相关疑难解决方法(0)

在ScalaActors.pdf上的这个幻灯片放映中,单个引号指示何时将消息发送给pong actor？

class Ping(count: int, pong: Pong) extends Actor {
def act() {
   pong ! 'Ping // what does the single quote indicate???
      receive {
         case 'Pong =>
      }
   }
}