Exampale of using Comparable<> interface to sort objects.

FastCar.java

class CarsTest {

    public static void main(String[] args) {
        Ferrari ferrari = new Ferrari(200);
        Jaguar jaguar = new Jaguar(190);
        Lotus lotus = new Lotus(170);
        Lamborghini lamborghini = new Lamborghini(210);

        ArrayList<FastCar> fastCars = new ArrayList<>();
        fastCars.add(ferrari);
        fastCars.add(jaguar);
        fastCars.add(lotus);
        fastCars.add(lamborghini);

        System.out.println(fastCars.toString());

        Collections.sort(fastCars);

        System.out.println(fastCars.toString());
    }
}

/*
* This class is the super class for fast cars. It implements Comparable interface, when implementing compareTo() the return value defines sort
* order for a collection of FastCar's
* */


class FastCar implements Comparable<FastCar> {
    int topSpeed;
    String name;

    public FastCar(String name, int topSpeed) {
        this.name = name;
        this.topSpeed = topSpeed;
    }

    public int getTopSpeed() {
        return topSpeed;
    }

    @Override
    public String toString() {
        return name.toUpperCase() + " SPEED: " + topSpeed;
    }

    // Returns 1 if this object is > car, 0 if this object == car or -1 if this object < car.
    @Override
    public int compareTo(FastCar car) {
        return Integer.compare(this.topSpeed, car.topSpeed);
    }
}

class Ferrari extends FastCar {

    public Ferrari(int topSpeed) {
        super("Ferrari" , topSpeed);
    }
}

class Lotus extends FastCar {

    public Lotus(int topSpeed) {
        super("Lotus", topSpeed);
    }
}

class Lamborghini extends FastCar {
    public Lamborghini(int topSpeed) {
        super("Lamborghini", topSpeed);
    }
}

class Jaguar extends FastCar {
    public Jaguar(int topSpeed) {
        super("Jaguar", topSpeed);
    }
}