Help with exam question

Question

Please can someone help with this exam question I have. I have no idea about it I have been looking through loads of website and class documentation but I still have no answer.
Please can someone explain this to me thanks
Consider the following code:
Import java.util.hashmap;
Import java.util.map;
Public myClass {
     Private Map <Integer, Object> myMap = New HashMap <Integer, Object> ();
     Public void addStuff(int id, Object Stuff) {
               myMap.add(new Integer(id), stuff);
     Public Map getMyMap() {
               Return myMap;
Why is myMap declared as a Map and not a HashMap?

Answer 1

It's called programming against the interface.
The code using myMap doesn't need to know that it's a HashMap, but only needs to know that it's a Map (Map is an interface that HashMap implements, so every HashMap is-a Map).
Since that knowledge is not required it's better not to give it away.
If at some point we decide that HashMap is not the best choice here we can easily change it to any other Map implementation (such as TreeMap for example) and only need to change that one line.