/kotlin-tooling-java-to-kotlin

Dagger / Hilt Conversion Guide

When This Applies

This guide applies when the Java source contains imports matching dagger.* or dagger.hilt.*. This covers Dagger 2, Hilt for Android, and Hilt Jetpack integrations.

Key Rules

1. @Inject constructor syntax

Kotlin places @Inject before the constructor keyword in the primary constructor:

class Foo @Inject constructor(private val bar: Bar)

2. @Module classes with @Provides methods

Keep @Provides methods open, or use object for modules that contain only @JvmStatic provides methods (companion object pattern):

@Module
@InstallIn(SingletonComponent::class)
object NetworkModule {
    @Provides
    @Singleton
    fun provideOkHttpClient(): OkHttpClient = OkHttpClient.Builder().build()
}

3. @Binds abstract methods

@Binds methods work in abstract classes exactly as in Java. Convert the abstract class directly — no special Kotlin considerations.

4. Hilt Android annotations

@HiltAndroidApp, @AndroidEntryPoint, @HiltViewModel — preserve these exactly on Application, Activity, Fragment, and ViewModel classes.

5. Scoping annotations

@Singleton, @ActivityScoped, @ViewModelScoped, @FragmentScoped — preserve exactly. No annotation site target is needed.

6. @AssistedInject / @AssistedFactory

@AssistedInject replaces @Inject on the constructor. @Assisted parameters appear alongside regular injected parameters in the primary constructor:

class PlayerViewModel @AssistedInject constructor(
    @Assisted private val playerId: String,
    private val repository: PlayerRepository
) : ViewModel()

7. @Component / @Subcomponent interfaces

Convert directly to Kotlin interfaces. Dagger's annotation processing works identically with Kotlin interfaces via kapt or KSP.

Examples

Example 1: Hilt ViewModel with @Inject Constructor and a @Module

Java:

package com.acme.feature;

import androidx.lifecycle.ViewModel;
import dagger.Module;
import dagger.Provides;
import dagger.hilt.InstallIn;
import dagger.hilt.android.lifecycle.HiltViewModel;
import dagger.hilt.components.SingletonComponent;
import javax.inject.Inject;
import javax.inject.Singleton;

@HiltViewModel
public class UserProfileViewModel extends ViewModel {

    private final UserRepository userRepository;
    private final AnalyticsTracker analyticsTracker;

    @Inject
    public UserProfileViewModel(UserRepository userRepository, AnalyticsTracker analyticsTracker) {
        this.userRepository = userRepository;
        this.analyticsTracker = analyticsTracker;
    }

    public LiveData<User> getUser(String userId) {
        analyticsTracker.trackProfileView(userId);
        return userRepository.getUser(userId);
    }
}

@Module
@InstallIn(SingletonComponent.class)
public class AnalyticsModule {

    @Provides
    @Singleton
    public AnalyticsTracker provideAnalyticsTracker(Application app) {
        return new AnalyticsTracker(app);
    }
}

Kotlin:

package com.acme.feature

import androidx.lifecycle.LiveData
import androidx.lifecycle.ViewModel
import dagger.Module
import dagger.Provides
import dagger.hilt.InstallIn
import dagger.hilt.android.lifecycle.HiltViewModel
import dagger.hilt.components.SingletonComponent
import javax.inject.Inject
import javax.inject.Singleton

@HiltViewModel
class UserProfileViewModel @Inject constructor(
    private val userRepository: UserRepository,
    private val analyticsTracker: AnalyticsTracker
) : ViewModel() {

    fun getUser(userId: String): LiveData<User> {
        analyticsTracker.trackProfileView(userId)
        return userRepository.getUser(userId)
    }
}

@Module
@InstallIn(SingletonComponent::class)
object AnalyticsModule {

    @Provides
    @Singleton
    fun provideAnalyticsTracker(app: Application): AnalyticsTracker {
        return AnalyticsTracker(app)
    }
}

Key changes:

• @Inject moves before the constructor keyword in the primary constructor.
• Constructor parameters become private val in the primary constructor.
• The module class becomes an object since it contains only static-like provides methods.
• SingletonComponent.class becomes SingletonComponent::class (Kotlin class reference).
• Java getter method getUser becomes a regular function getUser (no get prefix convention change needed here since it takes a parameter).