我正在尝试在我的应用程序中实现mvvm设计模式，我已经创建了viewmodel和repository类，但是当我尝试在main活动中示例化viewmodel时，它会在下面显示错误红线 MainActivity 在下面的行中示例化时。 pdfViewModel = new ViewModelProvider(MainActivity.this).get(PdfViewModel.class); 下面是我的代码：
主活动.java

public class MainActivity extends AppCompatActivity {

    PdfViewModel pdfViewModel;

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

    pdfViewModel = new ViewModelProvider(MainActivity.this).get(PdfViewModel.class);

 }
}

pdfviewmodel.java文件

public class PdfViewModel extends AndroidViewModel {

private PdfRepository pdfRepository;

public PdfViewModel(@NonNull Application application) {
    super(application);
    pdfRepository = new PdfRepository(application);
}

public LiveData<List<Pdfs>> getAllPdfs(){
    return pdfRepository.getMutableLiveData();
}
}

pdfrepository.java文件

public class PdfRepository {

private ArrayList<Pdfs> list = new ArrayList<>();
private MutableLiveData<List<Pdfs>> mutableLiveData = new MutableLiveData<>();
private Application application;

public PdfRepository(Application application){
    this.application = application;
}

public MutableLiveData<List<Pdfs>>  getMutableLiveData(){

    SharedPreferences preferences = application.getSharedPreferences("Credentials", Context.MODE_PRIVATE);
    String email = preferences.getString("email",null);

    Retrofit retrofit = RetrofitClient.getInstance();
    ApiService apiService = retrofit.create(ApiService.class);

    Call<List<Pdfs>> call = apiService.getFiles(email);

    call.enqueue(new Callback<List<Pdfs>>() {
        @Override
        public void onResponse(Call<List<Pdfs>> call, Response<List<Pdfs>> response) {

            if(response.body() != null){
                list = (ArrayList<Pdfs>) response.body();
                mutableLiveData.setValue(list);
            }
        }

        @Override
        public void onFailure(Call<List<Pdfs>> call, Throwable t) {

            TastyToast.makeText(application,t.getMessage(),TastyToast.LENGTH_SHORT,TastyToast.ERROR).show();
        }
    });

    return mutableLiveData;
}

}

以上代码中需要更正什么？