MainActivity RecyclerView 和房间数据库崩溃 - Java [重复]

Question

有人可以提供一些帮助吗，我已经尝试在 Stackoverflow 上使用一些解决方案并搜索类似的主题，但没有任何效果。

我有一个使用底部导航栏循环浏览片段的项目。使用片段的主要活动有一个正在运行的回收器视图。实现了 Room 数据库，用于使用 RecyclerView 显示一个简单的笔记功能

启动 MainActivity 时启动屏幕后应用崩溃

MainActivity.java

public class MainActivity extends AppCompatActivity {

private NoteViewModel noteViewModel;
private RecyclerView mRecyclerView;
private RecyclerView.Adapter mAdapter;
private RecyclerView.LayoutManager mLayoutManager;
private ActivityMainBinding binding;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = ActivityMainBinding.inflate(getLayoutInflater());
    setContentView(binding.getRoot());




    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_logbook, R.id.navigation_settings, R.id.navigation_aircraft, R.id.navigation_pilots, R.id.navigation_totals)
            .build();
    NavController navController = Navigation.findNavController(this, R.id.nav_host_fragment_activity_main);
    NavigationUI.setupWithNavController(binding.navView, navController);



    RecyclerView recyclerView = findViewById(R.id.recyclerView);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    recyclerView.setHasFixedSize(true);
    NoteAdapter adapter = new NoteAdapter();
    recyclerView.setAdapter(adapter);
    noteViewModel = new ViewModelProvider.AndroidViewModelFactory(getApplication())
            .create(NoteViewModel.class);
    noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
        @Override
        public void onChanged(List<Note> notes) {


            //update RecyclerView
    adapter.setNotes(notes);



        }


    });



}

}

Logcat

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.company.swiftlogbook/com.company.swiftlogbook.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setLayoutManager(androidx.recyclerview.widget.RecyclerView$LayoutManager)' on a null object reference
    at com.company.swiftlogbook.MainActivity.onCreate(MainActivity.java:50)
    at android.app.Activity.performCreate(Activity.java:8000)
    at android.app.Activity.performCreate(Activity.java:7984)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
    at android.os.Handler.dispatchMessage(Handler.java:106) 
    at android.os.Looper.loop(Looper.java:223) 
    at android.app.ActivityThread.main(ActivityThread.java:7656) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

非常感谢您对此的任何建议或帮助。

Answer 1

这是因为在您的 activity_main.xml 视图中没有 ID 为 recyclerView 的小部件。 请检查您的视图中的ids 或通过从binding 变量而不是findViewById 调用小部件来使用数据绑定的类型安全模式

可能是这样的RecyclerView recyclerView = binding.recyclerView 而不是RecyclerView recyclerView = findViewById(...)