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

For

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

List and Map are the interfaces, so any class implementing those interfaces can be assigned to these references.

ArrayList is one of the several classes (another is LinkedList) which implement List interface.

Same with Map. HashMap, LinkedHashMap, TreeMap all implement Map.

It is a general principle To program for interfaces and not for implementations. Due to this, the programming task becomes easier. You can dynamically change the behavior of the references.

If you write

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

ints and map will be ArrayList and HashMap only, forever.