Retrofit skips init { } block of a data class while initializing - object

Question

I'm going to set variables in class Data based on the arguments of DClass
abstract class Data(p: String) {
var a1: String
var a2: String
init {
"""(\w+)(\d+)""".toRegex().find(p)!!.groupValues.run {
a1 = "a1 is ${get(1)}"
a2 = "a2 is ${get(2)}"
}
}
}
data class DClass(val p1: String, val p2: String) : Data(p1)
Then I'm able to get values of a1 and a2 after DClass is created:
DClass("string1", "string2").run { println("$a1 $a2") }
it returns "a1 is string a2 is 1" as it should
Next I'm trying to make retrofit initialize DClass from the proper JSON response:
#GET("loadDClass") suspend fun dClass(): DClass
and say the program to do the same output of a1 and a2, it just returns:
"a1 is null a2 is null".
So I found out that init{} block of Data is skipped as the retrofit just builds DClass but doesn't do the initialization
Is it possible to make abstract class' variables be initialized without doing it manually?

Answer 1

The init block is not called because GsonConverterFactory (which I assume you use since it is very common in this case) constructs the DClass in a special way that does not invoke its constructor. In particular, an internal class called UnsafeAllocator is used. The JavaDoc for that class states:
/**
* Do sneaky things to allocate objects without invoking their constructors.
* [...]
*/
You could write your own Retrofit Converter.Factory but that would IMO be overkill. I suggest you add a simple member function to DClass instead of inheriting from Data, and also remove the abstract keyword from Data. DClass should look like this:
data class DClass(val p1: String, val p2: String)
fun newData(): Data = Data(p1)
}
then you can construct a Data instance from the body() of the response:
call.execute().body()?.newData()?.run { println("$a1 $a2") }