How to serialize a Kotlin Set to a JSON object whose values are empty objects?

Question

I would like to make a Kotlin class serializable using Kotlin serialization.

The class is very simple, something like this:

@Serializable(with = CustomSerializer::class)
data class MyObject(val keys: Set<String>)

Now, I need the serialization format to be a JSON object where the keys are given by the Set<String> and the values are always empty JSON objects.

Example:

val example = MyObject(setOf("abc", "def"))

Should serialize to:

{ "abc": {}, "def": {} }

The reason is that this object is being sent to an API where that's how they want the JSON to look like... the empty objects could contain some directives but I don't want or need to use those.

Having trouble doing that by just reading the documentation.

Renato · Accepted Answer · 2022-12-15 14:31:27Z

I've found one way to do it... and it seems simple enough!

I realized that I can get a serializer of empty Objects almost for free with this:

@Serializable
private object EmptyMap

Now, I can write a custom serializer in a straightforward way:

object MyObjectSerializer : KSerializer<MyObject> {
    private val _delegate = MapSerializer(String.serializer(), EmptyMap.serializer())

    override val descriptor: SerialDescriptor = _delegate.descriptor

    override fun serialize(encoder: Encoder, value: MyObject) {
        val data = value.keys.associateWith { EmptyMap }
        encoder.encodeSerializableValue(_delegate, data)
    }

    override fun deserialize(decoder: Decoder): MyObject {
        val value = decoder.decodeSerializableValue(_delegate)
        return MyObject(value.keys)
    }

}

Now all that's left to do is to apply the serializer on the type, which can be done with:

@Serializer(with = MyObjectSerializer)
data class MyObject(val keys: Set<String>)

Running Json.encodeToString(example) on the examples works perfectly.

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