public class Pack implements Product
{
  private int quantity;
  private Product product;

  public Pack(int qty, Product p)
  {
    quantity = qty;
    product = p;
  }

  public double getPrice()
  {
    return quantity * product.getPrice();
  }
}

public class Bundle implements Product
{
  private List<Product> items; 1

  public Bundle()
  {
    items = new LinkedList<Product>(); 1
  }

  public void add(Product p)
  {
    items.add(p);
  }

  public double getPrice()
  {
    double price = 0.0;

    for (Product item : items)
      price += item.getPrice();

    return price;
  }
} 3

1. Or declare a specific type of list:

     private LinkedList<Product> items;

   or

     private ArrayList<Product> items;

2. Or new ArrayList<Product>().
   
   
3. In an alternative solution, this class would simply extend ArrayList<Product> or LinkedList<Product>.  Then neither the constructor nor the add method would be needed:

   public class Bundle extends LinkedList<Product> implements Product
   {
     public double getPrice()
     {
       double price = 0.0;
   
       for (Product item : items)
         price += item.getPrice();
   
       return price;
     }
   }