Convert Simple JSON to a Kotlin Data Class

From JSON to a Kotlin Data Class

The simplest JSON-to-Kotlin conversion maps a flat JSON object to a data class. Each JSON key becomes a val property with an inferred Kotlin type.

Example JSON

{
  "id": 42,
  "name": "Alice",
  "email": "alice@example.com",
  "active": true,
  "score": 98.5
}

Generated Kotlin

data class User(
    val id: Int,
    val name: String,
    val email: String,
    val active: Boolean,
    val score: Double
)

Type Mapping Rules

JSON type	Kotlin type
string	String
integer	Int (or Long for large values)
decimal	Double
boolean	Boolean
null	String? (nullable)

Why Data Classes?

Kotlin data class automatically generates equals(), hashCode(), toString(), copy(), and destructuring declarations. This makes them ideal for holding JSON data because you get value-based comparison and easy cloning out of the box.

Deserialization with Kotlinx.serialization

@Serializable
data class User(
    val id: Int,
    val name: String,
    val email: String,
    val active: Boolean,
    val score: Double
)

val user = Json.decodeFromString<User>(jsonString)

Deserialization with Gson

val user = Gson().fromJson(jsonString, User::class.java)

This is the foundation of all JSON-to-Kotlin conversions. Understanding this basic mapping is essential before moving on to nested objects, nullable fields, and collections.