今天想學(xué)BottomNavigationView弄一個(gè)底部欄的導(dǎo)航杯聚，然后我在New->Activity->BottomNavigationActivity創(chuàng)建了一個(gè)例子，然后直接運(yùn)行隧期。發(fā)現(xiàn)app直接崩潰閃退了，run里面也沒(méi)有錯(cuò)誤赘娄，我就找了一下error日志：

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
        at androidx.navigation.ui.ActionBarOnDestinationChangedListener.setTitle(ActionBarOnDestinationChangedListener.java:48)
        at androidx.navigation.ui.AbstractAppBarOnDestinationChangedListener.onDestinationChanged(AbstractAppBarOnDestinationChangedListener.java:104)
        at androidx.navigation.NavController.addOnDestinationChangedListener(NavController.java:233)
        at androidx.navigation.ui.NavigationUI.setupActionBarWithNavController(NavigationUI.java:228)
        at com.example.college.BottomNavActivity.onCreate(BottomNavActivity.java:26)
        at android.app.Activity.performCreate(Activity.java:7224)
        at android.app.Activity.performCreate(Activity.java:7213)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:201) 
        at android.app.ActivityThread.main(ActivityThread.java:6810) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 

我就從出錯(cuò)的方法androidx.navigation.ui.ActionBarOnDestinationChangedListener.setTitle(ActionBarOnDestinationChangedListener.java:48)進(jìn)去：

    @Override
    protected void setTitle(CharSequence title) {
        ActionBar actionBar = mActivity.getSupportActionBar();
        actionBar.setTitle(title);
    }

這里報(bào)了空指針錯(cuò)誤仆潮，就是getSupportBar()沒(méi)有獲取到標(biāo)題欄，我就找了一下應(yīng)用的主題遣臼，發(fā)現(xiàn)：

NoActionBar

然后我把標(biāo)題欄改為：DarkActionBar鸵闪，就OK了！