具体错误如下
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.zjm.myapplication, PID: 6588
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.zjm.myapplication/com.zjm.myapplication.footNavigation.FootNavActivity}: 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:2668)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2729)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1480)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6176)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:893)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:783)
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:103)
at androidx.navigation.NavController.addOnDestinationChangedListener(NavController.java:233)
at androidx.navigation.ui.NavigationUI.setupActionBarWithNavController(NavigationUI.java:237)
at com.zjm.myapplication.footNavigation.FootNavActivity.onCreate(FootNavActivity.java:31)
at android.app.Activity.performCreate(Activity.java:6692)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2621)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2729)?
at android.app.ActivityThread.-wrap12(ActivityThread.java)?
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1480)?
at android.os.Handler.dispatchMessage(Handler.java:102)?
at android.os.Looper.loop(Looper.java:154)?
at android.app.ActivityThread.main(ActivityThread.java:6176)?
at java.lang.reflect.Method.invoke(Native Method)?
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:893)?
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:783)?
布局：
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_foot_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".footNavigation.FootNavActivity">
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/bottomNavigationView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:menu="@menu/menu_foot_nav" />
<androidx.fragment.app.FragmentContainerView
android:id="@+id/fragmentContainerView2"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="match_parent"
android:layout_height="0dp"
app:defaultNavHost="true"
app:layout_constraintBottom_toTopOf="@+id/bottomNavigationView"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/nav_graph_foot" />
</androidx.constraintlayout.widget.ConstraintLayout>代码：
public class FootNavActivity extends AppCompatActivity {
private ActivityFootNavBinding binding;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = ActivityFootNavBinding.inflate(getLayoutInflater());
setContentView(binding.getRoot());
BottomNavigationView bottomNavigationView = findViewById(R.id.bottomNavigationView);
NavHostFragment navHostFragment = (NavHostFragment)getSupportFragmentManager().findFragmentById(R.id.fragmentContainerView2);
NavController navController = navHostFragment.getNavController();
AppBarConfiguration appBarConfiguration = new AppBarConfiguration.Builder(navController.getGraph()).build();
NavigationUI.setupActionBarWithNavController(this,navController,appBarConfiguration);
NavigationUI.setupWithNavController(bottomNavigationView,navController);
}
}其实就是空指针异常，一般是找不到对应的组件或者布局文件，突然想到我之前在Style.xml文件中设置过标题栏的样式是没有标题栏，而我的代码里对标题栏进行了设置? 肯定会因为找不到标题栏而报空指针啦
?
?解决办法：把导航栏改成默认有状态栏显示的主题就好了
?
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