Why do we first declare subtypes as their supertype before we instantiate them?

Question

Reading other people\'s code, I\'ve seen a lot of:

List ints = new ArrayList();
Map map = new HashMap();

Answer 1

This is programming to the interface not the implementation, as per the Gang of Four. This will help to stop the code becoming dependent on methods that are added to particular implementations only, and make it easier to change to use a different implementation if that becomes necessary for whatever reason, e.g. performance.