MainActivity

public class MainActivity extends AppCompatActivity {

    private EditText mTodoEditText;
    private TextView mResultTextView;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTodoEditText=findViewById(R.id.todo_edit);
        mResultTextView=findViewById(R.id.result_text);

        MainViewModel viewModel= new ViewModelProvider(this).get(MainViewModel.class);


        //UI갱신
        viewModel.getAll().observe(this, todos -> {
            mResultTextView.setText(todos.toString());
        });

        //버튼 클릭시 DB에 insert
        findViewById(R.id.add_buton).setOnClickListener(view -> {
            viewModel.insert(new Todo(mTodoEditText.getText().toString()));

        });

    }


}

MainViewModel

public class MainViewModel extends AndroidViewModel {

    private AppDatabase db;

    public MainViewModel(@NonNull Application application) {
        super(application);
        db= Room.databaseBuilder(application,AppDatabase.class,"todo-db")
                .build();
    }

    public LiveData<List<Todo>> getAll(){
        return db.todoDao().getAll();
    }

    public void insert(Todo todo){
        new InsertAsyncTask(db.todoDao()).execute(todo);
    }


    private static class InsertAsyncTask extends AsyncTask<Todo,Void, Void> {

        private TodoDao mTodoDao;

        public InsertAsyncTask(TodoDao todoDao) {
            this.mTodoDao = todoDao;
        }

        @Override
        protected Void doInBackground(Todo... todos) {
            mTodoDao.insert(todos[0]);
            return null;
        }
    }


}

계속 ViewModel instanc를 생성할수 없다고 나오네요 ViewModel Constructor도 public으로했는데도 에러가 나네요

오류내용입니다

FATAL EXCEPTION: main Process: com.corona.room_exam, PID: 27875 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.corona.room_exam/com.corona.room_exam.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.corona.room_exam.MainViewModel at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 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$H.handleMessage(ActivityThread.java:1808) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:193) at android.app.ActivityThread.main(ActivityThread.java:6669) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) Caused by: java.lang.RuntimeException: Cannot create an instance of class com.corona.room_exam.MainViewModel at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) at com.corona.room_exam.MainActivity.onCreate(MainActivity.java:31) at android.app.Activity.performCreate(Activity.java:7136) at android.app.Activity.performCreate(Activity.java:7127) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)  at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)  at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)  at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)  at android.os.Handler.dispatchMessage(Handler.java:106)  at android.os.Looper.loop(Looper.java:193)  at android.app.ActivityThread.main(ActivityThread.java:6669)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)  Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor at java.lang.Class.newInstance(Native Method) at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219) at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)  at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)  at com.corona.room_exam.MainActivity.onCreate(MainActivity.java:31)  at android.app.Activity.performCreate(Activity.java:7136)  at android.app.Activity.performCreate(Activity.java:7127)  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)  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$H.handleMessage(ActivityThread.java:1808)  at android.os.Handler.dispatchMessage(Handler.java:106)  at android.os.Looper.loop(Looper.java:193)  at android.app.ActivityThread.main(ActivityThread.java:6669)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.2"
    defaultConfig {
        applicationId "com.corona.room_exam"
        minSdkVersion 15
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility = 1.8
        targetCompatibility = 1.8
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation 'androidx.room:room-runtime:2.2.3'
    annotationProcessor 'androidx.room:room-compiler:2.2.3'

    def lifecycle_version = "2.2.0"
    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
}