Moshi error: @JsonClass can't be applied to [class]. must not be sealed

Question

Imagine this data sample

    "meta_data": [
                {
                    "id": 40097,
                    "key": "_wcf_frm_created",
                    "value": ""
                },
                {
                    "id": 40098,
                    "key": "_wcf_custom_degin_checkbox",
                    "value": ""
                },
                {
                    "id": 40099,
                    "key": "_wcf_frm_data",
                    "value": {
                        "1": {
                            "1": "",
                            "2": "",
                            "3": "chk_box"
                        }
                    }
                },
                {
                    "id": 40119,
                    "key": "_vendor_select",
                    "value": "6484"
                },
                {
                    "id": 40120,
                    "key": "_vendor_percentage",
                    "value": "1"
                },
                {
                    "id": 40121,
                    "key": "_vendor_pro_cat",
                    "value": "Accessories"
                }
            ]

the Value in Meta_data can have multiple types. In the generator I used shown that the data type should be created like this.

sealed class Value {
        class StringMapMapValue(val value: Map<String, Map<String, String>>) : Value()
        class StringValue(val value: String)                                 : Value()
    }

With Moshi, I understand you have to add @JsonClass(generateAdapter = true) on top of the data class. Thus I have something like this

    @JsonClass(generateAdapter = true)
    data class MetaDatum (
            val id: Long,
            val key: String,
            val value: Value
    )
    @JsonClass(generateAdapter = true)
    sealed class Value {
        class StringMapMapValue(val value: Map<String, Map<String, String>>) : Value()
        class StringValue(val value: String)                                 : Value()
    }

I would like to note that the full json is much bigger than this. However, this is the only issue I have. I had some Enum issues as well, but those can be replaced with String

The error I received is error: @JsonClass can't be applied to net......Activity.Value: must not be sealed public static abstract class Value

Thus my question is, how do i decode the json with multiple enum types.

Ill add this here, In xCode(swift) this was how i was manage to do it.

enum Value: Codable {
    case string(String)
    case stringMapMap([String: [String: String]])
    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if let x = try? container.decode([String: [String: String]].self) {
            self = .stringMapMap(x)
            return
        }
        if let x = try? container.decode(String.self) {
            self = .string(x)
            return
        }
        throw DecodingError.typeMismatch(Value.self, DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Wrong type for Value"))
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
        case .string(let x):
            try container.encode(x)
        case .stringMapMap(let x):
            try container.encode(x)
        }
    }
}

Calling the data

fun retrieveMenu(sku: Int, SSLAuth: String)
    {

        doAsync {

            val client = OkHttpClient().newBuilder()
                .build()
            val formBody: RequestBody = FormBody.Builder()
                .build()
            val request: Request = Request.Builder()
                .url("https://carteapp.net/..................")
                .method("Get", formBody)
                .build()
            client.newCall(request).execute().use { response ->
                if (!response.isSuccessful) throw IOException("Unexpected code $response")

                val gist =
                        gistJsonAdapter.fromJson(response.body!!.source())
                println(gist)


            }
        }

    }

private val moshi = Moshi.Builder().build()
private val gistJsonAdapter = moshi.adapter(BarcodeScannerActivity.WcProductCall::class.java)
@JsonClass(generateAdapter = true)
    data class WcProductCall (
            val id: Long,
            ...........
            val metaData: List<MetaDatum>,
           ...
    )

score 0 · Answer 1 · answered Jan 13 '21 at 08:10

This is a bit tricky as Moshi won't allow sealed/abstract classes, my idea would be to use the DTO pattern (This is a pattern for using a Data Transfer Object to pass as much data in a single network request as possible)

Create a data class (your DTO) that contains both / all the data

@JsonClass(generateAdapter = true)
data class MetaDatumDTO (
        val id: Long,
        val key: String,
        val value1: Value1DTO?,
        val value2: Value2DTO?
)

@JsonClass(generateAdapter = true)
data class Value1DTO(val value: Map<String, Map<String, String>>)

@JsonClass(generateAdapter = true)
data class Value2DTO(val value: String)

And then inside your repository / model, when you retrieve the data, use a mapper function to map the data to the data classes you want to use

data class MetaDatum(
    val id: Long,
    val key: String,
    val value: Value?
)

sealed class Value {
    data class Value1(val value: Map<String, Map<String, String>>) : Value()
    data class Value2(val value: String) : Value()
}

fun MetaDatumDto.mapFromNetworkRequest(): MetaDatum {
    return MetaDatum(
        id = id,
        key = key,
        value = getValueFromDTO()
    )
}

fun MetaDatumDto.getValueFromDTO(): Value? {
    return (value1 as? Value1) ?: (value2 as? Value2)
}

Hello Brandon, Thanks for responding. I am fairly new with android kotlin, could you please tell me how you would intergrate the mapper function when using calls? — Zack Cheang Weng Seong, Jan 13 '21 at 22:06

Moshi error: @JsonClass can't be applied to [class]. must not be sealed

1 Answers1