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name: kotlin description: | Kotlin 2.3.0 language patterns, coroutines, and Spring integration Use when: Writing backend services, JPA entities, DTOs, controllers, or any Kotlin code in casino-b allowed-tools: Read, Edit, Write, Glob, Grep, Bash

Kotlin Skill

Kotlin 2.3.0 with Spring Boot 3.2.5 and Java 21 for the casino backend. This codebase uses data classes extensively for entities and DTOs, relies on constructor injection, and follows strict BigDecimal handling for financial operations. Null safety is critical—leverage Kotlin's type system to prevent runtime errors.

Quick Start

Data Class for DTOs

Copydata class CreateBonusRequest(
    @field:NotBlank(message = "Name is required")
    val name: String,

    @field:NotNull(message = "Amount is required")
    val amount: BigDecimal,

    val category: BonusCategory? = null
)

Service with Constructor Injection

Copy@Service
@Transactional
class BonusService(
    private val bonusRepository: BonusRepository,
    private val playerRepository: PlayerRepository,
    private val eventPublisher: EventPublisher
) {
    private val logger = LoggerFactory.getLogger(javaClass)

    fun create(request: CreateBonusRequest): BonusDto {
        logger.info("Creating bonus: ${request.name}")
        val bonus = Bonus(name = request.name, amount = request.amount)
        return BonusDto.from(bonusRepository.save(bonus))
    }
}

Entity with JPA Annotations

Copy@Entity
@Table(name = "bonuses")
data class Bonus(
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Long? = null,

    @Column(nullable = false)
    val name: String,

    @Column(precision = 19, scale = 2)
    val amount: BigDecimal = BigDecimal.ZERO,

    @Enumerated(EnumType.STRING)
    val category: BonusCategory? = null
)

Key Concepts

Common Patterns

Repository Lookup with Null Handling

When: Finding entities that may not exist

Copyfun findById(id: Long): PlayerDto {
    val player = playerRepository.findById(id)
        .orElseThrow { PlayerNotFoundException("Player not found: $id") }
    return PlayerDto.from(player)
}

Safe BigDecimal Comparison

When: Comparing monetary values

Copy// CORRECT - use compareTo
if (balance.compareTo(BigDecimal.ZERO) > 0) {
    processWithdrawal()
}

// WRONG - equals checks scale too
if (balance == BigDecimal.ZERO) { } // May fail unexpectedly

See Also

• patterns
• types
• modules
• errors

Related Skills

• See the spring-boot skill for REST controllers and service configuration
• See the jpa skill for entity mapping and repository patterns
• See the kafka skill for event publishing patterns
• See the postgresql skill for database migrations and queries