Simple Traffic Light Simulation with Java Threads

Question

As the questions states, I am trying to make a very, very simple traffic light simulation of a four way intersection. What my problem is, is that I am using two different threads and trying to bounce back and forth between the two using wait and notify while inside a for loop. I have some psuedo code below.

public class Processor 
{
    public static int CarCounter = 1;

    public void lightOneAndTwo() throws InterruptedException
    {
        synchronized(this)
        {
            for(int carsOne = 0; carsOne < 50; carsOne++)
            {
                //Light One---------------------------->


                //Light Two---------------------------->

                wait();
            }
        }
    }
    public void lightThreeAndFour() throws InterruptedException
    {
        Thread.sleep(1000);

        synchronized(this)
        {
            for(int carsTwo = 0; carsTwo < 50; carsTwo++ )
            {
                //Light Three---------------------------->

                notify();

            }
        }
    }
}

But when I call notify, it does not reactivate the the first thread. Am I doing something wrong or is there a better way to do it?

Answer 1

Your program have multiple issues - no conditional variable, no while loop to check condition etc. Below is one possible solution. I have four classes:

MyLock - (Lock Object)

package com.test;

public class MyLock {

    private volatile boolean condition;

    public MyLock(boolean condition) {
        this.condition = condition;
    }

    public boolean condition() {
        return condition;
    }

    public void flipCondition() {
        condition = !condition;
    }
}

TrafficLightWorkerOne (Traffic Worker)

package com.test;

public class TrafficLightWorkerOne implements Runnable {

    private int cars;
    private MyLock lock;

    public TrafficLightWorkerOne(MyLock lock, int cars) {
        this.lock = lock;
        this.cars = cars;
    }

    @Override
    public void run() {
        while (true) {
            synchronized (lock) {
                while (!lock.condition()) {
                    for (int carsOne = 0; carsOne < cars; carsOne++) {

                        System.out.println(Thread.currentThread().getName()
                                + " car no : " + carsOne);
                        // Light One---------------------------->

                        // Light Two---------------------------->
                    }
                    lock.notifyAll();
                    lock.flipCondition();
                }
                try {
                    lock.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }
}

TrafficLightWorkerTwo (Another Traffic Worker)

package com.test;

public class TrafficLightWorkerTwo implements Runnable {

    private int cars;
    private MyLock lock;

    public TrafficLightWorkerTwo(MyLock lock, int cars) {
        this.lock = lock;
        this.cars = cars;
    }

    @Override
    public void run() {

        while (true) {
            synchronized (lock) {
                try {
                    while (!lock.condition()) {
                        lock.wait();
                    }
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

                for (int carsOne = 0; carsOne < cars; carsOne++) {

                    System.out.println(Thread.currentThread().getName()
                            + " car no : " + carsOne);
                    // Light One---------------------------->

                    // Light Two---------------------------->
                }
                lock.flipCondition();;
                lock.notifyAll();
            }
        }

    }
}

TrafficLightSimulator (Main Class)

package com.test;

public class TrafficLightSimulator {

    public static void main(String[] args) {
        boolean  condition = false; 
        MyLock lock = new MyLock(condition);
        Thread threadOne = new Thread(new TrafficLightWorkerOne(lock, 5), "One");
        Thread threadTwo = new Thread(new TrafficLightWorkerTwo(lock, 4), "Two");

        threadOne.start();
        threadTwo.start();

    }
}