Buyer/Seller multi-threading Assignment in java eclipse

package Assignment2Multithreading;

import java.util.Random;

public class Car {

private String regName;
private int saleValue;
private static final String [] colour = {"blue","red","yellow","green","silver","black"};

private String colourChoice;

private static final String[] countyCodes = {"C","CE","CW","D","DL","G","KE","KK","KY",
        "L","LD","LH","LM","LS","MH","MN","MO","OY","RN","SO","T","W","WH","WX","WW"};


Random rand = new Random(System.currentTimeMillis());

public Car(){

    this.regName = String.valueOf(rand.nextInt(100) + "-" + 
    countyCodes[rand.nextInt(countyCodes.length)] + "-" + String.valueOf(rand.nextInt(10000)));

    colourChoice = colour[rand.nextInt(colour.length)];

    saleValue = Integer.valueOf(rand.nextInt(20000 + 1));

}

public String getRegistration() {

    return regName;
}


private int getSaleValue() {

    return this.saleValue;
}

private String getColour() {

    return colourChoice;
}

public String toString() {

    return getColour() + " car with registration " + getRegistration() + " worth " +  "€" +  getSaleValue();

}

}

package Assignment2Multithreading;

import java.util.ArrayList; import java.util.Random;

public class CarShowroom {

public static final int fullCapacity = 10;

public ArrayList<Car> carList= new ArrayList<Car>();

private static long idCounter = 0;


public static synchronized String createID()
{
    return String.valueOf(++idCounter);
} 


Car car;


public CarShowroom() {

    //carList = new ArryList<Car>();



    this.car = new Car();
}

public boolean isEmpty() {

    if(this.carList.isEmpty()) {

        return true;


    }
    return false;
}

public boolean isFull() {

    if(carList.size() == fullCapacity) {

        return true;
    }

    return false;
}

public void addCar(Car c) {

    carList.add(c);

    System.out.println("We now have " + c.toString() + " in stock!");

}

public Car takeCar() {

    car = carList.get(0);

    carList.remove(carList.get(0));

    System.out.println("Buyer #" + createID() + " bought " + car);

    return car;
}

}

package Assignment2Multithreading;

import java.util.Random; import java.util.UUID; import java.util.concurrent.atomic.AtomicInteger;

public class Buyer implements Runnable{

CarShowroom carshowroom;

Car car;

int numBuyers;


private AtomicInteger ai  = new AtomicInteger(1);

Random rand = new Random(System.currentTimeMillis());



public Buyer() {

    this.carshowroom = new CarShowroom();
     //car = new Car();


}

public Buyer(CarShowroom c) {

    this.carshowroom = c;
}

public void init() { // we create the threads from within a method in the task class (that's fine)
    Thread t;


    int n = rand.nextInt(3) + 1;


        for(int j=0 ; j< n; j++) {



            t = new Thread(this); 
            t.start();




    }
}




public void run() {


    //while(true) {



        try {


            buyCar();
            Thread.sleep(1000);

        }

        catch(Exception e) {
            e.printStackTrace();
        }

    //}


}

public Car buyCar() {

    synchronized(carshowroom.carList){


        numBuyers = ai.getAndIncrement();


        while(carshowroom.isEmpty()) {



                try {
                        carshowroom.carList.wait();

                } catch (InterruptedException e) {

                    e.printStackTrace();
                }

            }


            car = carshowroom.takeCar();

            carshowroom.carList.clear();

            carshowroom.carList.notifyAll();

            if(numBuyers >1) {

                System.out.println(numBuyers + " buyers bought this car.");
            }

            else {

                System.out.println(numBuyers + " buyer bought this car.");
            }



            return car;

        }


}

}

package Assignment2Multithreading;

import java.util.Random;

public class Seller implements Runnable{

CarShowroom carshowroom;
Car car;

Random rand = new Random(System.currentTimeMillis());


public Seller() {

    this.car = new Car();
    this.carshowroom = new CarShowroom();
}


public Seller(CarShowroom c) {


    this.carshowroom = c;
}

public void init() {

    Thread t;

    int n = rand.nextInt(3)+ 1;

    for(int i =0 ; i< n; i++) {

        t = new Thread(this);
        t.start();
    }

}


public void upDateStock(Car car){


    synchronized(carshowroom.carList) {

        while(carshowroom.isFull()) {


            try {

                carshowroom.carList.wait();

            } catch (InterruptedException e) {

                e.printStackTrace();
            }

        }

        carshowroom.addCar(car);


        carshowroom.carList.notify();




    }


}



public void run() {





        try {

            upDateStock(car);


            Thread.sleep(1000);

        }catch(Exception e) {

            e.printStackTrace();
        }


}

}

I am getting null-pointer exception errors and trying to figure out where they come from Below is the assignment so you can understand what is going on in this question.

CT545 Enterprise Java Programming, Semester 2 2019/2020 Dr Patrick Mannion, School of Computer Science, National University of Ireland Galway Assignment 2 – Implementing a Car Showroom using Multithreading • Please create a separate file for each of your Java classes. • Please copy the output from a run of your completed code into a .txt file and submit this also. • Put all files into a single zip-file named “Assignment2-YourName.zip” and submit via Blackboard. • Deadline (strict): Wednesday, 19th February, 23:59. Late submission only with a medical certificate. • Please note that the graders are advised to check all submissions for plagiarism. Assignments need to be solved individually (no group work). • If you have questions about the assignment or need help with it, the best approach is to ask a tutor during one of the lab sessions (it is recommended to visit all lab sessions). • Use Java comments to explain your code. Marks will be awarded for detailed comments (10%) and clear legible code style (10%). In this assignment you will implement a simulation of car dealership in Java. This simulation will be an example of the Producer-Consumer model (see Lecture 3b), where there are multiple producers (car sellers) and consumers (car buyers). Each producer and consumer will run on a separate thread, and there can be multiple producer and consumer threads active in the application at any one time. You should create 5 .java files in total, as detailed below: Car.java, CarShowroom.java, Buyer.java, Seller.java and Main.java. Part 1: Car.java (10%) Create a class Car. This class should have private fields for the following details: registration, sale value and colour. When the constructor of this class is called, it should create a new instance of the Car class that has randomly generated values for these details. Registrations should be stored as strings and have a format like the following “00-G-2137”. Sale values can be rounded to the nearest euro and the generated values should be between €1,000 and €20,000. Colours should stored as strings and should be chosen randomly from one of a finite list of options (hint: create a static final array listing the colours, then select one of these at random for each new Car object that is created). It may be helpful to override the toString() method, so that the details of a Car may be easily printed to the console as per the code sample below: // create a new Car object with a randomly chosen registration, colour and sale value Car c = new Car(); // prints e.g. “yellow car with registration 15-G-5123 worth €18198” System.out.println(c.toString()); Part 2: CarShowroom.java (10%) Create a class CarShowroom. This class should have a private field that specifies its capacity (i.e. the maximum number of cars the showroom can hold at once). Try using a capacity of 5 or 10 cars. It should also have a private field that holds a list of cars (using e.g. an ArrayList). CarShowroom should have a method isEmpty() that returns a boolean indicating whether the showroom is currently empty, and a method isFull() that returns a boolean indicating whether the showroom is currently full. CarShowroom should also have an addCar() method to add a new Car to the list, and a takeCar() method to remove the first (oldest) Car in the list and return it to the caller. You may assume that a Buyer will always buy the first available Car in the ArrayList. CT545 Enterprise Java Programming, Semester 2 2019/2020 Dr Patrick Mannion, School of Computer Science, National University of Ireland Galway Part 3: Buyer.java (20%) This class will represent customers who wish to buy cars from the CarShowroom. Each instance of Buyer will run as a separate thread, and so the Buyer class should implement the Runnable interface. Each Buyer instance should be assigned a unique ID when it first tries to interact with the CarShowroom (i.e. when its run() method is called). This unique ID should be stored in a private integer field. Static thread safe fields (e.g. instances of AtomicInteger) should be used to count the total number of buyers created, and the total number of purchases successfully completed. The Buyer constructor should accept an instance of CarShowroom as an argument, and then store the reference to this object in a private field. The Buyer run() method should attempt to buy a Car from the showroom. It should first check that there is at least one Car available to buy in the showroom (by calling the isEmpty() method of the CarShowroom instance). If there are no cars available, the Buyer should wait until a car becomes available before proceeding. The Buyer should then proceed to take a car from the CarShowroom using the takeCar() method. Once a Buyer has successfully completed a purchase, its run() method can terminate (i.e. the thread has finished its work). Some of the operations in the run() method will need to be made be made thread safe to prevent race conditions (e.g. two buyers trying to buy the same Car). Your answer should use the wait() and notify()/notifyAll() methods. A message should be printed to the console each time a new Buyer reaches the dealership (directly after its run() method is called), each time a Buyer waits to buy a Car, and each time a Buyer successfully buys a Car from the CarShowroom (see example program output on the last page). Part 4: Seller.java (20%) This class will represent customers who wish to sell cars to the showroom. Each instance of Seller will run as a separate thread, and so the Seller class should implement the Runnable interface. Each Seller instance should be assigned a unique ID when it first tries to interact with the CarShowroom (i.e. when its run() method is called). Static thread safe fields (e.g. instances of AtomicInteger) should be used to count the total number of sellers created, and the total number of sales successfully completed. The Seller constructor should accept an instance of CarShowroom as an argument, and then store the reference to this object in a private field. The Seller run() method should attempt to buy a Car from the showroom. It should first check that the CarShowroom has space available for another Car (by calling the .isFull() method of the CarShowroom instance). If there is no space available, the Seller should wait until a space becomes available before proceeding. The Seller should then proceed to add its Car instance to the CarShowroom. Once a Seller has successfully completed a sale, its run() method can terminate (i.e. the thread has finished its work). Some of the operations in the run() method should be made thread safe to prevent race conditions (e.g. two sellers trying to sell cars at the same time and causing the CarShowroom capacity to be exceeded). Your answer should use the wait() and notify()/notifyAll() methods. A message should be printed to the console each time a new Seller reaches the dealership (directly after its run() method is called), each time a Seller waits to sell a Car, and each time a Seller successfully sells a Car to the CarShowroom (see example program output on the last page). CT545 Enterprise Java Programming, Semester 2 2019/2020 Dr Patrick Mannion, School of Computer Science, National University of Ireland Galway Part 5: Main.java (20%) This file will contain the main() method that will run your car dealership simulation. It should create a single instance of the CarShowroom class; this instance should be passed into the constructors of any Buyer or Seller objects that are created. Your main method should simulate a number of days (e.g. 30) at the car dealership (hint: use e.g. a while loop and repeat the same operations for each day). A message should print to the console announcing the number of cars available at the start of each day (see example program output on the last page). Each day a random number of Buyers (between 0 and e.g. 3) and Sellers (between 0 and e.g. 3) can arrive at the dealership. For each new Buyer/Seller a new thread must be created and started. You should also add a small delay at the end of each day (loop iteration) to slow down the execution of the program. Bonus harder features (these do not need to be implemented to be eligible for full marks) • Try limiting the purchasing power of Buyers, i.e. give them a random amount of money to spend. If there are no Cars within a Buyer’s budget, they would have to wait until an affordable Car shows up on the sales floor. • Try implementing randomly generated preferences for each Buyer that is created e.g. a preference for a certain colour of Car, or for Cars registered after a certain year. Buyers would then choose to buy only a Car that matches their preference (and perhaps wait until an appropriate Car shows up in the CarShowroom) • Adda markup for the dealership so that the sale price to Buyers is e.g. 20% higher than the price paid to Sellers Sample (truncated) program output: Day 1 beginning. There are 0 cars in the showroom today. A new buyer #1 just appeared. Buyer #1 wants to buy a car, but the showroom is empty. A new seller #1 just appeared with a yellow car with registration 11-G-7212 worth €3944 to sell. A new seller #3 just appeared with a black car with registration 18-G-2743 worth €14040 to sell. Seller #1 sold their yellow car with registration 11-G-7212 worth €3944 to the showroom. This is sale number 1. A new seller #2 just appeared with a silver car with registration 18-G-2469 worth €1449 to sell. Buyer #1 bought a yellow car with registration 11-G-7212 worth €3944 from the showroom. This is purchase number 1. Seller #3 sold their black car with registration 18-G-2743 worth €14040 to the showroom. This is sale number 2. Seller #2 sold their silver car with registration 18-G-2469 worth €1449 to the showroom. This is sale number 3. Day 2 beginning. There are 2 cars in the showroom today. A new seller #4 just appeared with a black car with registration 18-G-5011 worth €10859 to sell. CT545 Enterprise Java Programming, Semester 2 2019/2020 Dr Patrick Mannion, School of Computer Science, National University of Ireland Galway A new seller #5 just appeared with a black car with registration 15-G-8043 worth €11984 to sell. Seller #4 sold their black car with registration 18-G-5011 worth €10859 to the showroom. This is sale number 4. Seller #5 sold their black car with registration 15-G-8043 worth €11984 to the showroom. This is sale number 5. Day 3 beginning. There are 4 cars in the showroom today. A new seller #6 just appeared with a green car with registration 18-G-756 worth €19839 to sell. A new seller #7 just appeared with a black car with registration 16-G-2453 worth €18573 to sell. Seller #6 sold their green car with registration 18-G-756 worth €19839 to the showroom. This is sale number 6. Seller #7 is trying to sell a car, but the showroom is full. Day 4 beginning. There are 5 cars in the showroom today. A new buyer #2 just appeared. A new seller #8 just appeared with a green car with registration 10-G-5572 worth €11462 to sell. Buyer #2 bought a black car with registration 18-G-2743 worth €14040 from the showroom. This is purchase number 2. Seller #7 sold their black car with registration 16-G-2453 worth €18573 to the showroom. This is sale number 7. Seller #8 is trying to sell a car, but the showroom is full. Day 5 beginning. There are 5 cars in the showroom today. A new buyer #3 just appeared. A new seller #9 just appeared with a green car with registration 17-G-2593 worth €1501 to sell. Buyer #3 bought a silver car with registration 18-G-2469 worth €1449 from the showroom. This is purchase number 3. Seller #8 sold their green car with registration 10-G-5572 worth €11462 to the showroom. This is sale number 8. Seller #9 is trying to sell a car, but the showroom is full. Day 6 beginning. There are 5 cars in the showroom today. A new buyer #4 just appeared. A new buyer #5 just appeared. A new seller #10 just appeared with a silver car with registration 12-G-6077 worth €1122 to sell. Buyer #4 bought a black car with registration 18-G-5011 worth €10859 from the showroom. This is purchase number 4. Seller #9 sold their green car with registration 17-G-2593 worth €1501 to the showroom. This is sale number 9. A new seller #12 just appeared with a black car with registration 17-G-91 worth €1457 to sell. A new seller #11 just appeared with a silver car with registration 20-G-6505 worth €16467 to sell. Seller #10 is trying to sell a car, but the showroom is full. Buyer #5 bought a black car with registration 15-G-8043 worth €11984 from the showroom. This is purchase number 5. Seller #10 sold their silver car with registration 12-G-6077 worth €1122 to the showroom. This is sale number 10. Seller #11 is trying to sell a car, but the showroom is full. Seller #12 is trying to sell a car, but the showroom is full. Day 7 beginning. There are 5 cars in the showroom today. Day 8 beginning. There are 5 cars in the showroom today. A new buyer #6 just appeared. CT545 Enterprise Java Programming, Semester 2 2019/2020 Dr Patrick Mannion, School of Computer Science, National University of Ireland Galway A new buyer #7 just appeared. A new seller #13 just appeared with a black car with registration 12-G-8021 worth €5192 to sell. Buyer #6 bought a green car with registration 18-G-756 worth €19839 from the showroom. This is purchase number 6. Seller #12 sold their black car with registration 17-G-91 worth €1457 to the showroom. This is sale number 11. Seller #11 is trying to sell a car, but the showroom is full. Seller #13 is trying to sell a car, but the showroom is full. Buyer #7 bought a black car with registration 16-G-2453 worth €18573 from the showroom. This is purchase number 7. Seller #13 sold their black car with registration 12-G-8021 worth €5192 to the showroom. This is sale number 12. Seller #11 is trying to sell a car, but the showroom is full. Day 9 beginning. There are 5 cars in the showroom today. Day 10 beginning. There are 5 cars in the showroom today. A new buyer #8 just appeared. Buyer #8 bought a green car with registration 10-G-5572 worth €11462 from the showroom. This is purchase number 8. A new buyer #9 just appeared. A new seller #15 just appeared with a green car with registration 20-G-4476 worth €13058 to sell. A new seller #14 just appeared with a yellow car with registration 11-G-8395 worth €8989 to sell. Seller #11 sold their silver car with registration 20-G-6505 worth €16467 to the showroom. This is sale number 13. A new seller #16 just appeared with a yellow car with registration 10-G-5035 worth €8286 to sell. Seller #14 is trying to sell a car, but the showroom is full. Seller #15 is trying to sell a car, but the showroom is full. Buyer #9 bought a green car with registration 17-G-2593 worth €1501 from the showroom. This is purchase number 9. Seller #15 sold their green car with registration 20-G-4476 worth €13058 to the showroom. This is sale number 14. Seller #14 is trying to sell a car, but the showroom is full. Seller #16 is trying to sell a car, but the showroom is full.