Serialization in Scala - scala

Question

Im trying to understand basics of serialization in Scala. When i run the first example below I get the following output on the last line: res1: A.Mao = A$$anonfun$main$1$Mao$1#78e67e0a
#SerialVersionUID(1L)
class Poo(val aa:Int) extends Serializable {
override def toString() = "Hola"
}
#SerialVersionUID(1L)
class Mao(val hi: Poo) extends Serializable
def serialize() = {
val test = new Mao(new Poo(1))
try{
val fout = new FileOutputStream("c:\\misc\\address.ser");
val oos = new ObjectOutputStream(fout);
oos.writeObject(test);
oos.close();
System.out.println("Done");
}catch {
case ex => ex.printStackTrace();
}
}
serialize()
def ReadObjectFromFile[A](filename: String)(implicit m:scala.reflect.Manifest[A]): A = {
val input = new ObjectInputStream(new FileInputStream(filename))
val obj = input.readObject()
obj match {
case x if m.erasure.isInstance(x) => x.asInstanceOf[A]
case _ => sys.error("Type not what was expected when reading from file")
}
}
ReadObjectFromFile[Mao]("c:\\misc\\address.ser")
If I change the example and use case classes instead things works as expected with the output
res1: A.Mao = Mao(Hola)
case class Poo(val aa:Int) {
override def toString() = "Hola"
}
case class Mao(val hi: Poo)
def serialize() = {
val test = new Mao(new Poo(1))
try{
val fout = new FileOutputStream("c:\\misc\\address.ser");
val oos = new ObjectOutputStream(fout);
oos.writeObject(test);
oos.close();
System.out.println("Done");
}catch {
case ex => ex.printStackTrace();
}
}
def ReadObjectFromFile[A](filename: String)(implicit m:scala.reflect.Manifest[A]): A = {
val input = new ObjectInputStream(new FileInputStream(filename))
val obj = input.readObject()
obj match {
case x if m.erasure.isInstance(x) => x.asInstanceOf[A]
case _ => sys.error("Type not what was expected when reading from file")
}
}
ReadObjectFromFile[Mao]("c:\\misc\\address.ser")
So my questions are:
What do I need to do to get class to give the same output as case class?
Why does case class work without adding any explicit information about serialization?

Answer 1

This has nothing to do with the de/serialization (which seems correct) - it's just the way the result is displayed:
Scala REPL (and Worksheets) use the value's toString method to display it. Case classes override the default toString() method, therefore the output is displayed nicely (as expected). For non-case classes, the defeault implementation of Object.toString() is called, and results in the class name and address that you see.
You can implement toString for the non-case class too, to get the same result:
class Mao(val hi: Poo) extends Serializable {
override def toString = s"Mao($hi)"
}
// program prints:
// Done
// Mao(Hola)