TrafficLightDemo.java

// An enumeration of the colors of a traffic light.
enum TrafficLightColor {
    RED, GREEN, YELLOW
}

// A computerized traffic light.
class TrafficLightSimulator implements Runnable {
    private TrafficLightColor tlc; // holds the traffic light color
    private boolean stop = false; // set to true to stop the simulation
    private boolean changed = false; // true when the light has changed

    TrafficLightSimulator(TrafficLightColor init) {
        tlc = init;
    }

    TrafficLightSimulator() {
        tlc = TrafficLightColor.RED;
    }

    // Start up the light.
    public void run() {
        while(!stop) {
            try {
                switch(tlc) {
                    case GREEN:
                        Thread.sleep(10000); // green for 10 seconds
                        break;
                    case YELLOW:
                        Thread.sleep(2000); // yellow for 2 seconds
                        break;
                    case RED:
                        Thread.sleep(12000); // red for 12 seconds
                        break;
                }
            } catch(InterruptedException exc) {
                System.out.println(exc);
            }
            changeColor();
        }
    }

    // Change color.
    synchronized void changeColor() {
        switch (tlc) {
            case RED:
                tlc = TrafficLightColor.GREEN;
                break;
            case YELLOW:
                tlc = TrafficLightColor.RED;
                break;
            case GREEN:
                tlc = TrafficLightColor.YELLOW;
        }
        changed = true;
        notify(); // signal that the light has changed
    }

    // Wait until a light change occurs.
    synchronized void waitForChange() {
        try {
            while(!changed)
                wait(); // wait for light to change
            changed = false;
        } catch(InterruptedException exc) {
            System.out.println(exc);
        }
    }

    // Return current color.
    synchronized TrafficLightColor getColor() {
        return tlc;
    }

    // Stop the traffic light.
    synchronized void cancel() {
        stop = true;
    }
}

public class TrafficLightDemo {
    public static void main(String[] args) {
        TrafficLightSimulator tl = new TrafficLightSimulator(TrafficLightColor.GREEN);
        Thread thrd = new Thread(tl);
        thrd.start();
        for (int i = 0; i < 9; i++) {
            System.out.println(tl.getColor());
            tl.waitForChange();
        }
        tl.cancel();
    }
}