In Kotlin you can create a data class:
data class CountriesResponse(
val count: Int,
val countries: List<Country>,
val error: String)
然后可以使用它来解析JSON，例如，"{n:10}".在本例中，您将有一个从Retrofit、Fuel或Gson接收的对象val countries: CountriesResponse，其中包含以下值:count = 0, countries = null, error = null.
In Kotlin + Gson - How to get an emptyList when null for data class you can see another example.
当您稍后try 使用countries时，您将在这里得到一个异常:val size = countries.countries.size:"kotlin.TypeCastException:null不能转换为非null类型kotlin.Int".如果您编写代码并在访问这些字段时使用?，Android Studio将突出显示?.并警告:Unnecessary safe call on a non-null receiver of type List<Country>.
So, should we use ? in data classes? Why can an application set null to non-nullable variables during runtime?