TestApp.java

public class TestsApp extends App {
  
    @Override
    protected AppComponent initializeAppComponent() {
        return DaggerAppComponent.builder()
                .applicationModule(new MockApplicationModule(this))
                .databaseModule(new DatabaseModule())
                .networkModule(new MockNetworkModule())
                .picassoModule(new PicassoModule())
                .androidModule(new AndroidModule())
                .androidImplModule(new AndroidImplModule())
                .schedulersModule(new SchedulersModule())
                .daoModule(new DaoModule())
                .utilsModule(new UtilsModule())
                .build();
    }

    @Module
    private class MockApplicationModule extends ApplicationModule {

        MockApplicationModule(@Nonnull final App app) {
            super(app);
        }

        @Override
        public AppPreferences appPreferences(@Dagger.ForApplication final Context context) {
            final AppPreferences mock = Mockito.mock(AppPreferences.class);
            when(mock.isUserValid()).thenReturn(true);
            return mock;
        }
    }

    @Module
    private class MockNetworkModule extends NetworkModule {

        @Override
        public ApiService provideApiService(@Nonnull final Retrofit retrofit,
                                            @Nonnull final Gson gson) {
            return new ApiService() {
              
                @Override
                public Observable<UserResponse> user() {
                    return Observable.fromCallable((Func0<UserResponse>) () -> 
                         gson.fromJson(TestUtils.getFromAssets(context, "user.json"), UserResponse.class));
                }

                @Override
                public Observable<LoginResponse> login(@Field("username") final String userName, 
                                                       @Field("password") final String password, 
                                                       @Field("scope") final String scope) {
                    return userName.equals("user123") ? 
                      Observable.just(gson.fromJson(TestUtils.getFromAssets(context, "sing_in.json"), UserResponse.class)) :
                      Observable.error(new Throwable("Error"));
                }
            };
        }
    }
}