Can't understand why i'm getting a null pointer error

Question

I'm new enough to java coding and new to stack exchange. I have been trying to do this car dealership program as practice. i keep getting a null pointer error when trying to add either a car or a bike to an arraylist of vehicles and i cannot seem to figure out why, im almost positve that i have things mixed up and in the wrong place but i cant figure out which goes where. I'm using a driver, a Vehicles superclass and Cars and Bikes as two subclasses inheriting from Vehicles. Apologies if im unclear on anything, any help is appreciated. Thanks!

public class BMWdriver {

ArrayList<Vehicles> list;

public void Driver() {
    list = new ArrayList<Vehicles>();

}

public void inputCarDetails() {
    Scanner scan = new Scanner(System.in);
    String model, colour, fuelType, layout, frame, vehicleType;
    int doors, stock, displacement, topSpeed, stroke, noSeats, noVehicles;
    double price, fuelMpg;
    boolean sunroof;
    Vehicles car;

    System.out.println("----Entering car details----");
    System.out.println("\nEnter model");
    model = scan.nextLine();
    System.out.println("Enter Price");
    price = scan.nextDouble();
    scan.nextLine();
    System.out.println("Enter colour");
    colour = scan.nextLine();
    System.out.println("Enter no. in stock");
    stock = scan.nextInt();
    System.out.println("Enter MPG");
    fuelMpg = scan.nextDouble();
    System.out.println("Enter displacement");
    displacement = scan.nextInt();
    System.out.println("Enter top speed");
    topSpeed = scan.nextInt();
    System.out.println("Enter no. of doors");
    doors = scan.nextInt();
    System.out.println("Enter fuel type");
    fuelType = scan.nextLine();
    System.out.println("Enter wheel layout");
    layout = scan.nextLine();
    System.out.println("Enter sunroof (true/false)");
    sunroof = scan.nextBoolean();

    car = new Cars(model, price, colour, stock, fuelMpg, displacement, topSpeed, doors, fuelType, layout, sunroof);
    list.add(car);
}

public void inputBikeDetails() {
    Scanner scan = new Scanner(System.in);
    String model, colour, fuelType, layout, frame, vehicleType;
    int doors, stock, displacement, topSpeed, stroke, noSeats, noVehicles;
    double price, fuelMpg;
    boolean sunroof;
    Vehicles bike;

    System.out.println("----Entering bike details----");
    System.out.println("\nEnter model");
    model = scan.nextLine();
    System.out.println("Enter Price");
    price = scan.nextDouble();
    scan.nextLine();
    System.out.println("Enter colour");
    colour = scan.nextLine();
    System.out.println("Enter no. in stock");
    stock = scan.nextInt();
    System.out.println("Enter MPG");
    fuelMpg = scan.nextDouble();
    System.out.println("Enter displacement");
    displacement = scan.nextInt();
    System.out.println("Enter top speed");
    topSpeed = scan.nextInt();
    System.out.println("Enter engine stroke");
    stroke = scan.nextInt();
    System.out.println("Enter no. of seats");
    noSeats = scan.nextInt();
    System.out.print("Enter the frame type");
    frame = scan.nextLine();

    bike = new Bikes(model, price, colour, stock, fuelMpg, displacement, topSpeed, stroke, noSeats, frame);
    list.add(bike);

}

public static void main(String[] args) // main method
{
    BMWdriver driver = new BMWdriver();
    driver.startMenu();
    driver.inputCarDetails();
    driver.inputBikeDetails();
}
}

Answer 1

ArrayList<Vehicles> list; 

you are not calling Driver method to initialize the arraylist

public static void main (String[] args) // main method
{
    BMWdriver driver = new BMWdriver();
    driver.Driver();
    driver.startMenu();
    driver.inputCarDetails();
    driver.inputBikeDetails();
}

}

you are trying to add data to your list which has not been initialized. you should make sure that it is initialized before adding any data.

Answer 2

You misspelled your constructor

public void Driver()

This should be

public BMWdriver()

Then your list property will be initialized and the NPE when adding to the list is gone.