JSON to Kotlin Data Class

About this tool

Paste a JSON response from your API and this converter writes the Kotlin data classes for it. Nested objects become their own classes, arrays become List types, and snake_case keys become camelCase properties with the original name kept in @SerialName, @SerializedName or @Json. A field that is null, or missing from some items of an array, becomes nullable, so the model doesn't crash on real data.

How to use JSON in Kotlin

On Android the usual way today is kotlinx.serialization. Add the Kotlin serialization Gradle plugin and the kotlinx-serialization-json library, mark your classes with @Serializable (the converter adds it when you choose kotlinx.serialization), then read and write JSON with a Json instance. ignoreUnknownKeys = true keeps the app working when the server adds new fields.

import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json

@Serializable
data class User(val id: Int, val name: String, val email: String? = null)

val json = Json { ignoreUnknownKeys = true }

// JSON to Kotlin
val user = json.decodeFromString(User.serializer(), """{"id":1,"name":"Sara"}""")

// Kotlin to JSON
val text = json.encodeToString(User.serializer(), user)   // {"id":1,"name":"Sara"}

Kotlin to JSON

Going the other way, from a Kotlin object to a JSON string, uses the same data class: encodeToString in kotlinx.serialization, toJson() in Gson, or adapter.toJson() in Moshi. kotlinx.serialization leaves out properties that still have their default value unless you set encodeDefaults = true.

Data classes with Gson and Moshi

A data class gives you equals(), hashCode(), toString() and copy() for free, which is what a model needs. With Retrofit, use the converter that matches your project: kotlinx.serialization for new Kotlin and Compose apps, Gson or Moshi when the project already uses them. Gson creates objects without calling the constructor, so it skips Kotlin's default values and can put null into a field declared as String; making such fields nullable avoids that crash.

The Android Studio plugin and other languages

Android Studio and IntelliJ IDEA have a JSON To Kotlin Class plugin that does the same job inside the IDE. This page needs no install and also works on a phone or a computer that isn't yours. Need the same model in another language? The JSON tools below convert to Java, Swift, Dart, TypeScript, Go, C# and Python. C++ has no class step: the nlohmann/json library reads JSON straight into its own json type.

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