Android ViewModel has no zero argument constructor

Question

I am following this documentation to learn about LiveData and ViewModel. In the doc, the ViewModel class has constructor as such,

public class UserModel extends ViewModel {   private MutableLiveData<User> user;    @Inject UserModel(MutableLiveData<User> user) {     this.user = user;   }    public void init() {     if (this.user != null) {       return;     }     this.user = new MutableLiveData<>();   }    public MutableLiveData<User> getUser() {     return user;   } } 

However, when I run the code, I get exception:

final UserViewModelviewModel = ViewModelProviders.of(this).get(UserViewModel.class); 

Caused by: java.lang.RuntimeException: Cannot create an instance of class UserViewModel Caused by: java.lang.InstantiationException: java.lang.Class has no zero argument constructor

Answer 1

In my case as I'm using HILT, it was lacking one annotation above the Fragment that has a ViewModel: @AndroidEntryPoint

@AndroidEntryPoint class BestFragment : Fragment() {  .... 

Of course in your ViewModel class you also need to Annotate with what HILT needs: @ViewModelInject

class BestFragmentViewModel @ViewModelInject constructor(var userManager: UserManager) : ViewModel() { .... } 

Answer 2

While initializing subclasses of ViewModel using ViewModelProviders, by default it expects your UserModel class to have a zero argument constructor. In your case your constructor has the argument MutableLiveData<User> user.

One way to fix this is to have a default no arg constructor for your UserModel.

Otherwise, if you want to have a non-zero argument constructor for your ViewModel class, you may have to create a custom ViewModelFactory class to initialise your ViewModel instance, which implements the ViewModelProvider.Factory interface.

I have not tried this yet, but here's a link to an excellent Google sample for this: github.com/googlesamples/android-architecture-components. Specifically, check out this class GithubViewModelFactory.java for Java code and this class GithubViewModelFactory.kt for the corresponding Kotlin code.