public abstract class ItemsHolder {

    Collection<Item> items;
    String params;
    
    protected ItemsHolder(String params) {
      this.params = params;
   }

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

    public Collectiont<Item> getItems() {
        return items;
    }
}

class ListHolder extends ItemsHolder {
    public A(String params) {
       super(params);
        this.items = new ArrayList<Item>();
    }

    public List<Item> getItems() {
        return items;
    }

}

class SetHolder extends ItemsHolder {
    public A(String params) {
        super(params);
        this.items = new HashSet<Item>();
    }

    public Set<Item> getItems() {
        return items;
    }

}