Question

ArrayList aList = new ArrayList();

List aList = new ArrayList();

What’s the difference between these two and which is better to use and why?

Answer 1

List<T> is interface, where ArrayList<T> is class, and this class implements List<T> interface.

I would prefer 2nd form, which is more general, ie if you do not use methods specific to ArrayList<T> you can declare its type as interface List<T> type. With 2nd form it will be easier to change implementation from ArrayList<T> to other class that implements List<T> interface.

EDIT:
As many of SO users commented both forms can be arguments to any method that accept List<T> or ArrrayList<T>. But when I declare method I would prefer interface:

 void showAll(List <String> sl) ...

and use:

 void showAllAS(ArrayList <String> sl) ...

only when my method uses method specific to ArrayList<T>, like ensureCapacity().

Response with information that we should use typed List<T> instead of just List is very good (of course if we do not use ancient Java).