java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to X error when call to X error when call Gson fromJson for generic type

Question

Hi I have this code in Kotlin Android I try to parse JSON string to data clas, but the data is unknown, I mean the data class is genric type T

class JsonFeatureFlag<T>(
        key: String,
        @StringRes nameResId: Int,
        category: DevOptionCategory,
        val defaultValue: T
    ): FeatureFlagV2<T>(key, nameResId, category) {
    .....
    override fun getValue(): T {
            val json = FirebaseRemoteConfig.getInstance().getString(key)
            return try {
                Gson().fromJson(json, object: TypeToken<T>(){}.type)
            } catch (e: Exception) {
                reportInvalidJson(json)
                defaultValue
            }
        }
    ....
    }

And I'm getting this error: java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to X

While I understand that TypeToken should be used for generic, something isn't working.

Due to type erasure at runtime the T is just java.lang.Object (respectively Kotlin's Any, I assume). Gson deserializes Object by default to a Map. If you can change your JsonFeatureFlag class to include the runtime class of T, then you can directly pass that class to the Gson.fromJson call. — Marcono1234
– Marcono1234, Commented Jan 29, 2022 at 22:46
Does this answer your question? Gson, ClassCastException with Generics — Marcono1234
– Marcono1234, Commented Jan 29, 2022 at 22:51
thanks @Marcono1234 , I also read regarding the type erasure. I have change it to Gson().fromJson(json, className) and I passing the className: Class<T> to the method — Keren Pastov
– Keren Pastov, Commented Jan 30, 2022 at 9:53
If that solved your issue, would you mind adding your own answer to this StackOverflow question to indicate that it has been answered? — Marcono1234
– Marcono1234, Commented Jan 30, 2022 at 19:44
hi @Marcono1234 I sow this answer stackoverflow.com/questions/25431859/… but it does not answer my question. I do not have a list or arrylist. My T generic can be any class — Keren Pastov
– Keren Pastov, Commented Jan 31, 2022 at 11:59

Keren Pastov · Accepted Answer · 2022-01-31 11:57:08Z

0

To solve this problem I provide a class name i.e. className: Class Although I used genric, when somebody wants to use my API and call getValue the type is known, and he can provide the class name to the get value method.

class JsonFeatureFlag<T>(
    key: String,
    @StringRes nameResId: Int,
    category: DevOptionCategory,
    val defaultValue: T
): FeatureFlagV2<T>(key, nameResId, category) {


     fun getValue(className: Class<T>): T {
        val json = FirebaseRemoteConfig.getInstance().getString(key)
        return try {
            Gson().fromJson(json, className)
        } catch (e: Exception) {
            reportInvalidJson(json)
            defaultValue
        }
    }

answered Jan 31, 2022 at 11:57

Keren Pastov

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