JSONStudio — Free Online JSON Formatter, Viewer & Validator

JSON to Kotlin: a worked example

This is real output from the converter on this page, not an illustration. The input below deliberately includes the cases that trip up naive converters — a float, a null, an empty array, an array of mixed types, and a nested object.

Input JSON

{
  "id": 101,
  "name": "Ada Lovelace",
  "active": true,
  "score": 9.5,
  "deletedAt": null,
  "tags": [
    "admin",
    "ops"
  ],
  "scores": [],
  "mixed": [
    1,
    "two"
  ],
  "createdAt": "2026-01-15T08:00:00.000Z",
  "profile": {
    "city": "Lisbon",
    "zip": 1000
  },
  "addresses": [
    {
      "line1": "1 Rua Augusta",
      "country": "PT"
    }
  ]
}

Generated Kotlin

data class User(
  val id: Long,
  val name: String,
  val active: Boolean,
  val score: Double,
  val deletedAt: String??,
  val tags: List<String>,
  val scores: List<Any>,
  val mixed: List<Any>,
  val createdAt: String,
  val profile: {
  val city: String,
  val zip: Long
},
  val addresses: List<{
  val line1: String,
  val country: String
}>
)

How each JSON type maps to Kotlin

Every row is generated by running that JSON value through the converter, so the table always matches what the tool actually emits.

JSON typeExample valueGenerated Kotlin
string"Ada Lovelace"val field: String
integer101val field: Long
float9.5val field: Double
booleantrueval field: Boolean
nullnullval field: String??
array of strings["admin", "ops"]val field: List<String>
empty array[]val field: List<Any>
mixed-type array[1, "two"]val field: List<Any>
nested object{ "city": "Lisbon" }see the full example above
date-like string"2026-01-15T08:00:00.000Z"val field: String

JSON to Kotlin Converter – Free Online Tool

Convert JSON to Kotlin instantly with JSONStudio's free online converter. Whether you're a backend developer working with configuration files, a DevOps engineer managing infrastructure as code, or a data analyst processing API responses, ourKotlin converter provides instant, accurate conversions with zero data uploads.

JSONStudio's JSON to Kotlin converter runs entirely in your browser, ensuring maximum security and privacy. All processing happens client-side, so your sensitive JSON data never leaves your device. No signup, no tracking, no storage—just fast, reliable conversion when you need it.

What is JSON?

JSON (JavaScript Object Notation) is a lightweight data interchange format that's become the de facto standard for API communication, configuration files, and structured data storage. Its human-readable syntax and language-agnostic design make it ideal for modern web development, mobile apps, and microservices architectures.

JSON is used everywhere—from REST APIs and GraphQL responses to configuration files and database exports. Its simplicity and widespread support across programming languages have made it the preferred format for data exchange in modern software development.

What is Kotlin?

Kotlin is a modern, concise language that runs on the Java Virtual Machine (JVM) and is the preferred language for Android development. Kotlin's `data class` feature is perfect for holding data parsed from JSON.

This converter takes your JSON and instantly turns it into Kotlin data classes, leveraging Kotlin's concise syntax to create readable and type-safe data models.

Why Convert JSON to Kotlin?

  • Concise code: Generate compact Kotlin data classes instead of verbose Java code
  • Null safety: Detects nullable fields and uses Kotlin's nullable types (`String?`)
  • Android ready: Perfect for Android apps consuming REST APIs with Retrofit/Moshi/Gson
  • Modern syntax: Uses `val` for immutable properties by default

How to Convert JSON to Kotlin Online

  1. Paste your JSON: Copy your JSON data from any source—API response, configuration file, or database export—and paste it into the left panel.
  2. Automatic validation: JSONStudio instantly validates your JSON syntax and highlights any errors, ensuring your data is properly formatted before conversion.
  3. Instant conversion: Select "Kotlin" from the converter dropdown and watch as your JSON transforms into Kotlin format in real-time.
  4. Copy or download: Use the one-click copy button to copy the converted Kotlin output, or download it directly to your device.

All conversion happens entirely in your browser using client-side processing. Your JSON data is never uploaded to any server, ensuring complete privacy and security for sensitive information.

Features of JSONStudio JSON to Kotlin Converter

  • Instant conversion: Real-time conversion with no delays or waiting
  • Large file support: Handle JSON files of any size without performance issues
  • Syntax highlighting: Color-coded output makes it easy to read and verify
  • Error detection: Automatic validation catches JSON syntax errors before conversion
  • One-click copy: Copy converted output to clipboard instantly
  • No signup required: Start converting immediately without creating an account
  • Free forever: All features available at no cost, with no usage limits
  • Dark mode support: Comfortable viewing in any lighting condition

Example JSON to Kotlin Conversion

Input JSON:

{
  "name": "John Doe",
  "age": 30,
  "email": "john@example.com",
  "active": true
}

Converted Kotlin:

data class Root(
  val name: String,
  val age: Long,
  val email: String,
  val active: Boolean
)

Is JSONStudio Secure?

100% client-side processing: All JSON conversion happens entirely in your browser using JavaScript. Your data never leaves your device, ensuring complete privacy and security.

No logging or tracking: JSONStudio doesn't log, store, or track any of your JSON data. We don't use analytics that capture your content, and we don't maintain any server-side storage of your files.

No data uploads: Unlike many online converters that upload your files to servers, JSONStudio processes everything locally. This means even if you're working with sensitive API keys, passwords, or proprietary data, it remains completely private.

No server involvement: JSONStudio is a fully static site with no backend server. There is no server to send your data to, even accidentally.

Who Should Use This Tool?

  • Android developers
  • Kotlin backend developers (Ktor, Spring Boot with Kotlin)
  • Java developers migrating to Kotlin

Frequently Asked Questions

Does it handle nullable types?

Yes, if the converter detects null values or missing fields in the JSON structure, it will generate nullable types (e.g., `String?`) in Kotlin.

Is this compatible with serialization libraries?

Yes, the generated data classes work well with libraries like `kotlinx.serialization`, Moshi, and Gson.

Is it free?

Yes, completely free and runs in your browser.

Related Tools

JSONStudio offers a complete suite of JSON tools to help you work with JSON data more effectively: