Car.java

public class Car {

    public String brandName;
    public int horsepower;
    public String licensePlate;
    public int mileage;

    public Car(String brandName, int horsepower, String licensePlate, int mileage) {
        this.brandName = brandName;
        this.horsepower = horsepower;
        this.licensePlate = licensePlate;
        this.mileage = mileage;
    }

    // Some code to see the information
    public void printInformation() {
        System.out.println(this.brandName + ", " + this.horsepower + " HP, License Plate: " + this.licensePlate + ", " + this.mileage + " miles");
    }
}

Customer.java

public class Customer {

    public String firstName;
    public String lastName;
    public String city;

    public Customer(String firstName, String lastName, String city) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.city = city;
    }

    // Some code to see the information
    public void printInformation() {
        System.out.println(this.firstName + " " + this.lastName + " from " + this.city);
    }
}

Main.java

import java.util.ArrayList;

public class Main {

    public static Car[] cars = new Car[15];
    public static ArrayList<Customer> customers = new ArrayList<>();

    public static void main(String[] args) {
        addCar(new Car("Honda", 120, "AC-HI 1234", 1000));
        addCar(new Car("Fiat", 200, "AC-HI 2345", 2000));
        addCar(new Car("Volkswagen", 150, "AC-HI 3456", 0));
        addCar(new Car("BMW", 200, "AC-HI 4567", 3000));

        addCustomer(new Customer("Justus", "Jonas", "Rocky Beach"));
        addCustomer(new Customer("Peter", "Shaw", "Rocky Beach"));
        addCustomer(new Customer("Bob", "Andrews", "Rocky Beach"));

        // Some code to see the information
        cars[3].printInformation();
        customers.get(1).printInformation();
    }

    public static void addCar(Car car) {
        for (int i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                cars[i] = car;
                break;
            }
        }
    }

    public static void addCustomer(Customer customer) {
        customers.add(customer);
    }
}