v1
Single-threaded

v2
Threaded AbstractDispatchHandler

v3
DailyDispatchHandler uses sleep() instead of a tight loop

v4
Can be stopped and interrupted

v5a
Synchronization of dispatch() and makeVehicleAvailable()

   Given the current implementation, there isn't a deadlock problem because
   dispatch() doesn't "retry" (it returns false)

v5b
Synchronized collection rather than synchronized methods in Dispatcher


v6a
Have dispatch "retry" and fix the race condition with a call to wait()
  Acquire the monitor, call availableVehicles.size(), if not > 0 then wait()

v6b
Use a LinkedBlockingQueue




NOT DONE YET

v7/v8/v?
Use an Executior and ThreadPool

v??
Have the message actually sent using a thread (in case the communication itself is a performance bottleneck)