import java.util.concurrent.Semaphore;

class BarberShop {

   // ...

   BarberShop( ... ) {
      // ...
   }

   void customerEnters(int id) {
      // ...
   }

   private void service(int id) {
      System.out.println ("Customer " + id + " is being serviced.");
      SleepUtilities.nap();
      System.out.println ("Customer " + id + " is done being serviced.");
   }
}