Making a given Java class more generic and flexible

Question

I am given a class Shop that maintains a collection of items of type T. It provides buy and sell functions for single or multiple items and uses a List<T> as a container for buying and selling:

import java.util.*;

public class Shop<T> {
    List<T> stock;

    public Shop() { stock = new LinkedList<T>(); }
    public T buy() { 
        return stock.remove(0);
    }
    void sell(T item) {
        stock.add(item);
    }

    void buy(int n, List<T> items) {
        for (T e : stock.subList(0, n)) {
            items.add(e);
        }
        for (int i=0; i<n; ++i) stock.remove(0);
    }

    void sell(List<T> items) {
        for (T e : items) {
            stock.add(e);
        }
    }
}

Now, I need to modify this class so that I can buy/sell items with any Collection type...not just List. I figured I would start by commenting out most everything and trying to convert things one by one, beginning with the stock :

 public class Shop<T> {
   // List<T> stock;
      Collection<T> stock;

   // public Shop() { stock = new LinkedList<T>(); }
      public Shop() { stock = new Collection<T>(); }            
      ...
      ... 
  }  

The first declaration works, but trying to instantiate the interface in the constructor, as expected, does not work. But as far as I can tell, stock needs to be a Collection so that I can use any Collection subtype in the other functions that deal with it. And I'm pretty sure I cannot use a wildcard as a type parameter in this case. So what exactly can I do to construct the stock here...or how should I declare the stock differently in the first place?

Answer 1

The actual collection implementation you use inside your class is completely immaterial to the clients/users of the class as long as your interface definition accepts Collection, i.e.

void buy(int n, List<T> items)
void sell(List<T> items)

should be

void buy(int n, Collection<T> items)
void sell(Collection<T> items)

This will not restrict anyone to use only List types. Then, your internal member stock can be and should be instantiated with any concrete subtype of Collection.