java-to-kotlin

Java to Kotlin Migration

Systematic conversion of Java codebases to idiomatic Kotlin, with framework-aware transformations for Nav services. Covers the full journey from faithful translation through nullability audit, collection migration, and idiomatic transforms — plus framework-specific patterns for Spring→Ktor, JPA→Kotliquery, JUnit→Kotest, and Lombok elimination.

When to Use

• Migrating existing Java services to Kotlin
• Converting Java files in mixed Java/Kotlin codebases
• Onboarding teams from Java to Kotlin patterns
• Planning a phased migration strategy for a Nav service

4-Step Conversion Methodology

Step 1: Faithful Translation

Direct Java → Kotlin conversion preserving exact behavior. Use IntelliJ's built-in converter as a starting point, then fix compilation errors. Keep all existing tests passing. No idiomatic changes yet — correctness first.

// Java — before
public class UserService {
    private final UserRepository repository;
    private final Logger log = LoggerFactory.getLogger(UserService.class);

    public UserService(UserRepository repository) {
        this.repository = repository;
    }

    public User findById(Long id) {
        User user = repository.findById(id);
        if (user == null) {
            throw new NotFoundException("User not found: " + id);
        }
        log.info("Found user: {}", user.getName());
        return user;
    }

    public List<User> findActive() {
        return repository.findAll().stream()
            .filter(u -> u.getStatus().equals("active"))
            .collect(Collectors.toList());
    }
}

// Kotlin — Step 1: faithful translation (no idiomatic changes)
class UserService(private val repository: UserRepository) {
    private val log = LoggerFactory.getLogger(UserService::class.java)

    fun findById(id: Long): User {
        val user = repository.findById(id)
        if (user == null) {
            throw NotFoundException("User not found: $id")
        }
        log.info("Found user: {}", user.name)
        return user
    }

    fun findActive(): List<User> {
        return repository.findAll().stream()
            .filter { u -> u.status == "active" }
            .collect(Collectors.toList())
    }
}