How to use the JSON to Kotlin
- Paste a JSON sample in the input — Kotlin is already selected.
- Set the root type name and options such as optional fields and date detection.
- Copy the generated Kotlin code into your project.
JSON to Kotlin data classes
Each JSON object becomes a Kotlin data class with val properties; nested objects become their own data classes and arrays become List<T>. Optional fields are nullable with a default of null, and @SerialName keeps the original JSON keys.
Parsing
Mark the classes @Serializable and call Json.decodeFromString<Root>(json) with kotlinx.serialization. Use Json { ignoreUnknownKeys = true } when the API may add fields. The same classes work with Moshi or Gson.
Private by design
The conversion runs entirely in your browser — API responses with customer or internal data are never uploaded.
Tools that pair well with the JSON to Kotlin
- JSON to Types — Paste an API response and get ready-to-use models in 8 languages: TypeScript, Python (dataclass, TypedDict or Pydantic), Go, Java, C#, Kotlin, Swift and Rust.
- JSON Validator — Paste JSON to check whether it is valid.
- JSON Schema Validator — Paste a schema and some JSON to see exactly which values break which rule — or generate a starting schema from example data.
- Mock Data Generator — Create up to 10,000 rows of realistic test data — users, products, orders, addresses, companies or nested API records — or build your own schema.
Frequently asked questions
How are nested JSON objects converted to Kotlin?
Each nested object becomes its own named data classe, named after its key, and referenced from the parent. Arrays become typed lists of the element type.
What happens with fields that are missing or null in some items?
When you paste an array of objects, fields that are missing or null in some items are marked optional (nullable), so the generated code matches real API responses.
Is my JSON uploaded?
No. The conversion runs entirely in your browser, so API responses with private data never leave your device.
Can I install JSON to Kotlin as a web app and use it offline?
Yes. JSON to Kotlin is an installable web app (PWA) — installed from your browser, not downloaded from an app store. Click Install app at the top of this page in Chrome or Edge (Windows, macOS, Linux, ChromeOS) or on Android, use Share → Add to Home Screen on iPhone and iPad, or File → Add to Dock in Safari 17+ on a Mac. It then opens in its own window with its own icon, and because it runs entirely in your browser it keeps working offline. Firefox on desktop can’t install web apps. How installing works.