具有多个对象或列表的类

public class Order {
    private List<Item> items;

    public void addItem(Item item) {
        if (items == null) {
            items = new ArrayList<>();
        }
        items.add(item);
    }

    public Item getItem(int index) {
        if (items != null && index >= 0 && index < items.size()) {
            return items.get(index);
        }
        return null;
    }
}

public class Order {
    private List<Item> items = Collections.synchronizedList(new ArrayList<>());

    public void addItem(Item item) {
        items.add(item);
    }

    public Item getItem(int index) {
        return items.get(index);
    }
}

public class Order {
    private List<Item> items = new LinkedList<>();

    public void addItem(Item item) {
        items.add(item);
    }

    public Item getItem(int index) {
        return items.get(index);
    }
}