How to program to interface?

Question

What does it mean exactly to "program to an interface"?
I had always learned to establish an interface and then write an implementation. However, is that what programming to an interface means?

Answer 1

Even if you "know" you're going to have an ArrayList,
the code only knows that it has a List, so it should
not be written as if it assumes a particular
implementation.The problem with the List interface (abstraction) is that it combines the weaknesses of two common data structures: the linked list and the dynamic array.
List has slow random accesses like a linked list and slow insertions at the beginning like a dynamic array. It should be very seldom one would prefer such a data structure.
(There's also a RandomAccess marker
interface, but that doesn't extend List, so, since
you still need List's functionality, you can't just
declare your variables to be of type RandomAccess.)RandomAccess still can be used to "cook up" an interface for a dynamic array,
public interface RandomList<T> extends List<T>, RandomAccess {
}Users of RandomList are guaranteed that random accesses are fast. It's easy to provide a concrete RandomList implementation like this,
class RL<T> extends ArrayList<T> implements RandomList<T> {
}