I was just astonished to find out that Map.getValue(key) does not guarantee non-null value. If a map has explicit null value for a key, this method will return null !

Yes,

null

is still a value in the map.

Hmmm... Now I understand that this if the map's type is <String, String?> the compiler treats the value as nullable. The problem is when a json map with null value is deserialized into Map<String, String> which is not type-safe. Then you can bypass the compiler's protection, and get an exception only when you try to something with the value, like using it as a constructor parameter of a non-nullable value. Then you'll get exception in Runtime.

Yes, generics are erased. This is not limited to nullability. You can unsafely cast

Map<String, Any>

to

Map<String, String>

and then get an exception at runtime when you try to use the values as strings.

The problem is when a json map with null value is deserialized into Map<String, String> which is not type-safe.

I’ve seen that problem with Gson. On the other hand moshi-kotlin works with non-nullable types properly.