JSON to Kotlin Data Class

JSON → Kotlin data classes. Int/Long are chosen by magnitude, nested objects become their own class, and identical shapes are reused instead of duplicated.

kotlinx.serialization annotations (@Serializable, @SerialName)

JSON{
  "id": 42,
  "user_name": "ada",
  "is_active": true,
  "score": 91.5,
  "profile": {
    "email": "ada@example.com",
    "avatar_url": null
  },
  "roles": ["admin", "editor"],
  "sessions": [
    { "device": "ios", "expires_at": 1735689600 }
  ]
}

Kotlinimport kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable

@Serializable
data class Root(
    val id: Int,
    @SerialName("user_name")
    val userName: String,
    @SerialName("is_active")
    val isActive: Boolean,
    val score: Double,
    val profile: Profile,
    val roles: List<String>,
    val sessions: List<Session>
)

@Serializable
data class Session(
    val device: String,
    @SerialName("expires_at")
    val expiresAt: Int
)

@Serializable
data class Profile(
    val email: String,
    @SerialName("avatar_url")
    val avatarUrl: String? = null
)

JSON to Kotlin Data Class — Free Online Tool

Generates Kotlin data classes from a JSON sample with kotlinx.serialization annotations, choosing Int or Long by magnitude, Double for decimals, nullable types with defaults for JSON nulls, and adding @SerialName only when the camelCase property differs from the wire key, while reusing one class for repeated object shapes.

How to use the JSON to Kotlin Data Class

Open the tool above — it runs entirely in your browser, so your input never leaves this page.
Paste or type your input into the field, then press the relevant button (e.g. Encode / Decode, Generate, or Convert).
Copy the result from the output area with the copy button orCtrl/Cmd + C.
No signup, no upload, and no tracking — repeat as often as you need.

Frequently asked questions

When do I need the @SerialName annotation?

Only when the JSON key differs from the Kotlin property. Kotlin properties are camelCase, so a user_name key needs @SerialName("user_name") on userName; keys that are already camelCase need no annotation and none is emitted.

How does it choose between Int and Long?

By magnitude. Whole numbers that fit in a signed 32-bit range become Int; anything larger becomes Long, which matters for epoch-millisecond timestamps and Snowflake-style IDs.

What happens to null values in the sample?

A null gives no type information, so the property becomes String? with a null default. Replace it with the real type once you know it — a sample payload cannot tell you what a null field would have contained.

Free & private — why use this JSON to Kotlin Data Class

The JSON to Kotlin Data Class runs 100% client-side in your browser. Your data is never uploaded to a server, no account is required, and the tool is completely free. It works offline once the page has loaded and is part of a growing collection of privacy-first developer utilities.

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