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

 List ints = new ArrayList();

If you write some code that deals only with List then it will work for any class that implements List (e.g. LinkedList, etc). But, if your code directly deals with ArrayList then it's limited to ArrayList.

CharSequence s = new String("String");

Manually instantiating a String object is not good. You should use string literal instead. I am just guessing the reason that you don't see CharSequence might because it's quite new and also, strings are immutable.