How to use gson deserialize to ArrayList in Kotlin

Question

I use this class to store data

public class Item(var name:String,
                  var description:String?=null){
}

And use it in ArrayList

public var itemList = ArrayList<Item>()

Use this code to serialize the object

val gs=Gson()
val itemListJsonString = gs.toJson(itemList)

And deserialize

itemList = gs.fromJson<ArrayList<Item>>(itemListJsonString, ArrayList::class.java)

But this method will give me LinkedTreeMap, not Item, I cannot cast LinkedTreeMap to Item

What is correct way to deserialize to json in Kotlin?

Answer 1

Try this code for deserialize list

val gson = Gson()
val itemType = object : TypeToken<List<Item>>() {}.type
itemList = gson.fromJson<List<Item>>(itemListJsonString, itemType)

Answer 2

You can define a inline reified extension function like:

internal inline fun <reified T> Gson.fromJson(json: String) =
    fromJson<T>(json, object : TypeToken<T>() {}.type)

And use it like:

val itemList: List<Item> = gson.fromJson(itemListJsonString)

By default, types are erased at runtime, so Gson cannot know which kind of List it has to deserialize. However, when you declare the type as reified you preserve it at runtime. So now Gson has enough information to deserialize the List (or any other generic Object).