Android 底部导航 Activity 在 onCreate 中给出空指针异常

Question

我在我的项目中添加了一个底部导航 Activity ，但是当我开始相关 Activity 时，它给出了一个空指针异常。

它说异常在第 28 行被触发

at lk.apiit.eea.mobile.Activities.CompanyProfile.onCreate(CompanyProfile.java:28)

以下是异常和代码请帮我解决这个问题。谢谢

java.lang.RuntimeException: Unable to start activity ComponentInfo{lk.apiit.eea.mobile/lk.apiit.eea.mobile.Activities.CompanyProfile}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setTitle(java.lang.CharSequence)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2720)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2781)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1508)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:241)
    at android.app.ActivityThread.main(ActivityThread.java:6274)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
 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:100)
    at androidx.navigation.NavController.addOnDestinationChangedListener(NavController.java:218)
    at androidx.navigation.ui.NavigationUI.setupActionBarWithNavController(NavigationUI.java:220)
    at lk.apiit.eea.mobile.Activities.CompanyProfile.onCreate(CompanyProfile.java:28)
    at android.app.Activity.performCreate(Activity.java:6720)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2673)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2781) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1508) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:241) 
    at android.app.ActivityThread.main(ActivityThread.java:6274) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

这是onCreateMethod中的代码

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_company_profile);
    BottomNavigationView navView = findViewById(R.id.nav_view);
    // Passing each menu ID as a set of Ids because each
    // menu should be considered as top level destinations.
    AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(
            R.id.navigation_home, R.id.navigation_dashboard, R.id.navigation_notifications)
            .build();
    NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment);
    NavigationUI.setupActionBarWithNavController(this, navController, appBarConfiguration);
    NavigationUI.setupWithNavController(navView, navController);
}

Answer 1

如果您没有调用 setSupportActionBar() 并且您使用的是 NoActionBar 主题，那么您就没有 ActionBar。 setupActionBarWithNavController() 假设您有一个 ActionBar，因此会出现 NullPointerException。

在调用 setupActionBarWithNavController() 之前，您应该切换到提供 ActionBar 的主题或使用布局中的 Toolbar 调用 setSupportActionBar() .