A Map is a collection objects. But Map interface is not compatible

with Collection interface in Java.

A Map requires key as well as a value. So it requires two

parameters to add an element to a HashMap.

But Collection interface provides add(Object o) method with only

one parameter.

Map collection has to provide methods like valueSet, keySet etc.

These methods are specific to Map collection. Where as methods in

Collection interface can be reused by a List, Set, Queue etc.

https://agrawalsuneet.github.io/blogs/why-map-does-not-extend-collection-interface/