In our "hello world" example, we constructed an actor that is expected to receive a string as message. Any object can be passed as a message, provided it is immutable. It is very common to use case classes to represent messages. This is better than using strings because of the additional type safety: the compiler will catch a typo in a case class but not in a string.
Let's rewrite our EchoActor to accept instances of case classes as messages. We will make it accept two different messages: EchoMessage(message) and EchoHello, which just echoes a default message. The examples for this section and the next are in the chap09/hello_akka_case_classes directory in the sample code provided with this book (https://github.com/pbugnion/s4ds).
A common Akka pattern is to define the messages that an actor can receive in the actor's companion object:
// EchoActor.scala
object EchoActor {
case object EchoHello
case class EchoMessage(msg:String)
}Let's change the actor definition...