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public class Swapper extends AbstractLoggingActor {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
      .matchEquals(Swap, s -> {
        log().info("Hi");
        getContext().become(receiveBuilder().
          matchEquals(Swap, x -> {
            log().info("Ho");
            getContext().unbecome(); // resets the latest 'become' (just for fun)
          }).build(), false); // push on top instead of replace
      }).build();
  }
}

public class SwapperApp {
  public static void main(String[] args) {
    ActorSystem system = ActorSystem.create("SwapperSystem");
    ActorRef swapper = system.actorOf(Props.create(Swapper.class), "swapper");
    swapper.tell(Swap, ActorRef.noSender()); // logs Hi
    swapper.tell(Swap, ActorRef.noSender()); // logs Ho
    swapper.tell(Swap, ActorRef.noSender()); // logs Hi
    swapper.tell(Swap, ActorRef.noSender()); // logs Ho
    swapper.tell(Swap, ActorRef.noSender()); // logs Hi
    swapper.tell(Swap, ActorRef.noSender()); // logs Ho
    system.terminate();
  }
}