Boat.java

public class Boat extends Vehicle {

    public Boat(String brand){
        super(brand);
    }
    public String doStuff() {
        return ("Je suis " + getBrand() + " et je fais glou glou!");
    }
}

Car.java

public class Car extends Vehicle {

    public Car(String brand){
        super(brand);
    }
    public String doStuff() {
        return ("Je suis " + getBrand() + " et je fais vroum vroum!");
    }

}

Hangar.java

class Hangar {
    Hangar() {
    }

    public static void main(String[] var0) {
        Car var1 = new Car("307");
        Boat var2 = new Boat("Betty");
        System.out.println(var1.doStuff());
        System.out.println(var2.doStuff());
    }
}

Vehicle.java

public abstract class Vehicle {

    // attributes
    private String brand;
    private int kilometers;
    // constructors
    public Vehicle(String brand) {
        this.brand = brand;
        this.kilometers = 0;
    }
    // method
    public abstract String doStuff();
    // getters and setters
    public String getBrand() {
        return this.brand;
    }
    public void setBrand(String brand) {
        this.brand = brand;
    }
    public int getKilometers() {
        return this.kilometers;
    }
    public void setKilometers(int kilometers) {
        this.kilometers = kilometers;
    }
}