Why is Kotlin accepting a null value in an attribute declared as a non-nullable string?

Question

I declared a data class like this:

data class Product(val name: String = "", val price: Float = 0f)

My code is:

val json = "{'name': null, 'price': 50.00}"
val gson = GsonBuilder().create()
val p = gson.fromJson(json, Product::class.java)
println("name is ${p.name}")

The console output is: name is null

How is this possible? The name attribute is not a nullable string.

Answer 1

That's common problem when using Gson with Kotlin - and the runtime errors occur far too late here, which may make your program unstable and crash-friendly. For example, write:

val name: String = p.name

Boom! Crash.

Gson simply, as per super hacky implementation, allocates memory for the class without calling the constructor, and later fills fields with values that are present in JSON using reflection.

This makes it possible to store null in Kotlin's not-null properties, and that can cause NPE at runtime. You can provide custom TypeAdapter to disable reflection for your class.