You could see a Map as a collection of key-value pairs. The designers of the
Java collections library could have chosen to implement a Map like that, but they didn't. I know other programming languages in which the Map is indeed a Collection of key-value pairs. So, ultimately there's no logical reason why a Map could not possibly be a Collection, it comes down to a design choice that probably
Joshua Bloch made years ago. Rob already explained some of the reasons why it might have been designed like this.