Dagger 2 @Inject return Null

Question

I am creating multi module android project using java. I got Null when I inject a class. Followings are my codes.

SignupComponent.class

@FeatureScope
@Subcomponent(modules = SignupModule.class)
public interface SignupComponent {

    @Subcomponent.Factory
    interface Factory{
        SignupComponent create();
    }

    void inject(Activity_Signup activity_signup);

SignupModule.class

@Module
public class SignupModule {

    @FeatureScope
    @Provides
    public SignupContract.Presenter providePresenter(){
        return new SignupPresenter();
    }


}

Activity_Signup.class

SignupComponent signupComponent = ((SignupComponentProvider) getApplicationContext())
                .provideSignupComponent();
        //Inject the view
        signupComponent.inject(this);

SignupPresenter.class

 public class SignupPresenter implements SignupContract.Presenter {

    // this injection return Null
    @Inject
    public SignupUseCase signupUseCase;  


}

SignupUseCase.class

public class SignupUseCase {
    @Inject
    public SignupUseCase(){}
...
}

Why am I getting NPE?

EDIT

SignupModule.class

@Module
public class SignupModule {

    @FeatureScope
    @Provides
    public SignupContract.Presenter providePresenter(){
        return new SignupPresenter();
    }

    @FeatureScope
    @Provides
    public SignupUseCase provideUseCase(){
        return new SignupUseCase();
    }



}

Answer 1

@FeatureScope
@Provides
public SignupContract.Presenter providePresenter(){
    return new SignupPresenter();
}

When you call new SignupPresenter(), Dagger isn't instantiating the object, you are. Therefore, Dagger doesn't inject the @Inject-annotated fields, because it assumes you want the object exactly as you returned it.

If you want Dagger to populate SignupUseCase, you'll need to do one of the following:

• Let Dagger create SignupPresenter and receive it in your @Provides method. You'll need to add an @Inject-annotated constructor to SignupPresenter, potentially with no args.

  @FeatureScope
  @Provides
  public static SignupContract.Presenter providePresenter(SignupPresenter presenter) {
    return presenter;
  }
  

• Let Dagger create SignupPresenter and bind it to SignupContract.Presenter using a @Binds method. This requires the @Inject-annotated constructor on SignupPresenter and also requires making your Module an abstract class.

  @FeatureScope
  @Binds
  abstract SignupContract.Presenter providePresenter(SignupPresenter presenter);
  

• Use a MembersInjector, which you can get via injection. You probably don't want this one unless you're receiving an instance of SignupPresenter that you can add @Inject methods and fields to, but that Dagger can't create.

  @FeatureScope
  @Provides
  public static SignupContract.Presenter providePresenter(
      MembersInjector<SignupPresenter> injector) {
    SignupPresenter presenter = new SignupPresenter();
    injector.injectMembers(presenter);
    return presenter;
  }