Why declare a variable its parent?

Question

I find somebody writing their codes like:

Map<String, Integer> map = new Hashmap<>();
List<String> list = new ArrayList<>();

instead of:

Hashmap<String, Integer> map = new Hashmap<>();
ArrayList<String> list = new ArrayList<>();

So is there any advantages?
Or it's just their habit?

Answer 1

https://www.sololearn.com/learn/Java/2164/

Maybe you should read this first.