Java - Thread Synchronization Questions

Question

I have the following classes:

Vehicle (Vehicle):

public class Vehicle {

    private String chassis;
    private String type;
    private double topSpeed;

    public Vehicle() {

    }

    public Vehicle(String chassis, String type, double topSpeed) {
        super();
        this.chassis = chassis;
        this.type = type;
        this.topSpeed = topSpeed;
    }

    public void clean() {
        System.out.println(">> O veículo do tipo [" + getType() + "] está sendo limpo.");
    }

    public void adjust() {
        System.out.println(">> O veículo do tipo [" + getType() + "] está sendo ajustado.");
    }

    public int incrementalSpeed(int value) {
        return (int) (value + getTopSpeed());
    }

    public String getChassis() {
        return chassis;
    }

    public String getType() {
        return type;
    }

    public double getTopSpeed() {
        return topSpeed;
    }

    public void setTopSpeed(double topSpeed) {
        this.topSpeed = topSpeed;
    }

}

Competitor (Competitor):

public class Competitor implements Runnable {

    private Vehicle vehicle;
    private String name;
    private int pitStopTime;

    public Competitor(Vehicle vehicle, String name) {
        super();
        this.vehicle = vehicle;
        this.name = name;
    }

    @Override
    public void run() {
        startRace();
    }

    public void startRace() {
        synchronized(this) {
            System.out.println(">> O veículo de nome " + getName() + " e do tipo " + vehicle.getType() + " iniciou a corrida.");

            try {
                for(int i = 1; i <= 5; i++) {
                    if(i == 1) {
                        System.out.println(">> O veículo de nome " + getName() + " e do tipo " + vehicle.getType() + " acabou de percorrer sua primeira volta.");

                        if(vehicle.getType().equalsIgnoreCase("Carro")) {
                            setPitStopTime(1500);
                            Thread.sleep(getPitStopTime());
                        } else {
                            setPitStopTime(950);
                            Thread.sleep(getPitStopTime());
                        }
                    } else {
                        System.out.println(">> O veículo de nome " + getName() + " e do tipo " + vehicle.getType() + " percorreu " + i + " voltas.");
                    }
                }
            } catch(InterruptedException exception) {
                System.out.println(">> O veículo de nome " + getName() + " e do tipo " + vehicle.getType() + " teve um problema durante a volta.");
            }

            System.out.println(">> O veículo de nome " + getName() + " e do tipo " + vehicle.getType() + " terminou a corrida.");
        }
    }

    public String getName() {
        return name;
    }

    public void setPitStopTime(int pitStopTime) {
        this.pitStopTime = pitStopTime;
    }

    public int getPitStopTime() {
        return pitStopTime;
    }

}

Race (Race):

public class Race {

    public static void main(String[] args) {
        Thread firstCompetitor = new Thread(new Competitor(new Vehicle(null, "Bicicleta", 5), "Aitor"));
        Thread secondCompetitor = new Thread(new Competitor(new Vehicle(null, "Carro", 5), "Anderson"));
        Thread thirdCompetitor = new Thread(new Competitor(new Vehicle(null, "Carro", 5), "Gabriel"));
        Thread fourthCompetitor = new Thread(new Competitor(new Vehicle(null, "Bicicleta", 5), "Eduardo"));
        Thread fifthCompetitor = new Thread(new Competitor(new Vehicle(null, "Bicicleta", 5), "Angela"));
        Thread sixthCompetitor = new Thread(new Competitor(new Vehicle(null, "Carro", 5), "José"));

        firstCompetitor.start();
        secondCompetitor.start();
        thirdCompetitor.start();
        fourthCompetitor.start();
        fifthCompetitor.start();
        sixthCompetitor.start();
    }

}

If it is a race with several competitors (which are vehicles), each competitor has a name and a time of Pitstop, if it is a car, the time is 1500 milliseconds, if it is a bike, 950 milliseconds.

I used a synchronization block in my startRace() method but nothing happens. According to my knowledge of Threads synchronization, the Thread calling the method will only run when the Thread calling the method first finishes running.

I managed to execute the way I’d like it to run through this way:

try {
    firstCompetitor.start();
    firstCompetitor.join();

    secondCompetitor.start();
    secondCompetitor.join();

    thirdCompetitor.start();
    thirdCompetitor.join();

    fourthCompetitor.start();
    fourthCompetitor.join();

    fifthCompetitor.start();
    fifthCompetitor.join();

    sixthCompetitor.start();
    sixthCompetitor.join();
} catch(InterruptedException exception) {
    System.out.println(exception.toString());
}

But I would like to sync through the syncronized(this) block. I read in some topics on the internet that to make it work you must lock the object that we want to synchronize, causing the instance that will run the method to traverse until the instance you called first finishes executing.

Answer 1

I’ll be brief that I’m on the cell phone.

With Join() it’s wrong because threads run one at a time and that’s not what you want.

Your code has two other errors.

First, the Synchronized block is being the entire race of each vehicle (it’s out of the for, it should be in).

Second, you have to synchronize on the same object. You are synchronizing on this, only that each Competitor is a different object and therefore there is no dispute for resources.