Car.java

public class Car {
	// instance variables
	private float fuel;
	private boolean started;
	private int distanceTravelled;

	// static variable to keep track how many cars there are GLOBALLY.
	private static int numCars = 0;

	private static final float mileage = 0.15; // constant for mileage in L/km

	private static void addCarCount(){
		// increments number of cars that exist; only accessible by members of this class.
		numCars++;
	}

	public static int getCarCount(){
		// Allows the outside world to get how many cars there are
		return numCars;
	}

	public Car(){
		// constructor for Car
		fuel = 100f; // literals are longs by default, and decimal literals are doubles by default
		distanceTravelled = 0;
		started = false;
		
		// increments global car count
		addCarCount();

		// just to show that car has been created.
		System.out.println("New car created.");
	}

	public float getFuel(){
		// just a getter for the private fuel level
		return fuel;
	}

	public int getDistanceTravelled(){
		return distanceTravelled;
	}

	public void startCar() throws Exception {
		// Attempts to start the car. If the car is out of fuel then it throws an exception.
		if(fuel > 0){
			started = true;
		} else {
			throw new Exception("Not Enough Fuel.");
		}
	}

	public void stopCar(){
		// Stops the car.
		started = false;
	}

	public void addFuel(float fuel){
		// this <-- a keyword signifying "this" instance of the class.
		// needed when variable names conflict like this.
		// There is too much "this" here.
		this.fuel += fuel;
	}

	public void travel(int distance) throws Exception{
		// checks if car started
		if(started){
			// calculates fuel needed to complete trip
			float fuelNeeded = distance * mileage;

			// checks if enough fuel
			if(fuelNeeded > fuel){
				// not enough fuel. Throw fit.
				throw new Exception("Not Enough Fuel.");
			} else {
				// Subtract fuel used for trip.
				fuel -= fuelNeeded;
				// Add mileage to car
				distanceTravelled += distance;

				// Print amount of distance travelled
				System.out.println("I traveled " + Integer.toString(distance) + "km");
			}
		} else {
			// Car is not started. Throw fit.
			throw new Exception("Car is not started!");
		}
	}

	public static void main(String[] args){
		// main method is static. That means it cannot access any instance vars without creating 
		// instance of a class

		System.out.println("Number of cars: " + Integer.toString(Car.getCarCount())); // 0

		Car car1 = new Car();
		System.out.println("Number of cars: " + Integer.toString(Car.getCarCount())); // 1
		Car car2 = new Car();
		Car car3 = new Car();

		System.out.println("Number of cars: " + Integer.toString(Car.getCarCount())); // 3
		
	}

}