLayout
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
>
<ImageButton android:gravity="right" android:layout_alignParentRight="true" android:src="@drawable/searchbutton" android:text="@string/search" android:id="@+id/searchbutton" android:layout_width="wrap_content" android:layout_height="wrap_content" android:visibility="visible"></ImageButton>
</RelativeLayout>
<ListView android:layout_height="wrap_content" android:layout_width="fill_parent" android:id="@+id/WantList"></ListView>
</LinearLayout>
提前致谢。