Limit permitted integer values for serialised class property with Kotlin

October 23, 2023

I’d like to limit the permitted integer values for a class property, in a way that after serialisation to Json (with gson) the value would be represented as a number and not as a string.

class Car(
@Expose val color: Color = Color.RED,
@Expose val seats: Seats = Seats.FIVE   // Using an enum class so only predefined values are accepted.
)

enum class Color{
     @SerializedName("red") RED,
     @SerializedName("blue") BLUE,
}

enum class Seats{
     @SerializedName("4") FOUR,
     @SerializedName("5") FIVE,
}

Actual json output:

{
   "color": "red",
   "seats": "5"   // This is a string. It's not what I want.
}

The json output I need:

{
   "color": "red",
   "seats": 5   // This is a number ✓
}

>Solution :

If you really want to stick to Gson, one way of doing this is to specify the integer as a property of the enum:

enum class Seats(val value: Int) {
    FOUR(4),
    FIVE(5);

    companion object {
        fun of(value: Int) = entries.firstOrNull { it.value == value }
    }
}

Then use a type adapter or custom serializer that uses this property for serialization, and the Seat.of(...) function for deserialization:

class SeatSerializer : JsonSerializer<Seats> {
    override fun serialize(src: Seats, typeOfSrc: Type, context: JsonSerializationContext): JsonElement =
        JsonPrimitive(src.value)
}

class SeatDeserializer : JsonDeserializer<Seats> {
    override fun deserialize(element: JsonElement, type: Type, context: JsonDeserializationContext): Seats =
        Seats.of(element.asInt) ?: error("Cannot deserialize ${element.asInt} as Seats")
}