Bulletproof

This post waited a long-long time to be written. The motivation to finally do it came from a discussion I had with Andreas Brink at OOPSLA'08. We both felt that TDD made us more productive. In the pre-TDD era we were always trying to make our API bullet-proof, i.e.: that it will be really difficult to write code that will fail at run time. With TDD this is not such a big concern. You write simpler code (which is much faster than writing bulletproof code) and you rely on the tests suite to keep from you doing mistakes.

I don't want to get into the can-tests-really-prevent-bugs debate. My intention here is just to show what I mean by a bulletproof design. Here is a simple (non-bulletproof) Flight class:

public class Flight {
  private final int numSeats;
  private final Set<Traveler> travelers 
    = new HashSet<Traveler>();

  public Flight(int numSeats_) {
    numSeats = numSeats_;
  }

  public void book(Traveler t) {
    if(travelers.size() >= numSeats)
      throw new RuntimeException("Fully booked");
    travelers.add(t);
  }

  public void unbook(Traveler t) { 
    if(!travelers.contains(t))
      throw new IllegalArgumentException("t=" + t);
    travelers.remove(t);
  }
}

public class Traveler {
   ...
}

The predicament lies in the Flight.unbook() method. If we pass the wrong argument, this method will fail:

Filght f1 = ...;
Filght f2 = ...;
Traveler t = ...;
f1.book(t);
f2.unbook(t); // Illegal argument error!

And now, let's see the bulletproof approach. The trick is to use static type checking - sometimes in conjunction with encapsulation - to make it impossible to issue an incorrect unbook() request:

public class Flight {

  private final int numSeats;
  private final Set<Traveler> travelers 
    = new HashSet<Traveler>();

  public Flight(int numSeats_) {
    numSeats = numSeats_;
  }

  public Traveler book(Traveler t) {
    if(travelers.size() >= numSeats)
      throw new RuntimeException("Fully booked");
    travelers.add(t);
    return new Reservation(t);
  }

  public class Reservation {
    public final Traveler traveler;
    public Reservation(Traveler t) { 
      traveler = t;
    }

    public void unbook() {
      travelers.remove(traveler);
    }
  }
}

With this design unbook requests (now issued through a Reservation object) cannot fail:

Filght f1 = ...;
Filght f2 = ...;
Traveler t = ...;
Flight.Reservation r = f1.book(t);
r.unbook(); // Always safe!