Generic method for convert json string to List of Objects

Question

I wanna create generic method for convert json string to List of objects.

fun  convertJsonStringToObjectList(jsonString: String , clazz: Class<Any>) :List<Any>{
        val gson = Gson()
        val objectList = gson.fromJson(jsonString, Array<clazz>::class.java).asList()
        return objectList
}

This code does not works. I know how to convert json string to a class like this:

 fun <T> convertJsonStringToObject(jsonString: String, clazz: Class<T>): T {
        val gson = Gson()
        val objectList = gson.fromJson(jsonString, clazz)
        return objectList
    }

The problem is i wan to get class type from method and then add Array type to it and get classtype again. I do not know how to handle this.

momvart · Accepted Answer · 2021-11-09 19:43:06Z

Because of the type erasure and the special reflection types of arrays in jvm, you cannot do this really clean but there is a way using reified keyword in kotlin in which it is possible:

inline fun <reified T> convertJsonStringToObject(jsonString: String): Array<T> =
    Gson().fromJson(jsonString, emptyArray<T>().javaClass)

(You may notice that I have used emptyArray<T> to get the type instead of Array<T>::class.java. Because the latter one will give you an Object[] not an Array<T>. This may be a bug in kotlin.)

(it may look appropriate here but is not sufficient for complicated cases, because we have made it inline.)

Also, here are two alternative methods:

Passing the array type to the function (like java):

fun <T> convertJsonStringToObject(
    jsonString: String, 
    clazz: Class<Array<T>>): Array<T> =
    Gson().fromJson(jsonString, clazz)

//and call it like this:
val result = convertJsonStringToObject(
    "[{\"test\":123}, {\"test\": 456}]", 
    Array<Test>::class.java)

Parsing the json array manually one by one:

fun <T> convertJsonStringToObjectList(
    jsonString: String, 
    clazz: Class<T>): List<T> {
    val gson = Gson()
    val objects = gson.fromJson(jsonString, JsonElement::class.java).asJsonArray
    return objects.map { gson.fromJson(it, clazz) }
}

(again you can omit the clazz param using reified)

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