import akka.actor.ActorSelection;
import akka.dispatch.OnComplete;
-import com.google.common.base.Optional;
import com.google.common.base.Preconditions;
import java.util.Collection;
+import java.util.Optional;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicLong;