Currently, for scala, add new field and deserialize from old binary data will get a null as new field value. But avoid null is a good practice in scala language.

  val personBytes = readBytesFromFile("person_v1") // there are old version `Person(1,true,some text)`
  //  append field as new person: case class Person(a: Int, b: Boolean, c: String, d: String = "default d")
  val deserPerson = fury.deserializeJavaObject(personBytes, classOf[Person])
  println(s"deserPerson: ${deserPerson}") // deserPerson: Person(1,true,some text,null)

I think it's better using default value our a empty value to set the new field. Such as give the result:

// deserPerson: Person(1,true,some text, "default d")

And if there not a default value in the field define, can give a empty value. For String is "" will be better for null.

If the new field is a structure, can using a default value to instance this one.
case class Foo(a: String, b: Int) can setting the default value as Foo("", 0)

However, for some performance care scenario. Using null should be better and handle by developer.

I'm advice add a new configuration to decide using the default value or null for new field.