What is the best way to inject a class of a child type with Dagger 2?

Question

Is it just to provide the child type in the module ? or is there a more direct way eg by using constructor injection with some parameter ?

@Module
class TestModule() {

  @Provides
  @Singleton
  fun provideDummy(): Dummy = DummyChild()

}

class MainActivity : AppCompatActivity() {
    @Inject
    lateinit var dummy: Dumy

    override fun onCreate(savedInstanceState: Bundle?) {
       ...
    }

    ...
}

Answer 1

That's how it needs to be done. The Module provides components that can be injected.

In the above case TestModule is the module which can provide DummyChild . It becomes a complicated if the constructor of DummyChild required a parameter. In that case the parameter needs to be injected as well.

@Module
class TestModule() {

@Provides
@Singleton
fun provideDummy(Context context): Dummy = DummyChild(context)
}

In the above case, context needs to be provided as well. You should have an exposed Provides method that can provide Context.

Answer 2

or is there a more direct way eg by using constructor injection with some parameter ?

Absolutely!

@Module
class TestModule() {

  @Provides
  @Singleton
  fun provideDummy(): Dummy = DummyChild() // bad, we construct it ourselves!

}

You should let Dagger care about where the object comes from, so calling the constructor is usually not a good idea (unless this is really what you want). The proper way to bind an implementation is by using a @Provides method and requesting the child, directly returning it, or just using the @Binds annotation which will do effectively the same, but possibly more optimized than @Provides .

@Module
class TestModule() {

  @Provides
  fun provideDummy(implementation : DummyChild): Dummy = implementation // better!

  // or with @Binds

  @Binds
  abstract fun provideDummy(implementation : DummyChild): Dummy // also good

}

@Singleton class DummyChild @Inject constructor()

Notice that I removed the @Singleton from the methods, as this is usually an implementation detail of DummyChild , rather than the interface, but you can of course still add it to the method if you wish to do so.