Map map=new HashMap (); Why is that so?

Map is an interface, and HashMap is an implementation of map. The interface cannot be instantiated.
Map map=new HashMap (); is to instantiate the map as a hashmap. The benefit of this is that the caller does not need to know the map specific implementation, map interface and the specific implementation of the map Java to help you do. For example, you are a leader, there are several people: Zhang San, John Doe and so on. You need to do one thing, you are concerned about whether there is someone to do, and do not care who is going to do, you just need to shout: "Somebody!" "Specifically, it doesn't matter who comes in and does things for you.
If this is said: HashMap map = new HashMap (), it is the equivalent of what you specify to Zhang San do: "To Zhang San!" ”
That's it.

Map map=new HashMap (); Why is that so?