Features

🎯 Idiomatic Kotlin

Data class generation
Proper Kotlin type mapping
Nested class support
Null-safe types with ?

📦 Multiple Libraries

kotlinx.serialization
Gson annotations
Moshi annotations
Automatic @SerialName

⚡ Smart Type Inference

Int vs Double detection
List<T> for arrays
Nested object classes
Optional nullable fields

Kotlin Data Class Guide

Kotlin Data Classes with Serialization

Kotlin data classes are the idiomatic way to model JSON data. With kotlinx.serialization, you get compile-time safe serialization.

import kotlinx.serialization.Serializable
import kotlinx.serialization.SerialName

@Serializable
data class User(
    val id: Int,
    val name: String,
    @SerialName("is_active")
    val isActive: Boolean
)

Using Generated Classes

import kotlinx.serialization.json.Json

fun main() {
    val jsonString = """{"id": 1, "name": "Alice"}"""
    val user = Json.decodeFromString<User>(jsonString)
    println(user.name) // Alice

    val encoded = Json.encodeToString(user)
    println(encoded)
}

Common Use Cases

Android Apps: Model API responses with Retrofit + kotlinx.serialization
Ktor Backend: Request/response models for Ktor server applications
Multiplatform: Shared data models for KMP projects
Spring Boot: DTO classes for Spring Boot Kotlin applications

FAQ

Q: When should I use @SerialName?

A: Use @SerialName when the JSON key doesn't match the Kotlin property name — for example when the JSON uses snake_case but your Kotlin uses camelCase.

Q: val vs var — which should I use?

A: Prefer val (immutable) for data classes. Use var only when you need to mutate fields after construction, which is rare for JSON model classes.

Making generated Kotlin data classes robust

Kotlin serialization libraries fail loudly when JSON does not match the declared types. That is useful, but it means a generated data class should be reviewed against the real range of data your app will receive.

Tips and common pitfalls

Mark fields nullable (String?) when the API can omit them or send null, and give them a default of null.
With kotlinx.serialization, configure Json { ignoreUnknownKeys = true } so new server fields do not crash older app versions.
Use Long rather than Int for IDs and counters that can exceed about 2.1 billion.
Map fixed string values to an enum class with @SerialName on each constant.

JSON to Kotlin Data Class