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.

Practice

1. What is the primary purpose of using @HiltAndroidApp annotation in an Android application?

easy

A. To inject dependencies into a ViewModel

B. To mark an Activity as injectable by Hilt

C. To trigger Hilt's code generation and setup the application-level dependency container

D. To define a module that provides dependencies

Solution

1. Step 1: Understand the role of @HiltAndroidApp

   This annotation is placed on the Application class to start Hilt's code generation and create the application-level dependency container.

2. Step 2: Differentiate from other annotations

   @AndroidEntryPoint marks Activities or Fragments, @Module defines providers, and @Inject is for injection points, so they do not setup the application container.

3. Final Answer:

   To trigger Hilt's code generation and setup the application-level dependency container -> Option C

4. Quick Check:

   @HiltAndroidApp = Setup app container [OK]

Hint: Remember: @HiltAndroidApp is only on Application class [OK]

Common Mistakes:

• Confusing @HiltAndroidApp with @AndroidEntryPoint
• Thinking @HiltAndroidApp is for Activities
• Using @HiltAndroidApp on modules

2. Which of the following is the correct way to inject a dependency into an Android Activity using Hilt?

easy

A. Annotate the Activity with @AndroidEntryPoint and use @Inject lateinit var for the dependency

B. Use @Module annotation on the Activity and call inject() manually

C. Add @HiltAndroidApp on the Activity and initialize dependencies in onCreate()

D. Declare dependencies as constructor parameters without annotations

Solution

1. Step 1: Identify how to enable injection in Activity

   Using @AndroidEntryPoint on the Activity allows Hilt to generate necessary code for injection.

2. Step 2: Inject dependencies inside Activity

   Use @Inject lateinit var to declare dependencies that Hilt will provide automatically.

3. Final Answer:

   Annotate the Activity with @AndroidEntryPoint and use @Inject lateinit var for the dependency -> Option A

4. Quick Check:

   @AndroidEntryPoint + @Inject lateinit var = Activity injection [OK]

Hint: Use @AndroidEntryPoint on Activity and @Inject lateinit var inside [OK]

Common Mistakes:

• Trying to use @Module on Activity
• Forgetting @AndroidEntryPoint annotation
• Injecting dependencies via constructor in Activity

3. Given the following Hilt module code, what will be the output when repository.getData() is called?

@Module
@InstallIn(SingletonComponent::class)
object DataModule {
    @Provides
    fun provideData(): String = "Hello Hilt"
}

class Repository @Inject constructor(private val data: String) {
    fun getData() = data
}

medium

A. "DataModule"

B. Compilation error due to missing @Inject on Repository

C. NullPointerException at runtime

D. "Hello Hilt"

Solution

1. Step 1: Understand the provided dependency

   The module provides a String "Hello Hilt" via the provideData() function.

2. Step 2: Check Repository injection

   Repository constructor is annotated with @Inject and receives the String dependency correctly.

3. Final Answer:

   "Hello Hilt" -> Option D

4. Quick Check:

   Provided String = "Hello Hilt" [OK]

Hint: Look for @Provides return value matching injected type [OK]

Common Mistakes:

• Assuming @Inject missing on Repository causes error (it is present)
• Confusing module name with provided value
• Expecting runtime null error without reason

4. Identify the error in the following Hilt setup:

@Module
@InstallIn(ActivityComponent::class)
object NetworkModule {
    @Provides
    fun provideRetrofit(): Retrofit {
        return Retrofit.Builder().baseUrl("https://api.example.com").build()
    }
}

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {
    @Inject lateinit var retrofit: Retrofit

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

medium

A. Retrofit cannot be provided in ActivityComponent scope because it should be Singleton

B. Missing @Inject constructor in Retrofit class

C. MainActivity must be annotated with @HiltAndroidApp

D. Retrofit instance must be created manually, not with @Provides

Solution

1. Step 1: Check the component scope for Retrofit

   Retrofit is usually a singleton because it is expensive to create and shared across app.

2. Step 2: Analyze the module's @InstallIn scope

   The module is installed in ActivityComponent, which creates a new instance per Activity, not singleton.

3. Step 3: Identify the mismatch

   Providing Retrofit in ActivityComponent scope causes multiple instances, which is incorrect for Retrofit.

4. Final Answer:

   Retrofit cannot be provided in ActivityComponent scope because it should be Singleton -> Option A

5. Quick Check:

   Retrofit = Singleton scope needed [OK]

Hint: Provide expensive objects in SingletonComponent scope [OK]

Common Mistakes:

• Thinking Retrofit needs @Inject constructor
• Confusing @HiltAndroidApp with @AndroidEntryPoint
• Believing manual creation is required with @Provides

5. You want to inject two different implementations of an interface Logger using Hilt in your Android app. Which approach correctly distinguishes these implementations for injection?

hard

A. Provide both implementations in the same module without qualifiers and inject List<Logger>

B. Use @Qualifier annotations with different names on each provider and injection site

C. Use @Singleton on both implementations and inject Logger directly

D. Create separate modules for each implementation without any annotations

Solution

1. Step 1: Understand multiple implementations injection

   When multiple implementations of the same interface exist, Hilt needs a way to distinguish them.

2. Step 2: Use @Qualifier annotations

   Custom @Qualifier annotations with different names allow Hilt to know which implementation to inject where.

3. Step 3: Reject other options

   Injecting List<Logger> is possible but does not distinguish single injection points; @Singleton does not differentiate implementations; separate modules without qualifiers cause ambiguity.

4. Final Answer:

   Use @Qualifier annotations with different names on each provider and injection site -> Option B

5. Quick Check:

   Multiple implementations = use @Qualifier [OK]

Hint: Use @Qualifier to name different implementations [OK]

Common Mistakes:

• Injecting interface without qualifiers causes errors
• Assuming @Singleton differentiates implementations
• Not using qualifiers when multiple providers exist