使用listview时遇到NullPointerException。 我已经初始化了我的适配器,我的listview和我的课程数组。
如果我的怀疑听起来含糊不清,请道歉。我是android的新手,无法找出错误
AboutActivity.java
public class AboutActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_about);
String[] lessons = new String[] { "Android Introduction","Android Setup/Installation","Android Hello World","Android Layouts/Viewgroups","Android Activity & Lifecycle","Intents in Android"};
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1,lessons);
ListView listview = (ListView)findViewById(R.id.ListView);
listview.setAdapter(adapter);
}
fragment_about.xml
<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.chatclient.AboutActivity$PlaceholderFragment" >
<ListView
android:id="@+id/ListView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_centerVertical="true" >
</ListView>
</RelativeLayout>
logcat的:
05-09 01:41:59.029: E/AndroidRuntime(1579): FATAL EXCEPTION: main
05-09 01:41:59.029: E/AndroidRuntime(1579): Process: com.example.chatclient, PID: 1579
05-09 01:41:59.029: E/AndroidRuntime(1579): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.chatclient/com.example.chatclient.AboutActivity}: java.lang.NullPointerException
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.os.Handler.dispatchMessage(Handler.java:102)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.os.Looper.loop(Looper.java:136)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.ActivityThread.main(ActivityThread.java:5017)
05-09 01:41:59.029: E/AndroidRuntime(1579): at java.lang.reflect.Method.invokeNative(Native Method)
05-09 01:41:59.029: E/AndroidRuntime(1579): at java.lang.reflect.Method.invoke(Method.java:515)
05-09 01:41:59.029: E/AndroidRuntime(1579): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-09 01:41:59.029: E/AndroidRuntime(1579): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-09 01:41:59.029: E/AndroidRuntime(1579): at dalvik.system.NativeStart.main(Native Method)
05-09 01:41:59.029: E/AndroidRuntime(1579): Caused by: java.lang.NullPointerException
05-09 01:41:59.029: E/AndroidRuntime(1579): at com.example.chatclient.AboutActivity.onCreate(AboutActivity.java:29)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.Activity.performCreate(Activity.java:5231)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-09 01:41:59.029: E/AndroidRuntime(1579): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-09 01:41:59.029: E/AndroidRuntime(1579): ... 11 more
答案 0 :(得分:3)
你应该改变这个
setContentView(R.layout.activity_about);
到
setContentView(R.layout.fragment_about);
这是因为ListView
文件中的fragment_about.xml
,您会在ListView
文件中找到activity_about.xml
ListView listview = (ListView)findViewById(R.id.ListView); //Cause NULL Pointer Exception
答案 1 :(得分:1)
错误:
在Activity
,您正在致电:
setContentView(R.layout.activity_about);
这会显示布局activity_about.xml
,而您在fragment_about.xml
内创建了自己的观看次数。它会导致 NullPointerException ,因为系统无法找到视图:
findViewById(R.id.ListView); // this 'id' (R.id.ListView) cannot be find
解决方案:
在activity_about.xml
内创建视图或更改目标布局,如下所示:
setContentView(R.layout.fragment_about);
答案 2 :(得分:0)
你得到空指针,因为你正在使用占位符,即用于UI的片段和在活动中编写java代码。以下是解决方案。
1.setContentView(R.layout.fragment_about);
2. change your activity like below:
public class AboutActivity extends ActionBarActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
ArrayAdapter<String> adapter;
public PlaceholderFragment() {
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
String[] lessons = new String[] { "Android Introduction","Android Setup/Installation","Android Hello World","Android Layouts/Viewgroups","Android Activity & Lifecycle","Intents in Android"};
adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1,lessons);
ListView listview = (ListView)rootView.findViewById(R.id.ListView);
listview.setAdapter(adapter);
return rootView;
}
}
}
答案 3 :(得分:0)
尝试替换此行:
setContentView(R.layout.activity_about);
使用:
setContentView(R.layout.fragment_about);
我希望这有帮助!