Android Kotlin - save and retrieve ArrayList of custom model with SharedPreferences-CodePudding

Before you mark it as duplicate, I already seen similar questions and none of the solutions work when I try them!

This is how I tried it based on the answers in the "solutions":

Saving:

val gson = Gson()
val hiddenUserList: ArrayList<HiddenUserModel> = ArrayList()
hiddenUserList.add(HiddenUserModel(777L, "steveee"))
hiddenUserList.add(HiddenUserModel(888L, "donbimbo"))
val json = gson.toJson(hiddenUserList)
prefs.edit().putString("hiddenUser", json).apply()

So far it's correct I guess?

Now when I try to retrieve and read it:

if(prefs.getString("hiddenUser", null) != null){
    val hiddenUserListBack: ArrayList<HiddenUserModel> = ArrayList()
    val gsonBack = Gson()
    val jsonBack: String = prefs.getString("hiddenUser", null)!!
    val obj: ArrayList<HiddenUserModel> = gsonBack.fromJson(jsonBack, hiddenUserListBack::class.java)

    Log.d(tagg, obj[0].nickname   " "   obj[1].nickname) 
}

I get:

java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to com.website.blaaa.models.HiddenUserModel

What is wrong here?

I'd also accept solutions without gson as long as it works and doesn't contain tons of workaround

CodePudding user response：

Try letting Gson to decide on the actual type of collection to deserialize the list to. For example, create a class with a List field:

data class UserBag(val users: List<HiddenUserModel>)

Then put your users there, and serialize and save:

...
val json = gson.toJson(UserBag(users = hiddenUserListBack))
prefs.edit().putString("hiddenUser", json).apply()

The just serialize/deserialize an instance of that class, so you don't care of the actual runtime of List used:

val userBag = gsonBack.fromJson(jsonBack, UserBag::class.java)
userBag.users.forEach {
    Log.d(tagg, "User nickname: ${it.nickname}") 
}