how to write Kotlin Base class for multiple data class for multiple similar API JSON response structure - CLOSED

Question

All the API JSON responses would have the following structure:

{
    "status": <Integer>
    "data": <Object or List of Objects>
    "message": <String>
}

the only property that changes is the 'data', which can be any object or list of object. So is there a way to create a BaseResponse class like

open class BaseResponse<T> (
    @SerializedName("status")
    val status: Int,
    @SerializedName("data")
    abstract val `data`: T,
    @SerializedName("message")
    val message: String
)

and the response classes

data class HelloResponse (
    override val `data`: Hello
) : BaseResponse<Hello> {
    data class Hello (
        @SerializedName("hello")
        val hello: String
    )
}


data class HellosResponse (
    override val `data`: List<Hello>
) : BaseResponse<List<Hello>> {
    data class Hello (
        @SerializedName("hello")
        val hello: String
    )
}

What i really want is to only override the data property, so that i don't have to write status and message property for each Response sub data class i write. I dont want to write status and message in my sub class and pass it to base class, cause i'd still write both the properties, so no difference than creating a data class with status and message.

so cannot be like

data class HelloResponse (
    val status: Int,
    override val `data`: Hello,
    val message: String
) : BasicResponse<Hello>(status, `data`, message) {
    data class Hello (
        @SerializedMessage("hello")
        val hello: String
    )
}

Edit: Own Answer

Well I realized that the HelloResponse is actually a waste since i'm only using it to access the actual Hello class. So what i did was to use the Base class directly in Retrofit2 service. Eg:

fun hello(): Call<BaseResponse<Hello>>

or

fun hellos(): Call<BaseResponse<List<Hello>>>

Well you have to directly specify the type with BaseResponse everywhere you use it. Maybe create typeallias

Or you can create alias

typealias HelloResponse = BaseResponse<Hello>

typealias HellosResponse = BaseResponse<List<Hello>>

To manually deserialize json string with Gson, you need to use TypeToken parameter instead of class type.

val hello = Gson().fromJson<BaseResponse<Hello>>(jsonStr, object: TypeToken<BaseResponse<Hello>>(){}.type)

If you use

val hello = Gson().fromJson<BaseResponse<Hello>>(jsonStr, BaseResponse::class.java)

The data property doesn't convert to Hello instead converts to LinkedHashMap

Note: Retrofit2's GsonConverterFactory uses TypeToken internally, so no problem.

Kudratillo · Accepted Answer · 2022-03-18 15:03:26Z

If you don't want to write status and message properties for data subclasses then you cannot expect subclass to have a constructor with status and message magically.

I strongly suggest you to make BaseResponse abstract and make subclasses like following

abstract class BaseResponse<T> {
    @SerializedName("status")
    abstract val status: Int

    @SerializedName("message")
    abstract val message: String

    @SerializedName("data")
    abstract val `data`: T
}

data class HelloResponse (
    override val status: Int,
    override val message: String,
    override val `data`: Hello,
) : BaseResponse<Hello>() {
    data class Hello (
        @SerializedMessage("hello")
        val hello: String
    )
}

You can achieve it in a way you don't need to write override val properties for subclass declarations by sacrificing data classes. However you lose all bounties provided by data class.

abstract class BaseResponse<T> {
    @SerializedName("status")
    abstract val status: Int

    @SerializedName("message")
    abstract val message: String

    @SerializedName("data")
    abstract val `data`: T
}

class HelloResponse: BaseResponse<Hello>() {
    data class Hello (
        @SerializedMessage("hello")
        val hello: String
    )
}

Just a kind reminder, you don't need to use @SerializedName annotation if class property name and json property name are same.

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