first working code

src/main/scala/org/sandbox/chat/ChatApp.scala

@@ -0,0 +1,21 @@
+package org.sandbox.chat
+
+import akka.actor.ActorSystem
+import akka.actor.Props
+
+object ChatApp extends App {
+
+  val system = ActorSystem("chat-app")
+
+  val server =
+    system.actorOf(Props[ChatServer], "ChuckNorris")
+
+  val client1 =
+    system.actorOf(Props(new ChatClient("client1", server)), "client1")
+  val client2 =
+    system.actorOf(Props(new ChatClient("client2", server)), "client2")
+  val client3 =
+    system.actorOf(Props(new ChatClient("client3", server)), "client3")
+
+  system.shutdown
+}

src/main/scala/org/sandbox/chat/ChatClient.scala

@@ -0,0 +1,15 @@
+package org.sandbox.chat
+
+import akka.actor.Actor
+import ChatServer._
+import akka.actor.ActorRef
+
+class ChatClient(name: String, chatServer: ActorRef) extends Actor {
+  def receive: Actor.Receive = {
+    case Broadcast(msg) =>
+      println(s"$name: received $msg")
+  }
+
+  chatServer ! Join(name)
+  chatServer ! Broadcast(s"Hi! I'm $name")
+}

src/main/scala/org/sandbox/chat/ChatServer.scala

@@ -0,0 +1,26 @@
+package org.sandbox.chat
+
+import akka.actor.Actor
+import akka.actor.ActorRef
+import ChatServer._
+
+class ChatServer extends Actor {
+  var chatters: Map[ActorRef,String] = Map.empty
+
+  override def receive: Actor.Receive = {
+    case Join(name) => chatters += sender -> name
+    case Leave => chatters -= sender
+    case Broadcast(msg) =>
+      for {
+        senderName <- chatters.get(sender)
+      }
+      chatters.keys foreach (_ ! Broadcast(s"$senderName: $msg"))
+  }
+}
+
+object ChatServer {
+  sealed trait ChatServerMsg
+  case class Join(name: String) extends ChatServerMsg
+  case object Leave
+  case class Broadcast(msg: String)
+}