How does the tool decide a property is nullable?

When the JSON root is an array of objects, the tool compares the keys present across every item. Any key missing from at least one item, or any key whose value is null on at least one item, becomes a nullable property with a `= null` default.

How does the tool name the Kotlin properties?

JSON keys are converted to camelCase, for example first_name becomes firstName. If the converted name differs from the original key, a comment above the property shows the original JSON key.

What Kotlin types does it use for numbers and strings?

Strings map to String, whole numbers map to Int (or Long once a value is too large for Int), decimal numbers map to Double, and booleans map to Boolean.

How are nested objects and arrays handled?

Each nested object gets its own named data class, generated from the property name, and the parent property references it by type. Arrays become List where T is inferred from the array elements.

Does the output include kotlinx.serialization or Moshi annotations?

No. The generated classes are plain Kotlin data classes with no external library dependency. Add @Serializable or @Json annotations yourself if your JSON keys and Kotlin property names differ and you use one of those libraries.

Is my JSON sent to a server?

No. Parsing and code generation run entirely in your browser with plain JavaScript. Your data never leaves your device.

JSON to Kotlin

Paste JSON and instantly get matching Kotlin data class declarations with nested classes and nullable-safe types.

Input JSON

Root class name

Use val (uncheck for var)

Kotlin Output

Paste JSON and click Generate

Copied!

Summary

JSON to Kotlin reads a JSON value and generates matching Kotlin data class declarations, including separate classes for nested objects and List types for arrays. When a field is missing from some entries in an array of objects, or the field can hold a null value, the generated property is typed nullable with a `= null` default so the class stays safe to construct from partial data. Field names are converted to camelCase and the original JSON key is noted in a comment whenever it differs from the Kotlin property name.

How it works

Paste or type JSON into the input panel, or load a sample.
Set a root class name (defaults to RootObject).
Click Generate to parse the JSON and build Kotlin data classes.
Nested objects become their own named data classes; arrays of objects are scanned across all items to detect optional fields.
Fields that are missing from some items, or that hold null, become nullable properties with a null default.
Click Copy to copy the Kotlin code to your clipboard.

Use cases

Scaffold data classes for a REST API response before writing a Retrofit or Ktor client.
Turn a sample config file into a typed Kotlin settings class.
Generate a starting model for kotlinx.serialization or Moshi without typing every field by hand.
Document a third-party API response shape as Kotlin types.
Check which fields are optional before writing null-handling logic by hand.
Speed up adding an Android or backend Kotlin model from sample JSON.

Frequently Asked Questions

Last updated: 2026-07-13 · Reviewed by Nham Vu