Dependency injection with Hilt in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Hilt-injected ViewModel initialization?

Consider a ViewModel injected with Hilt in an Android app. What will be the output when accessing the injected repository's data property?

Android Kotlin

class MyRepository @Inject constructor() {
    val data = "Hello from Repository"
}

@HiltViewModel
class MyViewModel @Inject constructor(
    val repository: MyRepository
) : ViewModel()

// In an Activity or Fragment
val viewModel: MyViewModel by viewModels()
println(viewModel.repository.data)

A Hello from Repository

B null

C Compilation error due to missing @Inject on ViewModel

D Runtime crash due to missing Hilt setup

💡 Hint

Remember that Hilt provides the dependencies automatically when properly set up.

The repository is injected into the ViewModel by Hilt, so accessing repository.data returns the string defined in the repository.

📝 Syntax

intermediate

Which option correctly provides a singleton repository with Hilt?

You want to provide a singleton instance of MyRepository using Hilt. Which code snippet correctly achieves this?

Android Kotlin

class MyRepository @Inject constructor() {
    // Repository implementation
}

@Module
@InstallIn(SingletonComponent::class)
object RepositoryModule {
    @Provides
    @Singleton
    fun provideRepository(): MyRepository {
        return MyRepository()
    }
}

A

@Provides
fun provideRepository(): MyRepository = MyRepository()

B

@Provides
@Singleton
fun provideRepository(): MyRepository = MyRepository()

C

@Singleton
fun provideRepository(): MyRepository = MyRepository()

D

@Provides
@Singleton
fun provideRepository(): MyRepository { MyRepository() }

💡 Hint

Look for correct annotations and function syntax.

Option B correctly uses @Provides and @Singleton annotations and returns the repository instance properly.

🔧 Debug

advanced

What error occurs when a Hilt-injected constructor is missing @Inject?

Given this class without @Inject on the constructor, what error will Hilt produce during compilation? class UserRepository { // No @Inject constructor }

A error: @Inject constructor is missing for UserRepository

B error: Duplicate bindings found for UserRepository

C error: Cannot find symbol UserRepository

D No error, Hilt will create the instance automatically

💡 Hint

Hilt requires @Inject on constructors to know how to create instances.

Without @Inject on the constructor, Hilt cannot generate the code to provide UserRepository, causing a compilation error.

❓ state_output

advanced

What is the scope of a dependency annotated with @ActivityScoped in Hilt?

If a repository is annotated with @ActivityScoped, how many instances will exist during the app's lifecycle?

Android Kotlin

@ActivityScoped
class ScopedRepository @Inject constructor() {
    var count = 0
}

A One instance for the entire app lifecycle

B A new instance every time it is injected

C One instance per activity instance

D One instance per fragment instance

💡 Hint

Think about what 'ActivityScoped' means in terms of lifecycle.

@ActivityScoped means the same instance is shared within one activity but different activities get different instances.

🧠 Conceptual

expert

Which statement best describes Hilt's component hierarchy?

Hilt uses a component hierarchy to manage dependency scopes. Which option correctly orders the components from the longest to shortest lifecycle?

A FragmentComponent > ActivityComponent > SingletonComponent > ViewComponent

B ActivityComponent > SingletonComponent > FragmentComponent > ViewComponent

C ViewComponent > FragmentComponent > ActivityComponent > SingletonComponent

D SingletonComponent > ActivityComponent > FragmentComponent > ViewComponent

💡 Hint

Consider which component lives the longest in the app.

SingletonComponent lives for the entire app, ActivityComponent for each activity, FragmentComponent for each fragment, and ViewComponent for each view.