Simply put, if we don’t provide any instance creator and the response class doesn’t have a default constructor, Gson will create the object without calling any constructor at all. If nothing works, it uses unsafe allocation.It tries to invoke the default/no-args constructor of the object.It tries to create an object using InstanceCreator implementation that we provide.How Gson works and tries to parse JSON into POJO is in the following order: It is intended for usage with pure Java and doesn’t interpret Kotlin primary constructors properly. It’s just the way the Gson library is implemented. The problem is neither in Kotlin data classes nor in Gson. Why was the default value of parameter not considered when the JSON response didn't have a key we specified in our data class? Theory Time There can be times that the app update is rolled out, but the backend changes are not deployed for any reason you don’t want the app to show unexpected default values across the app. Surprised? The data class we used as POJO for the API, clearly had a default value for field “number”, so logically if the API response does not contain a “ sameNumber”, the number should be set to 5 (default value we specified). Log for the response, the default value for the number was set to 5 but here we get 0

Data class kotlin serializable