The StopLight Class

StopLight.java

package test.concurrency.traffic;

import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;

/**
 * This class is thread safe (all methods synchronized) and represents a
 * StopLight The problem assumes only one StopLight on the pavement hence the
 * class is made singleton
 */
final public class StopLight {
  private static StopLight sLight;
	private static LightState state;

	public static synchronized StopLight getInstance() {
		if (sLight == null) {
			sLight = new StopLight();
		}
		return sLight;
	}

	private StopLight() {
		StopLight.state = LightState.GREEN;
		StopLight.StopWatch.getInstance().start();

	}
	public static synchronized void toggle() { // This method is called by Road
												// class to toggle the light
		if (StopLight.state == LightState.RED) {
			StopLight.state = LightState.GREEN;
		} else {
			StopLight.state = LightState.RED;
		}

	}
	public synchronized static LightState getState() {
		return StopLight.state;
	}

	public synchronized static void setState(LightState state) {
		StopLight.state = state;
	}

	static class StopWatch {
		private static AtomicLong startTime = new AtomicLong(0);
		private static AtomicLong stopTime = new AtomicLong(0);
		private static AtomicBoolean running = new AtomicBoolean(false);
		private static StopWatch sWatch = null;

		public static synchronized StopWatch getInstance() {
			if (sWatch == null) {
				sWatch = new StopWatch();
			}
			return sWatch;
		}

		private StopWatch() {

		}

		public boolean isRunning() {
			return running.get();
		}

		public void start() {
			startTime.set(System.nanoTime());
			running.set(true);
		}

		public void stop() {
			stopTime.set(System.nanoTime());
			running.set(false);
		}

		/* Returns Elapsed time in seconds referencing the point of call*/
		public long getElapsedTimeSecs() {
			long elapsed;
			if (running.get()) {
				elapsed = ((System.nanoTime() - startTime.get()) / 1000000000);
			} else {
				elapsed = ((stopTime.get() - startTime.get()) / 1000000000);
			}
			return elapsed;
		}

		public void aSecond() {
			try {
				Thread.sleep(Consts.ONE_SEC);
			} catch (InterruptedException e) {
				Thread.currentThread().interrupt();
			}
		}
	}

}