Using Java generics in interfaces that return collections. Best practice? Pitfalls?

Question

I ran into some code today that I found questionable. Here's a simplified example (not realistic).

public interface IListable {
    //returns first n items from list
    public ArrayList getFirstNThings(int n);

    //returns last n items from list
    public ArrayList getLastNThings(int n);
}

Then there's an implementor like so:

public GroceryList implements IListable {
    private ArrayList<GroceryItem> groceries;

    public GroceryList() {
        this.groceries = new ArrayList<GroceryItem>();
    }

    public ArrayList<GroceryItem> getFirstNThings(int n) {
        ArrayList<GroceryItem> firstNThings = new ArrayList<GroceryItem>();
        for (int i=0; i < n; i++) {
            firstNThings.add(this.groceries.get(i));
        }
        return firstNThings
     }

     public ArrayList<GroceryItem> getLastNThings(int n) {
         ArrayList<GroceryItem> lastNThings = new ArrayList<GroceryItem>();
         for (int i=this.groceries.size(); i < this.groceries.size()-n; i--) {
           lastNThings.add(this.groceries.get(i-1);
         }
         return lastNThings;
      }
}

Ignore any implementation problems you may find in that (I found a few too). What I'm getting at is that the interface does not use any generic type parameter for ArrayList (i.e ArrayList<?>), but the implementor of the interface's method does (i.e. ArrayList<GroceryList>). Other implementors may return ArrayLists with any other type parameters, no?

So my questions: Is this a problem? Should I refactor anything? Is it worth it? What's the advantage? What kind of problems can I run into if I have a method defined in an interface whose return type is a raw type, but the actual implementors of the method return various parameterized types?

Answer 1

if both methods of IListable always return the same type, use this instead:

public interface IListable<T> {
  //returns first n items from list
  public ArrayList<T> getFirstNThings(int n);

  //returns last n items from list
  public ArrayList<T> getLastNThings(int n);
}

if this isn't an option, try using ? instead. While it's basically the same, it avoids ugly warnings.

public interface IListable {
  //returns first n items from list
  public ArrayList<?> getFirstNThings(int n);

  //returns last n items from list
  public ArrayList<?> getLastNThings(int n);
}

Generally, it's not a problem to use a more specific return type in an implementation than in a super-type or interface. If you're dealing with IListable, you need to handle any object type in the returned list. If you're dealing with GroceryList, you expect only GroceryItems. That's not only true for genric type arguments of return types, but for the return type itself as well. So if an interface specifies List<Foo> get(), is okay to implement it as ArrayList<Foo> get().