Question

所以我有这个小代码应该打开标签布局：

public class FriendsList extends ActionBarActivity {
EditText search;
   ActionBar actionBar = getSupportActionBar();
    Activity activity=this;
    CallbackManager callbackManager=null;
ActionBar.TabListener tabListener = new ActionBar.TabListener() {
        @Override
        public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
            // show the given tab
        }
@Override
        public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {
            // hide the given tab
        }
@Override
        public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {
            // probably ignore this event
        }
    };



    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_friends_list);
        FacebookSdk.sdkInitialize(this);

       actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        // Add 3 tabs, specifying the tab's text and TabListener
        for (int i = 0; i < 3; i++) {
            actionBar.addTab(
                    actionBar.newTab()
                            .setText("Tab " + (i + 1))
                            .setTabListener(tabListener));
        }

问题是第二个我启动应用程序，它崩溃了。但是当我删除actionBar引用时，它完美地工作。崩溃时没有logcat，因为这是我的默认应用程序。如有必要，我可以分享我的清单和其余代码。

先谢谢

更新： logcat的;

 ss: com.inc.nicky.messengersayit, PID: 14114
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.inc.nicky.messengersayit/com.inc.nicky.messengersayit.FriendsList}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2297)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2452)
            at android.app.ActivityThread.access$900(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1302)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5586)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:70)
            at android.support.v7.app.AppCompatDelegateImplV7.<init>(AppCompatDelegateImplV7.java:141)
            at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:28)
            at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:29)
            at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:93)
            at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:77)
            at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:429)
            at android.support.v7.app.AppCompatActivity.getSupportActionBar(AppCompatActivity.java:77) 
            at com.inc.nicky.messengersayit.FriendsList.<init>(FriendsList.java:58)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1208)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1067)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2288)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2452)
            at android.app.ActivityThread.access$900(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1302)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5586)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invo

Answer 1

永远不要从数据成员初始化程序中调用从组件基类继承的方法（例如，AppCompatActivity）。等到onCreate()，特别是在super.onCreate()之后。

IOW，你不能在getSupportActionBar()打电话给你。在super.onCreate()返回之前不要打电话。

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