Java How to Program Fourth Edition - DCC

880 Multithreading Chapter 15
Method ride guarantees that only one Person may ride the Elevator at a time—
as described in Section 15.5, only one synchronized method may be active on an object
at once, so all other Person threads attempting to invoke ride must wait for the current
thread to exit method ride.
Method ride invokes static method sleep of class Thread to put the Person
thread into the sleep state, which represents the Person waiting for the ride to complete.
We must specify the maximum amount of time that a Person will wait for the Elevator
to complete traveling—however, there is no such information specified in the problem statement.
We introduce a new attribute that represents this time—maxTravelTime, to which
we arbitrarily assign a value of 10 minutes (i.e., a Person will wait 10 minutes for the
travel to complete). Attribute maxTravelTime is a safeguard in case the Elevator—
for whatever reason—never reaches the other Floor. The Person should never have to
wait this long—if the Person waits 10 minutes, then the Elevator is broken, and we
assume that our Person crawls out of the Elevator and exits the simulation.
Software Engineering Observation 15.6
In a software-development process, the analysis phase yields a requirements document (e.g.,
our problem statement). As we continue the design and implementation phase, we discover
additional issues that were not apparent to us at the analysis phase. As designers, we must
anticipate these issues and deal with them accordingly. 15.6
Software Engineering Observation 15.7
One false assumption to make is that the system requirements remain stable (i.e., they provide
all information necessary to build the system) throughout the analysis and design
phases. In large systems that have long implementation phases, requirements can, and often
do, change to accommodate those issues that were not apparent during analysis. 15.7
If our Elevator works correctly, the Elevator travels for five seconds—specifically,
invoking method sleep halts the Elevator’s thread for five seconds to simulate
the travel. When the Elevator thread awakens, it sends elevatorArrived events as
described in Section 10.22. The elevatorDoor receives this event and invokes method
doorOpened (message 3.3) of the ridingPassenger, as in:
public void doorOpened( DoorEvent doorEvent )
{
// set Person on Floor where Door opened
setLocation( doorEvent.getLocation() );
}
// interrupt Person's sleep method in run method and
// Elevator's ride method
interrupt();
Method doorOpened sets the ridingPassenger’s Location to the Floor at
which the Elevator arrived, then calls the ridingPassenger thread’s interrupt
method. The interrupt method terminates the sleep method invoked in method
ride, method ride terminates, and the ridingPassenger leaves the Elevator,
then exits the simulation. When the ridingPassenger exits the Elevator, the
ridingPassenger releases the monitor on the Elevator object, which allows the
waitingPassenger to invoke the ride method and obtain the monitor. Now, the
waitingPassenger may invoke method ride to ride the Elevator.