Add Companion Object Factory Methods to Kotlin Data Classes

Companion Objects in Kotlin Data Classes

After converting JSON to a Kotlin data class, you can enhance it with a companion object that provides factory methods, constants, and parsing utilities.

Basic Data Class from JSON

{
  "latitude": 37.7749,
  "longitude": -122.4194,
  "label": "San Francisco"
}

data class Location(
    val latitude: Double,
    val longitude: Double,
    val label: String
) {
    companion object {
        val ORIGIN = Location(0.0, 0.0, "Origin")

        fun fromJson(jsonString: String): Location =
            Json.decodeFromString(jsonString)

        fun parse(lat: String, lng: String, label: String = "Unknown"): Location =
            Location(lat.toDouble(), lng.toDouble(), label)
    }
}

Usage

val sf = Location.fromJson(jsonString)
val origin = Location.ORIGIN
val parsed = Location.parse("40.7128", "-74.0060", "New York")

Factory Method Patterns

Validation factory:

companion object {
    fun create(lat: Double, lng: Double, label: String): Location {
        require(lat in -90.0..90.0) { "Invalid latitude" }
        require(lng in -180.0..180.0) { "Invalid longitude" }
        return Location(lat, lng, label)
    }
}

Default instance:

companion object {
    val DEFAULT = Location(0.0, 0.0, "Unknown")
    val EMPTY = Location(0.0, 0.0, "")
}

The invoke Operator

companion object {
    operator fun invoke(jsonString: String): Location =
        Json.decodeFromString(jsonString)
}

// Looks like a constructor call:
val location = Location(jsonString)

Why Companion Objects?

• Encapsulation -- parsing logic lives with the data class, not scattered across the codebase
• Named constructors -- Location.fromJson(), Location.fromCsv(), Location.fromDatabase()
• Constants -- shared instances like EMPTY and DEFAULT avoid re-allocation
• Interop -- companion objects compile to Java static methods via @JvmStatic

Companion Object with Serialization

@Serializable
data class Config(
    val apiUrl: String,
    val timeout: Int,
    val retries: Int
) {
    companion object {
        val DEFAULT = Config(
            apiUrl = "https://api.example.com",
            timeout = 30,
            retries = 3
        )
    }
}

This pairs JSON deserialization with sensible defaults, useful for configuration objects that may partially exist in the JSON payload.