操作栏问题：NPE

Question

我是 Android 新手，我正在尝试设置操作栏。 当我运行应用程序时，我得到一个 NPE。

这是我的代码：

public class HomeActivity extends Activity implements TabListener {
private List<Fragment> fragList;

public HomeActivity() {
    super();
    fragList = new ArrayList<>();
    fragList.add(new HomeFragment());
    fragList.add(new SecondFragment());
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setUpActionBar();
}

protected void setUpActionBar() {
    // Set up the action bar to show a dropdown list.
    ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    // Set up the navigation tabs in the action bar.
    Tab homeTab = actionBar.newTab();
    homeTab.setText("Home");
    homeTab.setTabListener(this);
    actionBar.addTab(homeTab);

    Tab secondTab = actionBar.newTab();
    secondTab.setText("Second");
    secondTab.setTabListener(this);
    actionBar.addTab(secondTab);
}

@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
    int tabPosition = tab.getPosition();
    Fragment f = fragList.get(tabPosition);
    ft.replace(android.R.id.content, f);
}

@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    if (fragList.size() > tab.getPosition()) {
        ft.remove(fragList.get(tab.getPosition()));
    }
}

@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
    onTabSelected(tab, ft);
}
}

HomeFragment：

public class HomeFragment extends NavigationFragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.home_fragment, container);
    return v;
}
}

NavigationFragment，到目前为止什么都没做：

public abstract class NavigationFragment extends Fragment {
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

第二个片段：

public class SecondFragment extends NavigationFragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.second_fragment, container);
    return v;
}

那么这是我的清单文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.lh.mypackage"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="16"
    android:targetSdkVersion="21" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".HomeActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

活动 xml：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.lh.mypackage.HomeActivity"
tools:ignore="MergeRootFrame" >

<Button
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="I am the home button" />

</FrameLayout>

主页片段 xml：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.lh.mypackage.HomeActivity"
tools:ignore="MergeRootFrame" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="I am the home view" />
</FrameLayout>

还有第二个片段xml：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.lh.mypackage.HomeActivity"
tools:ignore="MergeRootFrame" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="I am the temperature view" />

</FrameLayout>

当我运行应用程序时，这是堆栈跟踪

FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lh.mypackage/com.lh.mypackage.HomeActivity}: java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2295)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2349)
at android.app.ActivityThread.access$700(ActivityThread.java:159)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1316)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:176)
at android.app.ActivityThread.main(ActivityThread.java:5419)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.IllegalStateException: The specified child already has a parent. You must call removeView() on the child's parent first.
at android.view.ViewGroup.addViewInner(ViewGroup.java:3739)
at android.view.ViewGroup.addView(ViewGroup.java:3610)
at android.view.ViewGroup.addView(ViewGroup.java:3555)
at android.view.ViewGroup.addView(ViewGroup.java:3531)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:914)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1075)
at android.app.BackStackRecord.run(BackStackRecord.java:682)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1455)
at android.app.Activity.performStart(Activity.java:5381)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2266)
... 11 more

Answer 1

换行：

View v = inflater.inflate(R.layout.second_fragment, container);

与：

View v = inflater.inflate(R.layout.second_fragment, container, false);

现在您正在将膨胀的视图添加到容器中。

Answer 2

愿你在HomeActivity 中忘记setContentView(yourLayoutId)

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    /***
        HERE You FORGOT setContentView();
    ***/

    setUpActionBar();
}