If I declare a new ArrayList as a parameter for the newly created Hippodrome which is assigned to game variable, where are the horses stored if I use the getter ?

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Line 15 - I declare a new Horse Arraylist. Yet in lines 17-19 I add those horses to the static variable from class Hippodrome. How does that work? How do the horses transfer from the static variable to the passed argument ?

public class Hippodrome {
    static Hippodrome game;
    private List<Horse> horses;

    public Hippodrome (List horses){
        this.horses = horses;
    }

    public List<Horse> getHorses() {
        return horses;
    }

    public static void main(String[] args) {

        game = new Hippodrome(new ArrayList<Horse>());

        game.getHorses().add(new Horse("BlackSabbath", 3, 0));
        game.getHorses().add(new Horse("BlueSteel", 3, 0));
        game.getHorses().add(new Horse("MagentaMarvel", 3, 0));

    }
}

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Nouser

Level 36 , Germany

26 March 2021, 07:35

useful

To access the horses field you'll need a Hippodrome object like:

Hippodrome hippodrome = new Hippodrome(new ArrayList<Horse>());

hippodrome is the reference variable pointing to a Hippodrome object and with that hippodrome ref. var. you can do the following now:

List<Horse> myHorses = hippodrome.getHorses();

Of course cause the getHorses returns the reference variable to a list (the horses field and that's a field, a reference variable inside your hippodrome object). Nothing new so far. You could, as the CG people die, save the reference variable to the Hippodrome object in a static variable inside the Hippodrome class. That's the same as when you're creating a singleton. You hava a static field and here you save the reference to the object.

Hippodrome hippodrome = new Hippodrome(new ArrayList<Horse>());
Hippodrome.game = hippodrome;

game and hippodrome are pointing to the same object now. So you're accessing the same object with both of them.

List<Horse> myHorsesA = hippodrome.getHorses();
List<Horse> myHorsesB = Hippodrome.game.getHorses();

myHorsesA and myHorsesB are pointing to the same object now. The advantage now is, that you can access that Hippodrome object from everywhere. If you eg. have a instance of a Horse object and you need to access an instance method of the Hippodrome object then you simple get the address of that object with Hippodrome.game and invoke the method on that reference variable. Of course you could pass the reference to your Hippodrome object to the class/ insstance of another class everytime you need it. But you need to pass it and make the code more complex.

Andrei

Level 41

26 March 2021, 09:19

Hi Nouser, thanks for the detailed explanation, I understand somethings better. In the end, I see you declare two new lists, myHorsesA and myHorsesB to which you assign the reference to the private list declared in the class, horses. So what happens with the list you declared earlier above, when you attributed a new reference to the hippodrome variable by creating a new Hippodrome Object. What happens to the list inside that object (new ArrayList<Horse>()), what does it store?

Nouser

Level 36 , Germany

26 March 2021, 11:30

solution

I do not declare lists, I declare reference variables. These are pointing to the list. In the end there is just one list but two reference variables pointing to the same list. myHorsesA and myHorsesB point to List<Horse> horses in line 3 of your code. (the list itself you created in line 15, the constructor argument, and inside the constructor you saved the reference to that list inside the horses variable. getHorses again will give me that reference variable to the list (but not the list itself, just the address inside the memory to that list). And that address you can save in another List variable as seen above, List<Horses> test = Hippodrome.game.getHorses();

Andrei

Level 41

26 March 2021, 11:54

OK, I understand completely now, thank you very much! 👍