Generate Kotlin Data Classes
from JSON

Kotlin data classes in four steps

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Common questions answered

Converting JSON to Kotlin data classes

Kotlin data classes provide equals(), hashCode(), toString(), and copy() automatically. Combined with kotlinx.serialization or Gson, data classes make JSON deserialization type-safe and concise. This tool generates Kotlin data class definitions with proper types (String, Int, Double, Boolean), nullable handling (String?), and @SerialName annotations.

The converter produces idiomatic Kotlin: val properties for immutability, nullable types for optional fields, List<T> for arrays, and nested data classes. When JSON keys use snake_case, it adds @SerialName annotations. The output works with kotlinx.serialization, Gson, Moshi, and Jackson.

{
  "user_name": "Alice",
  "age": 30,
  "roles": ["admin"],
  "bio": null
}

@Serializable
data class Root(
    @SerialName("user_name")
    val userName: String,
    val age: Int,
    val roles: List<String>,
    val bio: String?
)

Get the most out of this tool

• Kotlin data classes provide copy() for immutable updates — prefer val properties for safer data handling.
• Use nullable types (String?) for fields that may be null in API responses to avoid NullPointerException.
• @SerialName maps JSON keys to Kotlin properties — useful when JSON uses snake_case and Kotlin uses camelCase.

Kotlin data classes for JSON explained

Kotlin is the primary language for Android development and is increasingly used for server-side development with frameworks like Ktor and Spring Boot. Kotlin's data class feature is ideal for representing JSON: a single line declares a class with constructor parameters, equality checks, toString, and copy methods automatically. Combined with kotlinx.serialization or Gson, Kotlin data classes provide a clean, concise way to work with JSON APIs.

The converter generates Kotlin data classes with proper Kotlin type conventions. JSON strings become String, numbers become Int or Long (for integers) and Double (for floats), booleans become Boolean, null-capable fields use nullable types (String?), and arrays become List<T>. Field names are converted to camelCase to follow Kotlin naming conventions, while the @SerializedName or @JsonProperty annotations preserve the original JSON key for serialization.

Kotlin's null safety system is particularly important for JSON deserialization. JSON fields can be absent or null — Kotlin distinguishes these with nullable types (String? can be null) versus non-nullable types (String cannot be null). When a JSON field might be missing, marking it as nullable with a default null value prevents NullPointerExceptions during deserialization. The converter infers nullability from the data sample, but you should review and adjust based on your API's documented contract.

For Android development specifically, Kotlin data classes work with both Gson (Google's JSON library, widely used in Android) and kotlinx.serialization (JetBrains' multiplatform serialization library). The @SerializedName annotation is Gson-specific. The @SerialName annotation is for kotlinx.serialization. The converter generates Gson annotations by default — if you use kotlinx.serialization, replace @SerializedName with @SerialName from the kotlinx.serialization.SerialName import.

Retrofit, the most popular HTTP client library for Android, uses Gson or Moshi as its JSON converter. When you define a Retrofit API interface returning a data class, Retrofit automatically deserializes the JSON response into your data class. The generated Kotlin data classes from this tool are directly compatible with Retrofit + Gson — you can paste them into your Android project and use them as Retrofit response types immediately.

Kotlin multiplatform (KMP) projects can share data class definitions between Android, iOS (via Kotlin/Native), and server-side (JVM) code. Using kotlinx.serialization (which is multiplatform-compatible unlike Gson), you can define the data classes once and use them across all platforms. This is a major advantage of Kotlin over Java or Swift — a single source of truth for your data model shared across the entire application.

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