NotMyKindaModel.java

/* This is my interface, my API, my contract. */
interface IReservation {
  IList<IGuest> getGuests();
  Duration getLength();
  Amount getBalance();
  Amount getTotal();
  bool cancel();
  // By the way pay() could have been in IPaymentMethod as well but you know what?
  // I called first and so I win!
  // Or both of us would end up implementing pay() in which case we would smartly delegate 
  // the job of implementing pay() to a common helper class
  // Any similarity between the helper class and the IReservationManager is purely coincidental 
  PaymentStatus pay(IPaymentMethod method, Amount amount);
  PaymentStatus payInFull(IPaymentMethod method);
  // ... and much more ...
}

/*
 Hi, I'm Reservation. If you'd like to pay, let me
 know. Like a good waiter, I'll take your money to 
 the cashier and come back with your doggie bag.
 But there are caveats.. So hold on to your seat and look at 
 what I got to say below for the pay methods
 */
@serializable
class Reservation implements IReservation {
  // Dependency injection for the cashier...
  // Yes.. don't even think about instantiating me if you dont have the 
  // ITransactionGateway. I need one dead or alive. It would be dead if 
  // it is in a non service layer and alive if it is in the service layer.
  // And yes, you need to use the DI to obtain an instance of me since that is the 
  // guy who knows how to instantiate any object including domain objects. So for every
  // request you need to ask the DI framework to create me even if it is slow.
  // Sorry that is company policy!
  public Reservation(ITransactionGateway backend) {...}
  
  // Assume these do something useful
  public IList<IGuest> getGuests() {...}
  public Duration getLength() {...}
  public Amount getBalance() {...}
  public Amount getTotal() {...}
  public bool cancel() {...}

  /* 
   You want to pay in full? No problem, I know
   how much you owe. Your credit card, please...
   I might tempt you to call me from the presentation 
   layer but unless the service layer is co-resident in the same JVM I 
   am not going to work!
  */
  public PaymentStatus payInFull(IPaymentMethod method) {
    return pay(method, getBalance());
  }

  /* 
  The "cashier" (backend) handles the actual transaction;
  that's not my responsibility. I just tell the cashier
  what you want to purchase ("this") and bring you back
  the answer.
  Same comments as above.
  */
  public PaymentStatus pay(IPaymentMethod method, Amount amount) {
    return backend.applyPayment(this, method, amount);
  }
}