java.lang.NullPointerException: android.widget.ListView.setAdapter(android.widget.ListAdapter)' 在一个空对象引用上

如何解决java.lang.NullPointerException: android.widget.ListView.setAdapter(android.widget.ListAdapter)' 在一个空对象引用上

我在片段中创建一个列表，但我的 ArrayAdapter 出现 NullPointerException。 HomeFragment 中有错误，但我之前正在使用值初始化变量，所以我不明白为什么我会收到 NullPointerException。这是 HomeFragement.java。

import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.viewmodelProvider;

import com.test.mytext.R;
import com.test.mytext.Texts;

import java.util.ArrayList;

public class HomeFragment extends Fragment {

    private Homeviewmodel homeviewmodel;

    public View onCreateView(@NonNull LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
        homeviewmodel =
                new viewmodelProvider(this).get(Homeviewmodel.class);
        View root = inflater.inflate(R.layout.fragment_home,container,false);
        final ListView list = getView().findViewById(R.id.list);

        homeviewmodel.getText().observe(getViewLifecycleOwner(),new Observer<String>() {
            @Override
            public void onChanged(@Nullable String s) {
                ArrayList<String> arrayList = new ArrayList<>();
                arrayList.add("JAVA");
                arrayList.add("ANDROID");
                arrayList.add("C Language");
                arrayList.add("CPP Language");
                arrayList.add("Go Language");
                arrayList.add("AVN SYstemS");
                ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(getContext(),android.R.layout.simple_list_item_1,arrayList);
                //Error Here
                list.setAdapter(arrayAdapter);
                list.setonItemClickListener(new AdapterView.OnItemClickListener() {
                    @Override
                    public void onItemClick(AdapterView<?> parent,View view,int position,long id) {
                        String clickedItem=(String) list.getItemAtPosition(position);
                        Intent intent = new Intent(getActivity(),Texts.class);
                        intent.putExtra("User",clickedItem);
                        startActivity(intent);
                    }
                });
            }
        });
        return root;

    }
}

fragment_home.xml:

<?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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:scaleY="1"
    tools:context=".ui.home.HomeFragment">
        <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:scaleY="1">

        <ListView
            android:id="@+id/list"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:scaleY="1" />
        </LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.drawerlayout.widget.DrawerLayout;
import androidx.navigation.NavController;
import androidx.navigation.Navigation;
import androidx.navigation.ui.AppBarConfiguration;
import androidx.navigation.ui.NavigationUI;
import android.os.Bundle;
import android.view.Menu;
import com.google.android.material.navigation.NavigationView;

public class MainActivity extends AppCompatActivity {
    private AppBarConfiguration mAppBarConfiguration;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        DBHandler dbHandler = new DBHandler(this);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        DrawerLayout drawer = findViewById(R.id.drawer_layout);
        NavigationView navigationView = findViewById(R.id.nav_view);
        mAppBarConfiguration = new AppBarConfiguration.Builder(
                R.id.nav_home,R.id.nav_slideshow)
                .setDrawerLayout(drawer)
                .build();
        NavController navController = Navigation.findNavController(this,R.id.nav_host_fragment);
        NavigationUI.setupActionBarWithNavController(this,navController,mAppBarConfiguration);
        NavigationUI.setupWithNavController(navigationView,navController);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.navigation,menu);
        return true;
    }

    @Override
    public boolean onSupportNavigateUp() {
        NavController navController = Navigation.findNavController(this,R.id.nav_host_fragment);
        return NavigationUI.navigateUp(navController,mAppBarConfiguration)
                || super.onSupportNavigateUp();
    }
}

错误日志

03/12 14:31:20: Launching 'app' on Physical Device.
App restart successful without requiring a re-install.
$ adb shell am start -n "com.test.mytext/com.test.mytext.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER
Connected to process 16080 on device 'samsung-samsung_sm_g920a-03157df3bba77823'.
Capturing and displaying logcat messages from application. This behavior can be disabled in the "Logcat output" section of the "Debugger" settings page.
W/System: ClassLoader referenced unkNown path: /data/app/com.test.mytext-2/lib/arm64
W/art: Before Android 4.1,method android.graphics.PorterDuffColorFilter androidx.vectordrawable.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter,android.content.res.ColorStateList,android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
I/art: Rejecting re-init on prevIoUsly-Failed class java.lang.class<androidx.core.view.ViewCompat$2>: java.lang.NoClassDefFoundError: Failed resolution of: Landroid/view/View$OnUnhandledKeyEventListener;
        at void androidx.core.view.ViewCompat.setonApplyWindowInsetsListener(android.view.View,androidx.core.view.OnApplyWindowInsetsListener) (ViewCompat.java:2463)
        at android.view.ViewGroup androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor() (AppCompatDelegateImpl.java:938)
        at void androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor() (AppCompatDelegateImpl.java:806)
        at void androidx.appcompat.app.AppCompatDelegateImpl.setContentView(int) (AppCompatDelegateImpl.java:693)
        at void androidx.appcompat.app.AppCompatActivity.setContentView(int) (AppCompatActivity.java:170)
        at void com.test.mytext.MainActivity.onCreate(android.os.Bundle) (MainActivity.java:20)
        at void android.app.Activity.performCreate(android.os.Bundle) (Activity.java:6942)
        at void android.app.Instrumentation.callActivityOnCreate(android.app.Activity,android.os.Bundle) (Instrumentation.java:1126)
        at android.app.Activity android.app.ActivityThread.performlaunchActivity(android.app.ActivityThread$ActivityClientRecord,android.content.Intent) (ActivityThread.java:2880)
        at void android.app.ActivityThread.handleLaunchActivity(android.app.ActivityThread$ActivityClientRecord,android.content.Intent,java.lang.String) (ActivityThread.java:2988)
        at void android.app.ActivityThread.-wrap14(android.app.ActivityThread,android.app.ActivityThread$ActivityClientRecord,java.lang.String) (ActivityThread.java:-1)
        at void android.app.ActivityThread$H.handleMessage(android.os.Message) (ActivityThread.java:1631)
        at void android.os.Handler.dispatchMessage(android.os.Message) (Handler.java:102)
        at void android.os.Looper.loop() (Looper.java:154)
        at void android.app.ActivityThread.main(java.lang.String[]) (ActivityThread.java:6682)
        at java.lang.Object java.lang.reflect.Method.invoke!(java.lang.Object,java.lang.Object[]) (Method.java:-2)
        at void com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run() (ZygoteInit.java:1520)
        at void com.android.internal.os.ZygoteInit.main(java.lang.String[]) (ZygoteInit.java:1410)
    Caused by: java.lang.classNotFoundException: Didn't find class "android.view.View$OnUnhandledKeyEventListener" on path: DexPathList[[zip file "/data/app/com.test.mytext-2/base.apk"],nativeLibraryDirectories=[/data/app/com.test.mytext-2/lib/arm64,/system/lib64,/vendor/lib64]]
        at java.lang.class dalvik.system.BaseDexClassLoader.findClass(java.lang.String) (BaseDexClassLoader.java:56)
        at java.lang.class java.lang.classLoader.loadClass(java.lang.String,boolean) (ClassLoader.java:380)
        at java.lang.class java.lang.classLoader.loadClass(java.lang.String) (ClassLoader.java:312)
        at void androidx.core.view.ViewCompat.setonApplyWindowInsetsListener(android.view.View,java.lang.Object[]) (Method.java:-2)
        at void com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run() (ZygoteInit.java:1520)
        at void com.android.internal.os.ZygoteInit.main(java.lang.String[]) (ZygoteInit.java:1410)
D/TextView: setTypeface with style : 0
D/TextView: setTypeface with style : 0
D/TextView: setTypeface with style : 0
D/TextView: setTypeface with style : 0
D/AbsListView: Get MotionRecognitionManager
D/MotionRecognitionManager: mSContextService = com.samsung.android.hardware.context.ISemContextService$Stub$Proxy@23d1828
D/MotionRecognitionManager: motionService = com.samsung.android.gesture.IMotionRecognitionService$Stub$Proxy@84fa941
    motionService = com.samsung.android.gesture.IMotionRecognitionService$Stub$Proxy@84fa941
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.test.mytext,PID: 16080
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.mytext/com.test.mytext.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.listadapter)' on a null object reference
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:2927)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2988)
        at android.app.ActivityThread.-wrap14(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1631)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6682)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.listadapter)' on a null object reference
        at com.test.mytext.ui.home.HomeFragment.onCreateView(HomeFragment.java:43)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2698)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:320)
        at androidx.fragment.app.FragmentManager.movetoState(FragmentManager.java:1187)
        at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
        at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
        at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
        at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849)
        at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2629)
        at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:2577)
        at androidx.fragment.app.Fragment.performActivityCreated(Fragment.java:2722)
        at androidx.fragment.app.FragmentStateManager.activityCreated(FragmentStateManager.java:346)
        at androidx.fragment.app.FragmentManager.movetoState(FragmentManager.java:1188)
        at androidx.fragment.app.FragmentManager.movetoState(FragmentManager.java:1356)
        at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:1434)
        at androidx.fragment.app.FragmentManager.movetoState(FragmentManager.java:1497)
        at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2625)
        at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:2577)
        at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:247)
        at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:541)
        at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:210)
        at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1256)
        at android.app.Activity.performStart(Activity.java:6959)
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:2890)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2988) 
        at android.app.ActivityThread.-wrap14(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1631) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6682) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)