Hello. I have a question about polymorphic seriali...

# serialization

[Zakhar]

Hello. I have a question about polymorphic serialization for inner objects of a base json object. For example i have an object like this:

{
  "cars": []
}

Let’s name it just Base.
Next i have a base class for cars. Just Car class. And two subclasses. Truck and Tesla.

@Serializable
open class Car {
    @SerialName("ms")
    protected val maxSpeed: Int = 0
}

@Serializable
class Truck(
    @SerialName("lc")
    private val loadCapacity: Int = 0,
    
    @SerialName("ft")
    private val fuelType: String = ""
): Car()

@Serializable
class Tesla(
    @SerialName("rt")
    private val rechargeTime: Int = 0
): Car()

As you see these two subclasses have different properties.
Let’s add some cars to the array of the base class:

{ 
  "cars": [{ ms: 80, lc: 1000, ft: "diesel" }, { ms: 180, rt: 4 }]
}

And now i want to transform this json object into Base class.

class Base {
    val cars = listOf<Car>()
}

I know there is a polymorphic serializers module that allows me to register a hierarchy of classes.
Maybe i need to do something like this:

Register a hierarchy of classes.

val json = Json {
    serializersModule = SerializersModule {
        polymorphic(Car::class) {
            subclass(Truck::class)
            subclass(Tesla::class)
        }
    }
}

Add an information about types into the json

{ 
  "cars": [{ type: "truck", ms: 80, lc: 1000, ft: "diesel" }, { type: "tesla", ms: 180, rt: 4 }]
}

@Serializable
@SerialName("truck")
class Truck(
    @SerialName("lc")
    private val loadCapacity: Int = 0,

    @SerialName("ft")
    private val fuelType: String = ""
): Car()

@Serializable
@SerialName("tesla")
class Tesla(
    @SerialName("rt")
    private val rechargeTime: Int = 0
): Car()

And then just transform a string into the base class.

json.decodeFromString<Base>(string)

But this is not working. I see that Base class is not in the hierarchy so it can not be achieved by this way.

[Adam S]

But this is not working

Can you say more about what's not working? Is there an error, or does the decoded data not match in some way?

[andylamax]

you have to mark all the classes with @Serializable

[Zakhar]

Guys, sorry for this misunderstanding.
The Car class is not abstract, it is open.
Subclasses and the Car class already marked as @Serializable
The error i am getting is
Unexpected JSON token at offset 11: Encountered an unknown key 'type' at path

[Zakhar]

So if the Car class is abstract there is no problems. Everything works fine. But in my case the class is open.

My question is only about how to achieve this with a base class marked as open.

[Dominaezzz]

Your Car class is the issue. It needs to be abstract and maxSpeed needs to be abstract

[glureau]

I think you could add a CarInterface , Car will inherit this interface, and then your configuration becomes:

val json = Json {
    serializersModule = SerializersModule {
        polymorphic(CarInterface::class) {
            subclass(Truck::class)
            subclass(Car::class)
            subclass(Tesla::class)
        }
    }
}

Because if Car is open, it means it can be instantiated, so it's another possibility of the polymorphism (even if the naming is questionnable in this case).