JSON to Kotlin Data Class Converter

Data classes for Android and Ktor

Kotlin projects decode JSON with typed data classes, and kotlinx.serialization is the multiplatform standard for it: it works on Android, the JVM, Kotlin/JS and Kotlin Native, and Ktor uses it out of the box. Writing the classes for a big response is repetitive. Paste a response body from your network inspector or API docs and get classes you can use immediately.

What gets generated

• Every JSON object becomes a @Serializable data class with val properties, and nested objects become their own classes named after their key.
• JSON numbers become Long (whole numbers) or Double (anything with a decimal point or exponent); strings, booleans and arrays become String, Boolean and List<T>.
• Keys that are not idiomatic Kotlin names are converted to camelCase, and @SerialName("first-name") keeps the original key for decoding and encoding.
• Objects inside one array are merged. A property missing from some items, or null in some, becomes nullable with a default: val nick: String? = null. Thanks to the default, decoding succeeds when the key is absent.
• Values that are only ever null, and arrays that mix types such as [1, "a"], are typed as JsonElement. kotlinx.serialization cannot decode a plain union, so you inspect the element at run time instead of risking a crash.
• A JSON array at the root produces typealias Root = List<RootElement>.

The file starts with the kotlinx imports and a usage comment: Json.decodeFromString<Root>(jsonString).

Root type name

The only option, Root type name (default Root), names the top-level class or type alias. It is converted to PascalCase, so api response becomes ApiResponse. Nested classes are named after their keys, and items of a plural key are singularised, so badges holds Badge objects.

Setup and tips

The classes need the serialization plugin and runtime: add kotlin("plugin.serialization") to your Gradle plugins and org.jetbrains.kotlinx:kotlinx-serialization-json to dependencies. Real APIs add fields over time, so create your decoder with Json { ignoreUnknownKeys = true } to avoid failures when they do.

Inference is based only on your sample. Values written exactly as a calendar date (2024-03-11) become java.time.LocalDate, and the file then includes a small LocalDateSerializer registered with @file:UseSerializers — a JVM and Android type, so replace it with kotlinx.datetime.LocalDate in multiplatform code. Full timestamps stay String, and repeated strings are not turned into enums. Integers beyond 2^53 are flagged, since the sample cannot be read exactly; Long covers up to 2^63. For Jackson- or Gson-based projects, the Java class generator produces annotated POJOs instead. Generation runs locally in this tab, and the Kotlin formatter can restyle the result.

Examples

{
  "user_id": 42,
  "display_name": "Ben Okafor",
  "avatar_url": "https://cdn.example.com/u/42.png",
  "followers": 1280,
  "settings": { "dark_mode": true, "language": "en" },
  "badges": [
    { "code": "early", "awarded_at": "2024-03-11" },
    { "code": "mod", "awarded_at": "2025-01-20", "expires_at": null }
  ]
}

// Decode with kotlinx.serialization:
//   val data = Json.decodeFromString<Profile>(jsonString)

import kotlinx.serialization.*
import kotlinx.serialization.json.*
import kotlinx.serialization.descriptors.*
import kotlinx.serialization.encoding.*

@Serializable
data class Profile (
    @SerialName("user_id")
    val userID: Long,

    @SerialName("display_name")
    val displayName: String,

    @SerialName("avatar_url")
    val avatarURL: String,

    val followers: Long,
    val settings: Settings,
    val badges: List<Badge>
)

@Serializable
data class Badge (
    val code: String,

    @SerialName("awarded_at")
    val awardedAt: String,

    @SerialName("expires_at")
    val expiresAt: JsonElement? = null
)

@Serializable
data class Settings (
    @SerialName("dark_mode")
    val darkMode: Boolean,

    val language: String
)

[
  { "id": 1, "title": "Write release notes", "done": true },
  { "id": 2, "title": "Review pricing", "done": false, "due": "2026-10-01" }
]

// Decode with kotlinx.serialization:
//   val data = Json.decodeFromString<Todos>(jsonString)

import kotlinx.serialization.*
import kotlinx.serialization.json.*
import kotlinx.serialization.descriptors.*
import kotlinx.serialization.encoding.*

typealias Todos = List<Todo>

@Serializable
data class Todo (
    val id: Long,
    val title: String,
    val done: Boolean,
    val due: String? = null
)

Common errors and how to fix them

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