...
    private ArrayList<Ufo> ufos = new ArrayList<>();
    private ArrayList<Rocket> rockets = new ArrayList<>();
    private ArrayList<Bomb> bombs = new ArrayList<>();
...

public void moveAllItems() {
        // Get a list of all objects in the game and call the move() method on each of them.
        ...
    }

    /**
     * The method returns a single list that contains all objects in the game
     */
    public List<BaseObject> getAllItems() {
        // You need to create a new list and put all the game objects into it.
        ...
    }