@author trong0dn
@version 1.0
@date 21/03/23

---

State model of a PELICAN (PEdestrian LIght CONtrolled) crossing

Problem Description

The goal is to build a basic state machine used to control a pedestrian crossing. Based on the following blog post: Introduction to Hierarchical State Machines

Project Structure

PELICAN-STATE-MACHINE
|   .classpath
|   .gitignore
|   .project
|   README.md
|   README.txt
|
+---.settings
|       org.eclipse.jdt.core.prefs
|
\---src
    |   module-info.java
    |   state-machine.png
    |
    \---lab4
        |   Context.java
        |   State.java
        |   TestHarness.java
        |
        \---Operational
            |   Operational.java
            |
            +---Pedestrians
            |       PedestriansEnabled.java
            |       PedestriansFlash.java
            |       PedestriansWalk.java
            |
            \---Vehicles
                    VehiclesEnabled.java
                    VehiclesGreen.java
                    VehiclesGreenInt.java
                    VehiclesYellow.java

Context.java The Context of the State Design Pattern.

State.java State interface for performing state-specific behavior.

Operational.java Superstate of the state machine in operation.

VehiclesEnabled.java Implements the State interface for state-specific behavior for the VehicleEnabled state and substate of Operational.

VehiclesGreen.java Implements the State interface and substate of VehiclesEnabled.

VehiclesGreenInterrupt.java Implements the State interface and substate of VehiclesEnabled.

VehiclesYellow.java Implements the State interface and substate of VehiclesEnabled.

PedestriansEnabled.java Implements the State interface for state-specific behavior for the PedestrianEnabled state and substate of Operational.

PedestrianWalk.java Implements the State interface and substate of PedestriansEnabled.

PedestrianFlash.java Implements the State interface and substate of PedestriansEnabled.

Requirements and Dependencies

This application was created on Windows 10 OS using Eclipses IDE. Run on the latest version of jdk-17 using Java 17.

No other external dependencies required.

Compiling and Running the Application

Download and extract the .zip file. Then import the source code directly and run the program in local IDE. Or the file can be executed on Command Prompt using the following commands:

> cd C:\..\bin\                          // Navigate to the bin directory	
> java -cp . lab4.TestHarness            // Set classpath to run application

Questions/Answers

1. There is a defect in the design which will annoy law-abiding pedestrians.

The traffic light system operates by default with vehicles enabled and pedestrians disabled, but pedestrians can activate the traffic light switch by pushing a button (PEDESTRIAN_WAITING). When the button is pushed, vehicles are given a yellow light, followed by a red light, which triggers a WALK signal for pedestrians, which then changes to a flashing DONT_WALK signal. Once the flashing stops, vehicles are given a green light, and the cycle repeats. However, the traffic lights do not respond to the PEDESTRIAN_WAITING event immediately after this cycle. The system remembers that the button was pushed and ensures that vehicles receive a minimum of several seconds of green light before the cycle is repeated. For instance, a law abiding citizen will not walk during a pedestrianFlash states and will press the PEDESTRIAN_WAITING button. This will wait until all the transitions and states until vehiclesGreen TIMEOUT to be recognized.

The state model used in the system ensures the minimum green light time for vehicles by incorporating two states, vehiclesGreen and vehiclesGreenInt, and the isPedestrianWaiting flag. The vehiclesGreen state corresponds to the uninterrupted green light for vehicles. When the PEDESTRIAN_WAITING event occurs in this state, the system sets the isPedestrianWaiting flag to indicate that the button was pushed. The only condition for leaving the vehiclesGreen state is the TIMEOUT event, which triggers a transition to either the vehiclesYellow state (if the isPedestrianWaiting flag is set) or to the vehiclesGreenInt state, which represents the interruptible green light for vehicles.

Solution: Modify the initial TIMEOUT junction point in the vehiclesGreen state to exit upon triggering PEDESTRIAN_WAITING by the vehiclesGreenInt state. This way the pedestrians are not forced waiting the entire time duration when pressing the PEDESTRIAN_WAITING button.

2. There is second error in the design.

The TIMEOUT event choice junction point, which triggers the transition from the pedestrianFlash state should have signalPedestrian(DONT_WALK) set upon [pedestrianFlashCtr==0], as the last signalPedestrian(BLANK) remains. Hence, a BLANK signal upon exit from the pedestrianEnabled state means that there is no DONT_WALK signal being displayed to the pedestrian during the transition to the vehiclesEnabled state. This is a safety issue as the pedestrian signal remains BLANK while entry into the vehiclesEnabled state.

To be compliant with the Liskov substitution principle (LSP), none of the substates of vehiclesEnabled should disable the vehicles or enable the pedestrians. In particular, disabling vehicles (by switching the red light), or enabling the pedestrians (by displaying the WALK signal) in any of the nested states vehiclesGreen or vehiclesYellow would be inconsistent with being in the superstate vehiclesEnabled and would be a violation of the LSP (it will also be a safety hazard in this case).

Solution: Modify the state machine to implement the alternative design of switching the DONT_WALK signal in the exit action from pedestriansEnabled and switching the RED signal for vehicles in the exit action from vehiclesEnabled.

Disclaimer

Copyright disclaimer under section 107 of the Copyright Act 1976, allowance is made for fair use for purposes such as criticism, comment, news reporting, teaching, scholarship, education and research.

Fair use is a use permitted by copyright statute that might otherwise be infringing.